SEASONAL PREDICTION FOR WINTER 2014-15 FOR NORTHERN BRITAIN BASED ON LARGE-SCALE SST, SEA-ICE ANOMALIES IN AUTUMN 2014By iapennell
16th November 2014
Dear Fellow Forum Members;
I would like to express my sincere thanks for all those who appreciated my predictions of the prevailing weather -patterns likely through Winter 2013-14; I predicted a mild, wet and stormy winter and this is indeed what happened. I did also predict one or two cold snaps later in the winter last year but (with the exception of something a bit colder in mid-late February that resulted only in snow on high ground in the North) I do not think this developed to the intensity I expected it might. The persistent mild south-westerly flow held out far more consistently (and even the cold air off Greenland did not reach Britain against the strong south-westerly flow). The total rainfall in southern counties of England was also a god deal higher than I predicted. Aside from these details I am pleased that the general thrust of my predictions came true, even if some of the finer details were not quite right.
Buoyed by the success of last winter's forecast I will attempt to do the same for this year- based on macro-scale sea-surface temperature anomalies in the North Atlantic and the pattern of Arctic Ice extent in the autumn of 2014: I actually made this prediction over a month ago (and also cover October and November 2014) so you can judge for yourselves how accurate I have been to date.
So here it is:
PROGNOSIS FOR AUTUMN/WINTER 2014-15 FOR NORTHERN BRITAIN
Macroscale Physical Controls on the Weather in Britain
This is the long-term seasonal prediction for the general weather over the next six months for northern Britain: It is based on the macro-scale atmospheric circulations and temperatures likely to develop in the coming months from the time of writing (mid-September 2014). At this time of year patterns of temperature (including sea-surface temperature), Arctic ice-cover and net-radiation anomalies and (likely) global wind-patterns become decisive in determining the weather that is liable to occur over the coming months. The main direct controls on the prevailing weather during the winter half-year are:
The prevailing strength and direction of airstreams affecting the country. The prevailing wind during these seasons is normally south-westerly which means air is moist and transported from warmer parts of the North Atlantic (which being a large body of water never really becomes cold during the winter months). However, frequent spells of northerlies from the frigid Arctic or easterlies from Russia (which gets very cold in winter) would result in frequent spells of freezing weather, possibly with snow.
The amount of (and type of) cloud cover over the country, the height at which this forms and the humidity/temperature of the upper atmosphere that prevail throughout the winter. Throughout the winter half-year in northern Britain the Sun fails to reach as high as 30Â° above the horizon at noon and it is below the horizon for more than twelve hours each night; consequently given clear skies the solar energy absorbed at the surface is less than the long-wave radiative heat loss from the surface to space during each 24-hour period and so fine, dry and clear weather results in the low atmosphere getting progressively colder if there are no winds to bring milder air from elsewhere to forestall such cooling.
Thick cloud cover with warm low bases absorb heat radiated by the ground and re-radiate a similar degree of heat back to the surface, so thick cloud-cover tends to forestall the net cooling that would otherwise take place at the surface in the late autumn and winter months in northern Britain.
Sea surface temperatures up-wind of the prevailing airstreams reaching Britain clearly have a significant effect on their temperature. The seas around Britain, owing to the influence of the North Atlantic Drift and their large thermal heat capacity retain the warmth they receive from the summer sun through the autumn and into the winter. This means that winds from the west or south-west are mild and damp; it also means bitterly frigid airstreams starting out from the central Arctic or from northern Russia are moderated by the sea-surfaces they have to pass over en-route to northern Britain and this means that conditions are not as severe as they otherwise might be. Clearly, if the seas around Britain are warmer than usual in the autumn the greater their moderating influence on any cold airmasses reaching for Britain will persist well into winter because seas (having a large heat capacity) donâ€™t warm or cool rapidly.
In predicting the type of autumn/winter we are likely to get it is clearly fundamental that we have some indications of the basic controls on the weather in this country that are likely to unfold: There are another three basic predictors of these effects that are global rather than just affecting the immediate region around Britain. Three fundamental physical principles which underpin the entire Global Weather System control the prevailing winds in the winter half-year in this country, the dominance of fine and clear or cloudy/wet conditions and sea surface temperatures around Britain going into the winter. These physical principles are thus:
The changes in the density of air as it cools, warm air is less dense and it therefore expands. This is a fundamental Law of Physics called Charlesâ€™ Law which states that the volume of a given amount of gas at constant pressure is directly proportional to its absolute temperature. It is this fundamental law kicks into play to drive the Global Weather System as a consequence of the net heat loss in higher latitudes causing the atmosphere to get colder in the autumn and winter months whilst the strong sunshine heats the atmosphere over the deep tropics.
Where the air becomes more dense (due to cooling)- following Charlesâ€™ Law- there is an area of low pressure at high levels in the atmosphere which encourages air from surrounding regions to rush in to fill the void; this results in a sharp increase in atmospheric pressure at the surface (as air at high levels piles in to fill the void left by contracting denser and colder air); and there is a corresponding fall in surface pressure for those surrounding regions with warmer atmospheres. This is fairly simplistic, but it broadly explains why, during the Northern Hemisphere winter surface atmospheric pressure is relatively low near the equator but high over the frigid wastes of interior Siberia and Mongolia and also high over the cooling deserts of North Africa, Arabia and the southwest USA.
Clearly this is not the only control on the weather-patterns; if it were all mid and high-latitude locations would have dry, cold winters with hard frosts but with no rain or snow. Britain would be exposed to constant dry, bitter north-easterlies from northern Siberia in winter if there was not another important control on the global winds and weather patterns.
The second big control on the global weather patterns is the physical Law of Conservation of Angular Momentum: This is the principle by which ice-dancers can increase the speed of their pirouettes by drawing their arms into their bodies. This is an important physical law which governs the behaviour of every moving body in the universe, it enables planets to keep on orbiting without slowing down and crashing towards the Sun and it enables all planetary bodies to keep on spinning. Put simply this Law states that a body will conserve its absolute momentum without any outside force acting on it.
The Law of Conservation of Angular Momentum has a major control on the movement of airmasses on our rotating Earth: Air that moves from the subtropical-highs that intensify in winter over cooling continents towards the equator where hot, steamy air rises and where surface pressure is lower does not (and cannot) move directly from north to south. Because higher latitudes of the Earthâ€™s surface and atmosphere rotate closer to the Earthâ€™s axis of rotation they have a lower angular momentum than the equator which is furthest away. On moving to lower latitudes the Trade Winds (the name given to the winds that blow from the subtropical highs towards the low-pressure belt near the equator) retain their initial momentum and are not moving from west to east on the rotating Earth as fast as the ground beneath it. As a consequence to an observer on the ground the air is apparently moving from east to west, as well as blowing from the north; in other words our northerly Trade Winds become the NE Trade Winds. In the winter months, when temperature and pressure gradients between subtropical continents at their coolest and the hot, steamy equator reach a maximum these NE Trade Winds tend to be quite brisk in strength.
Now, if the air at low levels simply blew from cold areas of high-pressure in higher latitudes towards the equator- and were modified by the Conservation of Angular Momentum as just described- chilly, dry easterly or north-easterly winds would cover the entire Northern Hemisphere in the winter months: Clearly this does not happen, and again the reason why this does not happen is down to the Law of Conservation of Angular Momentum.
There could not be a system whereby north-easterly winds blew throughout the Northern Hemisphere in winter at low levels because the easterly winds would be blowing against the Earthâ€™s rotation and slowing down the Earth whilst at high elevations the air (moving from the equator to the arctic) would retain the west-to-east rotation it had at the equator with the result that there would be upper-atmosphere winds from the west of 1000â€™s of miles per hour on reaching high latitudes. The frictional loss of easterly momentum (a gain of westerly momentum) due to easterly winds at the surface would lead to the upper atmosphere over a wide range of latitudes whizzing from west to east faster and faster during the winter months; this would lead to the upper parts of the atmosphere reaching escape velocity and flying off into space! Whilst it is true that there are zones of very strong westerly winds at high elevation and that encircle the Arctic in winter (the jet-streams) these do not blow at thousands of miles per hour; usually they are 200-300 mph at most. It is also true that the rotating Earth does slow down very slightly during the Northern winter (adding perhaps one millisecond to the length of the day), and that this is indeed due to changes in pressure and global wind-patterns in the winter months; but it is nothing like what would occur if stiff north-east winds were permitted to blow over the entire Northern Hemisphere throughout the winter.
Clearly there are dynamical constraints on the global wind-patterns that prevent easterlies blowing over an entire hemisphere for any length of time, and that prevent upper westerlies blowing at 1000â€™s of miles per hour: When air moving at high levels from the deep tropics towards the subtropics starts moving a 100â€™s of mph from west to east (by virtue of the Earth rotating at a slower rate beneath it) it forms a barrier to air moving in from the south behind it. This results in the air â€œpiling upâ€ at high levels and being forced to sink in these subtropical latitudes (about 30 to 35Â°N in the winter months). Further north the strong upper westerlies largely prevent the poleward flow of air whilst at low levels cold, dense air still seeps south from cold land areas where pressure is high. This leads to the situation at middle latitudes whereby the troposphere (that part of the atmosphere in which weather happens) is much shallower than in the tropics; it also means that despite its lower temperature that overall the surface pressure tends to drop as one moves north from the subtropics (and this is especially true over warmer ocean surfaces as the warmed airmasses expand at lower levels and the surface atmospheric pressure drops further).
As a consequence of the piling up of rapidly easterly-moving air at high-levels over the subtropics and the restrictions on high-level air moving into ever higher latitudes, the surface atmospheric pressure reaches a maximum near 35Â°N in the winter months: North of this latitude surface pressure drops off rapidly and this leads to winds at low-levels blowing north from the subtropical high-pressure belt into middle latitudes. As these airmasses move north they are again modulated by the fact that the earth below rotates as a progressively slower rate from west-to-east and so the winds angle in from the south-west. These winds bring warm, moist air from the subtropical oceans towards the Arctic and, put simply, when these warm, moist winds meet very cold air seeping out from the highest latitudes the steep atmospheric temperature and pressure contrasts result in the formation of deep depressions: On the southern flank of these depressions the middle latitude south-westerlies are at their strongest and these zones form the sink of most of the accumulated westerly momentum imported to the atmosphere via both the NE Trade Winds and the zones of easterly winds in the highest latitudes. This is, in a nutshell, what prevents the upper air from reaching speeds enabling it to fly off into space and it also prevents the Earth slowing down in its rotation to any noticeable effect over the years!!
Therefore, depending on how strong the NE Trade Winds are in the tropics and subtropics and where, precisely the zones of strongest westerly winds occur in higher latitudes to counterbalance them, the weather patterns in Britain are dominated- to a greater or lesser degree- by stiff westerly or south-westerly winds. This does not preclude spells with icy north or east winds but it does mean that for Britain, at the eastern edge of a large body of relatively warm water for its latitude, they are hardly likely to persist for long.
The third major global control on the weather likely to affect Britain in the autumn and winter months is the direct and obvious implication of the macro-scale heat-budget in the months beforehand: In other words, if man has been polluting the atmosphere with CO2 for decades beforehand and this results in a small proportion less heat radiated from the Earthâ€™s surface making it to space over the entire planet the result will be a small net positive heat surplus at the Earthâ€™s surface as averaged over the entire planet. This heat surplus would lead to warmer temperatures worldwide, warmer seas and less snow and ice in the arctic (leading to less of an area of the Arctic suitable for the manufacture of intensely cold air in winter). This would then lead to warmer airstreams reaching Britain, any cold airstreams being modulated by warmer seas before reaching the country- and it would lead to milder winter conditions in this country with less likelihood of hard frost and snow. Over a shorter time-frame the Sun can vary its output by up to 0.5% over the course of a typical sunspot cycle (11.5 years) and this has significant effects on the climate. Major volcanic eruptions in summer can put so much dust and sulphur dioxide into the stratosphere that summer sunshine to the Earth and low atmosphere can be reduced by a few percentage points across middle and high latitudes so that the ensuing autumn and winter weather is moderated by seas cooler than usual and high latitude ice and snow get a head start. Some of Britainâ€™s most severe winters in the 17th and 18th centuries followed series of major volcanic eruptions (i.e. 1783-4 winter following eruption of Laki in Iceland).
As part of the global ocean circulation patterns the El Nino Southern Oscillation (ENSO) results in significant temperature changes throughout much of the tropical Pacific Ocean over periods from one to three years or more. This is really a part of the internal workings of the ocean-atmosphere circulation but the temperature and associated pressure changes in the tropical Pacific Ocean lead to variations in the rates at which heat is either stored within the ocean system or released into the Global Atmospheric Circulation: During strong El Nino phases very warm water extends right across the equatorial Pacific leading to a stronger zone of hot rising air near the equator (i.e. lower pressure), stronger NE Trade Winds in low latitudes of the Northern Pacific Ocean in winter and- by implication- stronger mid-latitude westerlies and more storms because of the need for stronger westerlies in higher latitudes to counterbalance the tropical easterlies. The hotter seas in the equatorial Pacific during El Nino episodes lead to warmer air rising over the equator and warmer air descending in the subtropical highs in the Northern Hemisphere. This warmer air then feeds into the stronger south-westerlies associated with El Ninoâ€™s that push into higher latitudes.
(CONTINUED FROM ABOVE)
Large-scale Patterns of Sea Surface Temperature, Arctic Sea Ice and Angular Momentum influences in autumn 2014
Let us now see how these macro-scale controls look at the end of September 2014, and from this we will see what kind of a winter this portends for northern Britain. There are a number of important factors to consider, most fundamental being the sea-surface temperatures across the North Atlantic and in the tropical Atlantic and Pacific Oceans. At the time of writing sea-surface temperatures are over 1Â°C above normal for the time of year across the North Atlantic between 50 and 60Â°N, with an area off the NE Canadian coast over 2Â°C warmer than usual and there is an area of very warm water further south to the west of Portugal (2Â°C warmer than usual here). Hudson Bay in Canada and the Davis Strait (between NE Canada and Greenland) is also very warm with sea surface temperatures between 2 and 4Â°C above normal for the time of year). Sea surface temperatures between Iceland and the west coast of Norway are up to 2Â°C warmer than normal for the end of September. The sea-surface temperatures in the North Pacific north of 50Â°N are also unusually warm (over 2Â°C above normal for the end of September).
However there is a patch of the mid-North Atlantic centred about 45Â°N with sea-surface temperatures up to 1Â°C colder than normal for the season. Sea surface temperatures just north of Norway and in the Norwegian Sea are not any warmer than usual for these locations for mid-September. This increases the odds of highest atmospheric pressure associated with the subtropical high occurring in the north-west Atlantic whilst depressions deepen more over the warm waters of the Davis Strait and move more west to east across southern Greenland and to the north of Norway: This pattern favours more westerly than south-westerly winds blowing towards the UK during the winter months, which marginally increases the chance of cold Arctic outbreaks in January-February (likely to come off a very cold Greenland by then). However, the unusual warmth across higher latitudes of the North Atlantic Ocean in general will encourage deep depressions moving north-east from Newfoundland to Iceland and then north of Norway; this tends to favour strong south-westerlies over Britain. That said, the patch of relatively cool water at 45Â°N in the mid North Atlantic will encourage a small semi-permanent ridge of high pressure to extend north from the subtropical high in the NW Atlantic and this will steer the prevailing winds north-east of this point to westerlies- it follows that the strong west or SW winds reaching Britain will come via somewhat higher latitudes than would occur otherwise.
The overall ocean temperatures upwind from the UK are fairly fundamental to the outcome of the late autumn and winter because warmer-than-usual ocean surfaces modulate any cold airstreams that may pass over them more than usual, a long warmer-than-normal ocean-track upwind of prevailing winter westerlies reaching Britain is naturally going to mean mild (and wet) winter conditions. If the ocean and the air passing over it is just 1C warmer the atmosphere can absorb 10% more water vapour so this is a predilection for warmer and wetter conditions through autumn and winter just for starters. However there are other major macro-scale considerations that are important:
The tropical North Atlantic and tropical North Pacific oceans are roughly 1Â°C warmer than usual and large areas of the North Atlantic between 10 and 35Â°N have sea-surface temperatures in excess of 28Â°C. The NW Pacific Ocean from the equator up to 35Â°N is also warmer than 28Â°C at the surface and a large area of the ocean surface east of the Philippines is hotter than 30Â°C. The equatorial Pacific is, at the time of writing, 1Â°C warmer than usual overall and a number of computer models forecasting the trend of ENSO hint at a weak El-Nino phase setting in over coming months. Taken together this points to more hurricanes and typhoons over the tropical North Atlantic and Pacific respectively (hurricanes require extensive areas of water over 27C well north of the equator- a condition that looks likely to be met until November), and also a stronger Intertropical Convergence Zone (ITCZ) of hot, moist rising air over the equatorial Pacific Ocean: All this portends stronger NE Trade Winds on the north flanks of tropical storms and north of the ITCZ being drawn into a deeper low-pressure zone near the equator- certainly in the late autumn but also extending into winter. Strong westerlies along the southern flanks of hurricanes ought to cancel out the effect of strong easterlies but much of this air tends to feed into the circulation south of the ITCZ and by implication the general circulation of the Southern Hemisphere.
Hotter tropical seas- provided there is still a substantial drop-off in sea-surface temperatures into the subtropics- therefore lend themselves to stronger tropical easterlies. This in turn means that the higher-latitude westerlies that affect Britain will need to blow stronger and more persistently to counterbalance the low-latitude easterlies. This also points to a mild, wet and (at times) stormy winter in this part of the world. The strength of hurricanes through the autumn also suggests that the circulation across the North Atlantic will need to degenerate to strong westerlies (with associated deep depressions in high latitudes) soon so that such winds can counterbalance the effect of easterlies associated with hurricanes as well as tropical NE Trade Winds arising from the strengthening of the subtropical highs over the northern subtropical landmasses as these cool down in the autumn.
At the time of writing the circulation looks fairly weak over the North Atlantic and a blocking high over Scandinavia is feeding easterlies towards the UK: From the above inference this looks set to change fairly soon.
The seasonal extent of the Arctic sea-ice is below the norm for the time of year but is not as much below normal as in 2012: At the time of writing the Arctic sea-ice covers an area of roughly 5.5 million square kilometres which is some 1.5 million squ. km below the long-term late-September normal extent. This pack-ice is centred roughly 250 miles on the Canadian side of the North Pole so the boundary between ice and open sea is close to its normal position for mid-September from Arctic NE Canada to Greenland and to Spitsbergen; however it is well north of its normal extent to the north of Siberia and Alaska.
The Arctic ice-cap does tend to grow and expand in size during the late autumn and winter months because the sun disappears from the High Arctic during these months allowing for uninterrupted net cooling of the surface and atmosphere here: However if the area covered by Arctic pack-ice is very low at the start of the autumn and seas in the sub-arctic are warmer than usual then the pack-ice is not likely to reach the areal extent it would otherwise reach during the winter months. This means that the area of frozen sea available for rapidly cooling airmasses to get very cold without modification by open water is reduced and thus the areal extent (and intensity) of very cold Arctic air is also lower. The strong atmospheric temperature gradient between the very cold Arctic air and the much warmer air from mid-latitude oceans is correspondingly pushed north; since it is this sharp atmospheric temperature and pressure gradient that spawns deep depressions these get pushed north too.
The seasonal extent of snow-cover remains below normal at the time of writing; only Greenland and the Arctic Canadian Islands along with far northern Siberia (north of 70Â°N) and the mountains of the Canadian and Alaskan Rockies have snow or ice-cover as yet. Snow-cover is important on a regional scale because it reflects away 70 to 85% of the Sunâ€™s heat back into space whilst emitting terrestrial (long-wave) radiation strongly to space under clear skies. The delay in the onset of seasonal snow-cover in the sub-arctic means the bare ground is still absorbing the Sunâ€™s heat,- even though the sun is weak in September in these latitudes the lack of snow-cover means the net seasonal cooling is reduced significantly and this further delays the cooling of the Arctic as a whole. At the time of writing the Greenland Icecap is already very cold,- below -25Â°C and the frozen Central Arctic Ocean and Canadian Arctic islands are about -15Â°C (much as would be expected in late-September). On the other hand most of northern Siberia has now only just reached 0Â°C (as averaged over day and night), whilst most of lowland Alaska and NW Canada is about 5Â°C,- itâ€™s going to be a few weeks before snow-cover is properly established throughout these locations.
All of this means that far northern Canada and Greenland are the only parts of the Arctic that will get very cold on time, whilst there will be a delay in the onset of very cold conditions by about three weeks over Alaska, central north-east Canada (because the warmth of Hudson Bay will retard the establishment of very cold conditions there) and the onset of very cold airmasses building up will be delayed by three weeks over Siberia. The winter zones of high-pressure that form over Siberia and NW Canada will be delayed- and it is likely that (because of warmer than usual water north of Siberia) depressions will push along the Arctic coasts of both Siberia and Alaska with spells of strong westerlies affecting regions immediately to the south until these waters eventually freeze (this does not look like happening until late November).
By December it is still likely that Hudson Bay and the Arctic waters just north of Alaska and Siberia will have frozen over. By this time, with the disappearance of the Sun altogether from northern Siberia, Alaska and northern Canada helping to establish intense cold over these continental regions, high-pressure will form over the northern continental interiors. This means that deep depressions will move from the still-open Davis Strait east to Iceland and then north-east to Spitsbergen (deflected by high-pressure over interior Russia).
The 11.5 year duration sunspot-cycle has now passed its maximum; the Sun is a little quieter and a tiny bit weaker: The mean six-monthly averaging for the Solar Constant has declined from 1361.6 Watts per squ. metre for January 2013 to 1361.3 Watts per squ. metre for August 2014; this is a change in solar heating of rather less than 0.05% and it would have a global cooling effect of less than 0.1C. This is undoubtedly counteracted by the weak El Nino phase in the equatorial Pacific Ocean where mean temperatures are close to 1Â°C warmer than usual; the total thermal forcing effect of the extra CO2 man has now put into the atmosphere over the last 100 years amounts to 4 Watts per squ. metre extra warming as averaged over the entire planet throughout the year, this will lead to 1.5Â°C global warming compared with the early 20th Century when the entire planet gains equilibrium with this thermal forcing and positive feedback effects are considered. With more CO2 being added to the atmosphere each year and without any major cooling influence from volcanoes likely over the next year the overall heat equation favours warmth for northern Britain.
There has been more than enough extra heat pumped into the Northern Hemisphere oceans during the spring and summer months of 2014 to keep the adjacent higher-latitude continents from getting spectacularly frozen in the coming months: Furthermore, clear skies associated with a northwards extent of the subtropical high-pressure belt allowed the strong June and July sunshine to heat the middle-latitude waters of the North Atlantic more than usual; this is heat that will subsequently be slowly released into westerly winds before reaching the United Kingdom during the autumn and winter months of 2014-15.
What is perhaps more important is that the more frequent (and powerful) solar storms at the peak of the sunspot cycle do have in impact on the general circulation in the autumn and winter months: The interactions of blasts of solar wind with the Earthâ€™s magnetic field at such times produce increases in the Aurora Borealis over the Arctic, but also the blasts from solar storms does impart increased westerly momentum to the atmosphere; deep depressions south of Alaska in winter have been observed to become 20% more intense than would otherwise be the case in the days following vigorous solar storms (â€œThe Climatic Threatâ€ Gribbin, 1977). This implies an outside force acting on the Earth-Atmosphere system to marginally increase its overall rate of rotation- and the clear implication is that at the Sunspot Maximum phases of the 11.5 year Solar Cycle the upper and near-surface higher-latitude Westerlies would blow stronger at such times.
We have now well past the peak of sunspot activity and of solar flares in the current 11.5-year Solar Cycle (it was maximum in early 2013). All things being equal this should lead to a winter in 2014-15 that is a bit less persistently mild, rainy and stormy than was winter 2013-14: There is a likelihood later in the winter of a couple of short blasts of much colder northerly and north-westerly winds reaching northern Britain when the Greenland Icecap and the Arctic are at their coldest- but the factors of Arctic Ice, higher than usual mid-latitude sea-surface temperatures and the Sunspot Cycle still in quite an active phase are not indicators of prolonged spells of really severe winter weather for the United Kingdom.
All these factors suggest that deep depressions will be forming and moving east further north than usual, at least until December. The reduced areal extent and intensity of cold airmasses over the Arctic compared to normal reduces the likelihood of unusually cold northerlies reaching Britain in the autumn months; but that warmth in the Davis Strait, the likelihood of high-pressure in the NW Atlantic and mild conditions over northern Eurasia precluding the Siberian High so that depressions can sweep along the Arctic Coasts of Scandinavia and Russia- all point to strong cooler westerlies rather than south-westerlies over Britain during the late autumn. The main tracks of deep depressions will remain well north of the UK; consequently southern Britain will enjoy spells of dry bright weather at times with temperatures near or marginally above normal.
Mild conditions compared to normal over eastern Canada (thanks to a still unfrozen Hudson Bay) and the patch of cool water around 45Â°N in the North Atlantic, but warm seas further east will probably lead (given the likely strength and northerly position of the jet-stream compared to normal) to a â€œthree-wave patternâ€ in the upper Westerlies but with the troughs and ridges positioned some 15 to 20 degrees longitude further east than normal over the North Atlantic through late autumn. The small surface ridge from the south in the NW Atlantic is likely to be complemented with an upper trough positioned from Iceland to west of Scotland- cue some very stormy, rainy and chilly (but not unseasonable cold) spells in both October and November; there will probably be substantial snow accumulations on the Scottish mountains above about 1,000 metresâ€™ elevation during these months. Further south, and to the east of the Pennines, such squally westerlies will carry less moisture and there will be a good deal of drier and brighter weather.
The overall strength of the wind is likely to be too high throughout the autumn to permit frosts to occur under clear skies. However occasional air frosts cannot be ruled out further north where the wake of some depressions will mean cooler north-westerlies and where; as winds drop and night skies clear (as the depressions move away into northern Scandinavia) nocturnal cooling is almost certain to lead to a few air frosts in Scottish glens and favoured Northumbrian valleys (i.e. Redesdale- a tributary of the North Tyne). Air frost is very unlikely to occur more generally across Scotland, northern England and the north Midlands until around the 25th October and the first air frost will probably be delayed until about the 10th November in the South of England and in west and south Wales.
From December onwards, it is certain that Hudson Bay will have frozen over and that areas of open water north of Siberia and Alaska will be frozen over by then- this will (in combination with the disappearance of the sun over both northern Alaska and northern Siberia) lead to the establishment of very cold conditions over all of Siberia and Alaska, as well as most of Canada by this time. However there will be a delay in the freezing of the Davis Strait because it is currently much warmer than usual, but cold west and north-west winds off North America into the NW Atlantic is as likely to mean that the patch of water near 45Â°N in the mid-north Atlantic currently cooler than the seasonal norm is liable to remain so for the winter months too. High-pressure is still likely to linger close to this part of the NW Atlantic with deep depressions more likely to form in the Davis Strait, move east towards Iceland and then be deflected north-east towards Spitsbergen by the influence of the Siberian High (this is likely to be well-established by December): Although strong Westerlies blowing into Europe are more likely to restrict the influence of the Siberian High to the Russian border for much of the winter there is a situation that could well unfold during December that would lead to this high-pressure extending right across Europe and bringing (for a time at least) some dry but much colder weather to the United Kingdom. The reasoning for this is discussed below.
It is probable, that the cold patch of water in the central North Atlantic combined with the Mediterranean Sea being considerably warmer than usual (at the time of writing), will lead to temporary split in the upper Westerlies that encircle the Arctic with some of these upper winds tracking south across the Mediterranean and with the rest blowing strongly from south of the Davis Strait across Iceland to north of Norway: This is likely to happen at some point in December 2014 whilst the Davis Strait is still ice-free and the cooling of continental Europe (combined with the cold-patch in the North Atlantic nudging the jet-stream southwards in the lee (east) of it) encourages a frontal zone to develop between the chilly continent and the even warmer-than-usual Mediterranean Sea to the south. This type of development would encourage some of the higher-latitude Westerlies to sweep over the Mediterranean in association with depressions moving along around 40Â°N latitude whilst the remainder of strong Westerlies would cross the North Atlantic but shift to be restricted to the sub-arctic on approaching Europe. This situation is likely to unfold at the end of the hurricane season in the tropical Atlantic and Pacific Oceans (end of November into December) which means there will be a temporary slight overall reduction to the strength of the tropical and sub-tropical easterlies- and hence the â€œneedâ€ from a Conservation of Angular Momentum standpoint for strong Westerlies in higher latitudes.
This development is important because it would displace the higher-latitude Westerlies from western Europe for much of December; this would allow mainland Europe to cool rapidly (the net radiation balance being strongly negative in December close to 50Â°N): This rapid cooling of the lower atmosphere over Europe would allow high-pressure to develop and to pull in much colder air from Russia to the east; particularly since over the southern flank of this high-pressure (but north of the Mediterranean depressions that would occur in this developing weather-pattern) winds will be easterly or north-easterly. How far west this cold Russian air gets depends on a number of factors, not least the overall strength, extent and position of Westerlies over the North Atlantic, but it looks certain that cold high-pressure will cover all central Europe for a couple of weeks in December due to the developments referred to above. However deep depressions will still be tracking east in the latitude of Iceland and northernmost Norway and with the strong westerlies extending 10 to 15 degrees south of these latitudes I am confident that north and west Scotland will remain under the influence of mild west or SW winds off the North Atlantic: An area south-east of a line running roughly from North Wales to Northumberland will almost certainly experience a week or more of dry, fine conditions but with progressively lower temperatures- caused by the progressive advection of colder air over southern and eastern Britain and the day-by-day net radiative heat loss from the surface under clear skies in December.
This weather pattern is likely to yield the coldest weather of the winter for the South and in Yorkshire- culminating in freezing fog and nighttime temperatures widely falling to about -5Â°C; this depends to some extent on how much clear skies accompany the high-pressure and whether the winds truly fall light because if all of Britain remains on the periphery of the European High a modest breeze could keep temperatures from falling below freezing even in the coldest locations. On the other hand if the continental high gets truly entrenched with Russian air reaching Britain followed by clear skies and flat calm for several days encouraging several days of strong near-surface cooling across England a very severe spell of weather could ensue. However, because air arriving off continental Europe is relatively dry and the high-pressure is likely to be centred around 50Â°N and certain to extend in a wide belt some 5 to 10 degrees either side of its centre I am confident that this will usher in a spell of dry, clear and progressively frosty weather for the south-east half of the country. I am not predicting snowfall to be associated with this cold weather because it would be associated with high-pressure which, by its nature, tends to limit the scope for precipitation.
Whilst I am confident of a dry, cold and frosty spell over England at some point in early-mid December 2014; this is certainly unlikely to extend to Scotland and also the continued cooling of the subtropical lands and seas (leading to stronger NE Trade Winds) and the increasing extent of Arctic ice leading to colder Arctic temperatures- all point to strong higher-latitude Westerlies resuming in force from late December onwards: The stronger NE Trade Winds will increase the â€œneedâ€ for stronger (and more extensive) Westerlies in higher latitudes to counter-balance them and the stronger atmospheric temperature and pressure gradients between the truly frigid Arctic in winter and the still-warmer-than-normal North Atlantic will encourage the deeper depressions to form in high latitudes capable of delivering such strong westerlies. Stormy conditions are therefore liable to resume across the whole of the British Isles over the Christmas Period and these conditions will last through January and February 2015.
On the whole the winter months of 2014-15 are likely to be dominated by strong westerlies: The North Atlantic as a whole is still likely to be warmer than usual during the winter months (the exception being the area near 45Â°N in the central North Atlantic); but the Arctic and Canada will be very cold by this point and the steep temperature gradient between the Arctic and mid-latitude North Atlantic atmospheres will trigger especially deep depressions. The Westerlies on their southern flank will therefore be strong- especially as they will be in higher latitudes than normal for the season: Blowing closer to the North Pole and therefore closer to the axis of the Earthâ€™s rotation the Westerlies will need to be stronger to exert a momental force capable of counterbalancing the effect of the North East Trade Winds in the tropics and sub-tropics and easterlies over the Arctic (all these will be stronger in mid-winter); this will be necessary to satisfy Conservation of Angular Momentum considerations. It follows that really stormy spells are likely to affect the northern half of Britain, with severe gales and heavy rain at times. With the winds coming from the west rather than south-west it will probably be colder than in winter 2013-14, but still warmer than usual because the prevailing wind will be off a North Atlantic a degree or two warmer than normal.
The south of England is unlikely to endure the severe and relentless battering of winter 2013-14 but the strong westerlies, gales and rain are still likely at times in these parts of the country this coming winter: The passage of depressions being further north than usual means that there will be spells of drier and more settled conditions across the south; consequently clear nights with light winds leading to frost and fog will occur from time to time- particularly for SE England, the Midlands and East Anglia (for this winter I am certain of the cold, frosty conditions in mid-December associated with a European High). However, most of the time the direction of winds- off the North Atlantic- means that even if skies do clear and winds drop at night temperatures are not likely to drop much below freezing point. Further north the general passage of depressions,- west to east but well north of the country- is likely to lead to a couple of cold Arctic outbreaks off the Greenland Icecap once this area has gotten very cold by late January-February.
The likely cause of such cold outbreaks would be the passage east of a particularly deep depression towards Arctic Norway pulling down very cold north and NW winds from the Arctic in its wake. A transient ridge-forming in the very cold air that had earlier pushed across the North Atlantic- could well extend from Greenland to the high in the NW Atlantic; a flow of very cold northerly or north-westerly winds lasting two or three days would affect the country. Despite the sea still being warmer than usual between Iceland and Scotland, the North Atlantic in this region will still have cooled down to about 6Â°C by this point in the season, and it would have less impact in moderating strong frigid north-westerlies off the Greenland Icecap. Consequently the two or three Arctic snaps likely to occur later in the 2014-15 winter will probably bring snow-showers to Scotland and northern England, daytime temperatures close to freezing and (as the transient ridge to the west moves over the country following the strong north-westerlies) expect nighttime minima close to -5Â°C for lowland Scotland and northern England.
During the winter months the strong upper Westerlies that encircle the Arctic can exist in either a two, three or four-wave pattern; usually with a ridge anchored to the North American Rocky Mountains. A four-wave pattern favours a ridge in the North Atlantic and an upper trough over Western Europe; this tends to favour frequent Arctic outbreaks over Britain and cold winter weather. A three-wave pattern places a trough over the eastern coast of Canada (bringing cold weather to eastern Canada and the north-east USA) and a weak surface ridge over central Europe- this pattern involves deep depressions forming near Newfoundland and moving northeast towards Iceland and then north of Norway and mild, damp weather over Britain with south-westerly winds prevail during winters dominated by the three-wave upper air pattern: The â€œthree-wave patternâ€ is what dominated in winter 2013-14 and it is by far the most common; although in 2013-14 it was with much stronger upper Westerlies than normal. This coming winter, with the Arctic Ice likely to be slow to expand to its normal winter extent, but with warmer seas further south across the North Pacific and North Atlantic and strong upper Westerlies liable to prevent the penetration of very frigid air south into the USA and likely to prevent the penetration of frigid Russian air south-west into Europe it is a real possibility that a â€œtwo-wave patternâ€ will develop with regard to the upper Westerlies as they encircle the Arctic; this is likely to kick-in around late-December and persist through to February. The upper Westerlies and the associated jet-stream are likely to be very strong and north of their normal latitude ranges because warm, open waters will persist unusually far north whilst the Arctic interior will still get very cold.
It is important if we get a two-wave pattern because a shallow â€œupper troughâ€ will persist in the vicinity of Iceland and depressions will deepen in this region rather than further west; this will be combined with a west-to-east track for the deep sub-arctic depressions rather than the more normal south-west to north-east track for depressions affecting NW Europe: It means the strong, stormy winds reaching northern Britain would be unlikely to have originated from much lower latitudes- if there is high-pressure in the NW Atlantic (because of a patch of cold water just to the east) this will most certainly be the case. With this sort of large-scale pattern there is a higher chance that, as each depression passes to the north-east of Britain, colder north-west winds from the sub-arctic will follow. Most of the time, however the strength of the higher-latitude Westerlies will preclude any direct incursion of frigid air from the Arctic over the United Kingdom, but as the Greenland Icecap reaches its coldest and Arctic Canada becomes very cold after mid-January there is almost certain to be some short-lived cold snaps bringing snow-showers, a thin snow-cover and a sharp night frost following it.
(CONTINUED NEXT POSTING)
Weather patterns predicted for the Winter half-year 2014-15
So, in summary, these are the predictions for the autumn and winter months of 2014-15:
Warm conditions associated with the subtropical high-pressure belt extending a ridge north over southern Britain bought warm and dry conditions in September to much of the United Kingdom; the strong upper Westerlies have been well to the north of the country. This pattern is likely to break down slowly during the first fortnight of October; winds will be south-westerly as the high-pressure slips south and east and progressively deeper depressions push east-northeast across the subarctic North Atlantic: The first half of October will still bring a good deal of warm, bright weather to the south (a good number of days will reach 20Â°C locally) and east of England but it will be progressively wetter and windier towards the north and west- but still very mild for early October (in Cumbria and Northumberland lowlands maxima of 15Â°C will typically occur).
Around the 15th October the upper Westerlies will strengthen and really deep depressions will race eastwards in the sub-arctic in response to the rapidly-intensifying atmospheric temperature and pressure gradients between the becoming-frigid Arctic and the still-warm North Atlantic and North Pacific Oceans: This will cause the winds to become generally westerly and strengthen across the United Kingdom. Hence chilly (but by no means unseasonal cold) westerly winds, gales and spells of heavy rain will affect the North West and Scotland, but there will be a good deal of dry and bright weather east of the Pennines and well as further south.
Frost will occur on cold, clear nights in sheltered valleys in Scotland and the North later in the month following occasional spells with north-west winds, but there is likely to always be too much wind and airstreams will not be cold enough for more widespread frost to affect the country until near the end of the month. No air-frost will affect the South or coastal areas at all this month.
The first major snowfalls of the season are likely to occur on Scottish mountains above about 800 metresâ€™ elevation during the second half of October with the arrival of the somewhat-colder and stormier conditions from the west. The first really severe gales of the season (with wind-gusts over 70 mph) are likely to affect the NW Scottish Coast and islands and the mountains by the end of October.
Overall mean monthly temperatures will be about 11Â°C for lowland northern England, but 10Â°C in Scotland- about 2Â°C above the long-term monthly norm. Rainfall likely to be well above average in the North West but near or fractionally below normal for the month east of the Pennines.
Increasingly stormier, chillier and wetter for northern regions as the month progresses; almost persistent strong west or SW winds bringing a good deal of rain to the North West. Really cold spells are unlikely to occur as the strong westerly flow over and north of Britain will keep out very cold Arctic air and the fact that the North Atlantic will still be warmer than usual will mean that prevailing Westerly winds will bring temperatures a couple of degrees above the November norm. A couple of really severe gales are likely to hit Scotland later this month.
Average temperatures are likely to be a little above normal for November on the whole with typical daytime temperatures of 8 to 10Â°C across most of lowland Scotland, colder in the mountains and far north (Cairngorm Summit will be below freezing point for most of the month). However, mild south-west winds will blow in advance of some of the deeper depressions to bring a few days well into double figures almost everywhere and in the lee of mountains near to the east coast the warming of such air forced over mountains and being warmed on descent will lead to the odd brighter day with temperatures as high as 15Â°C in favoured spots like Dundee and Aberdeen- but such conditions are liable to be accompanied by strong south-westerly winds.
The eastern side of mainland Scotland (being sheltered from the prevailing winds by mountains) will be rather drier and brighter than further west; not so much rain will fall although when the sun does shine it will bring little real warmth since it will not get more than 15 degrees above the horizon at noon anywhere in Scotland by mid-month. Clear skies at night will be rare in November anywhere in Scotland, though a bit more likely in the eastern lowlands- however fresh or strong westerly winds will keep the vast majority of nights frost-free. Averaged across lowland Scotland as a whole nighttime minima will be close to 4C during November: However there will be a few occasions during the month when clear night skies will follow the occasional veer in the wind to a more northerly quarter (direct blasts from the Arctic can be safely ruled out though) when temperatures fall widely to below -1Â°C across the country, with minima below -5Â°C in favoured frost-hollows (such as Braemar, Aberdeenshire) and in the mountains.
For northern England the weather regimÃ© will be similar with strong westerly winds, rain and gales being a major feature of the monthâ€™s weather: Locations over and west of the Pennines, Cumbria, parts of Northumberland west of the Cheviots will bear the brunt of the storms and get the heaviest rains. The North East lowlands, lowland Yorkshire and Lincolnshire are likely to escape really heavy rains, have somewhat less violent winds and enjoy more in the way of weak wintry sunshine at times. Overall temperatures will be close to 10Â°C on most days in the lowlands with most nights well above freezing. However clear skies at night will occasionally occur in between the passage of depressions and there is likely to be at least a couple of such nights when air temperatures drop below freezing point almost everywhere.
The warmest days are likely to accompany strong south-west winds ahead of some depressions, when temperatures will reach 13Â°C across the North; favoured locations along the East Coast in the lee of high ground (such as Alnwick, Northumberland) could reach 15Â°C.
Further from the storm tracks the South will be affected by ridges of high-pressure at times, especially between bouts of stormy weather further north. On such occasions clear skies at night will lead to the first air frosts in the south, fog may occur widely and linger in valleys but daytime temperatures should still recover to close to the seasonal norm in the weak November sunshine. Short fine spells will occur following the stormy spells further north too and on the clearest and coldest nights air temperatures will drop below freezing (but not by much).
Overall mean monthly temperatures will be about 8Â°C in the South of England and South Wales, 7Â°C in lowland northern England and the north Midlands and 6Â°C in Scotland. Occasional days with SW winds will reach about 13Â°C in Scotland and northern England. High rainfall- several inches or more- is anticipated for November this year over Scotland and the North West, but nearer three inches for lowland Yorkshire, the north Midlands and lowland NE England. Locations above 1000 metres in Scotland will be snow-covered from mid-month onwards; although there is a real likelihood of one or two snowfalls by the end of November for all land above 600 metresâ€™ elevation in both Scotland and the North.
For Scotland this month is likely to be very stormy as deep depressions race east-northeast from southernmost Greenland to Iceland to Arctic Norway; gales are likely to be almost-daily affairs off the north-west coasts and in the mountains where- more often than not- it will be cold enough for snow to fall and lie. In the lowlands of Scotland there is unlikely to be snow except when colder north-westerlies briefly follow the passage of the odd deeper depression; for the bulk of the time rain will fall rather than snow. Rain will be heavy on west-facing hills and coasts, falling in sufficient quantities to cause flooding at times. Some really severe gales and storm-force winds are likely to sweep Scotland over the Christmas Period.
The coldest weather in Scotland in December will be the occasions following the occasional north-westerlies as one of the deeper subarctic depressions move towards northern Norway; a short incursion of colder sub-arctic air will bring daytime temperatures down to 4Â°C across the lowlands and the squally showers (heaviest in the west) will then fall as hail, sleet and snow. Snow will fall and lie everywhere above about 400 metresâ€™ elevation which means that some of the highest settlements in Scotland will be experiencing their first major snow-storms of the winter. The odd clear night in Scotland is likely to follow such sub-arctic snaps and this will lead to minima falling widely to below -1Â°C (favoured frost hollows will get below -5Â°C on such occasions)
Further south strong westerlies will dominate the bulk of the monthâ€™s weather; gales and heavy rain are particularly likely to affect Cumbria, the Cheviots, the North Pennines and the Lancashire Moors along with North Wales and Dartmoor in southwest England. Snow will fall on higher ground-above about 500 metres in the North Pennines (above 700 metres on mountains in Wales) when winds veer slightly north of west following the passage of deep depressions far to the north. However, drier and brighter conditions will often occur east of high ground, here there will be rather less rain. On the mildest days with southwest winds blowing ahead of the passage of deep depressions well to the north temperatures will reach to over 10Â°C across the entire country (except in mountainous areas of Scotland and northern England).
There is confidence that almost all of England will come under the influence of a cold, dry high-pressure system from Europe for more than a week at some point in early or mid-December. This is because temperature patterns across southern Europe, in the Mediterranean and in the sub-arctic (combined with a pool of colder-than-usual water in the mid North Atlantic) looks set to split the upper Westerlies and weaken their passage towards Western Europe. To that end there is confidence that colder air from Eastern Europe will reach England and then high-pressure will bring clear skies and light winds. In December, with over 16 hours of night and with the sun not getting more than 15 degrees above the horizon even at noon, this means that a strong net radiation cooling of the surface and low atmosphere will occur day-by-day. The air coming off Europe (which itself will get very cold under clear skies with light winds at this time of year) will be relatively dry and this, combined with the strong atmospheric subsidence (sinking air in the atmosphere) which is associated with high-pressure systems, will keep the atmosphere largely cloud-free.
Now, the North Sea is still likely to be relatively warm for the season after the warm summer and autumn of 2014 but the airmass reaching the United Kingdom associated with the European High will come from the south-east, - it will therefore have a relatively short sea-track before reaching Britain. As such the cold winds ushering in this cold spell will bring air close to 4Â°C (colder in Yorkshire and over high ground) to begin with, but then clear nights with light winds will result in further strong cooling of this air as the high-pressure really moves in. Nighttime temperatures will drop to -3Â°C or lower right across England almost immediately, although some coastal areas such as around Barrow in Furness may escape with little frost: During the ensuing days the weak December sun will ensure daytime temperatures do not get above 3 to 4Â°C nationwide, but in frost hollows in the North, in upland locations further south or where freezing fog forms daytime temperatures will struggle to get above freezing point.
As the clear skies and light winds of the European High continue the net day-to-day radiative cooling over all land areas of England will make its presence felt in progressively lower nighttime (and daytime) temperatures and the development of a strong temperature inversion near the surface in which pollution and moisture will accumulate leading to the occurrence of widespread freezing fog and rime on the ground. At the climax of this cold, settled spell of weather minimum temperatures are likely to drop to or below -5Â°C right across England (except coastal areas); in some of the coldest frost-hollow locations where the deep ponding and concentration of cold air can occur (such as the Vale of York and the Trent Valley through the north Midlands and Yorkshire) I am confident we will see temperatures as low as -10Â°C. Freezing fog will occur widely near the culmination of this cold, settled spell and daytime temperatures will remain below 0Â°C over a wide area; in the coldest locations such as valleys in the North Midlands and Yorkshire the coldest days will be below -2Â°C and thick rime will build up on trees, on grass and on roads and pavements. Both the freezing fog and slippery surfaces (due to ice) will make outdoor travel quite dangerous for a time.
Snow will not really be a feature of this cold spell because the cold conditions would be associated with a high-pressure system that tends to bring dry, fine weather: This precludes the occurrence of precipitation of any sort. The decline of the high-pressure conditions- and the bitter cold with it- will happen as the high-pressure retreats south-eastwards into continental Europe allowing mild southwest winds off the North Atlantic to blow in along its northern flank. It is fairly likely that the cold air near the surface will be swept away and displaced by much milder air before rain-bearing frontal activity associated with cyclonic activity well to the north moves back across England.
However it is just possible that the cold air will hang on in Yorkshire and other eastern counties further south whilst mild, moist air associated with a warm front moves across England at the demise of this cold dry spell: This means that the cold air near the surface could stubbornly hang on and result in rain turning to snow in locations like Norwich or Scarborough. There could even be freezing rain whereby rain falls through a layer of freezing air near the ground and instantly turns to ice on hitting still-frozen objects. If there is snow or freezing rain at the demise of this cold, dry mid-December spell it is likely to be short-lived and liable to melt quickly as warmer air from the southwest arrives at the surface. The western side of England and all of Wales will most definitely not be getting any snow because the milder air will be established here quickly as the cold spell declines.
There is confidence that this cold spell will encompass all of England; except for western Cornwall, Cumbria and Northumberland that will remain in milder south-westerlies off the North Atlantic on the north-western periphery of this European High-Pressure Belt. Locations close to this limit such as Devon, parts of North Wales and Lancashire will probably have cold south-east winds for just a few days and with one or two clear nights; in such areas anything colder than -3Â°C is unlikely except in favoured frost hollows. Predictions as to the exact extent of such a high-pressure system from three months out is impossible; the very existence of this cold European High is based on the premise the likely behaviour of the jet-stream and upper Westerlies at the end of the hurricane season, the broad patterns of cold and unusually warm waters across the North Atlantic and Mediterranean, along with the extent of the Arctic ice far to the north: I have accordingly made a judgement accordingly that there will be a broad zone of cold high-pressure over central Europe for a couple of weeks in December, but that the zone around Iceland will be the focus of intense storm-tracks (and that some storms will develop in the Mediterranean). Strong west or south-west winds will extend 10 to 15 degrees south of the path of the storm tracks- based on observations of the distribution of such winds in relation to the sites of intense depressions. Accordingly, this puts the northern limit of the cold, dry and settled zone close to 55Â°N over North East England: It is therefore conceivable that the cold, dry and frosty conditions will also encompass much of Scotland and all of Wales too, but it is also possible that only the English Midlands and South East England will experience this pre-Christmas cold frosty spell whilst the remainder of the country remains under the influence of damp, mild south-westerlies.
However, because this high-pressure will be centred close to 50Â°N over mainland Europe,- largely thanks to cyclonic influence in the Med and northern Europe cooling more rapidly than further south- I remain confident that this cold, dry high-pressure will also encompass most of England. Because Iceland will almost certainly remain the focus of intense storm tracks Scotland is most unlikely to come under direct influence from this high-pressure: It means if anything is going to be different northern England and Wales will remain under mild south-westerlies but that the Midlands and SE England will still get very cold, or that northern England and Wales will just have a few days with very cold and frosty weather: Actually I incorporate this into the general forecast for how the cold spell should play out across the country with parts of the North West and North East just getting a few days of cold frosty weather; but with a week or more of increasingly bitter, frosty and foggy conditions across the Midlands and South.
However, we must not get carried away: We are looking at a week to ten days of cold dry weather in December; the rest of the month is almost certain to be dominated by strong westerly winds off the North Atlantic. We have already seen what this means for Scotland for the whole of December 2014; for England and Wales the regimÃ© before and after the cold, frosty spell will be similar: Strong winds and rain will affect the country on a regular basis with the wettest and stormiest conditions reserved for Cumbria, the Pennines, the Cheviots and North Wales. Drier conditions will affect lowland NE England, Yorkshire, Lincolnshire, the Midlands and southern England but even here there will be some rain on most days and there often be a strong wind. Typical prevailing daytime temperatures will range from 8 to 10Â°C in southern England, South Wales and Midlands to 6 to 8Â°C in the North and most nights will be free of air frost across the country.
Aside from the week or so of dry, cold and frosty weather over England, strong colder north-westerlies will periodically follow the passage of deep depressions towards northernmost Norway; these will bring gales and squally conditions across the country with daytime temperatures below 7Â°C even in the South. Showers of rain, hail and sleet will occur on such days and- as mentioned above- snow will fall (and lie) above 500 metres elevation in far northern England and on higher ground further south on such occasions. Clear night skies with lighter winds in between the passage of depressions will result in air frost across the country.
The Christmas Period is likely to see the upper Westerlies that encircle the Arctic intensifying once more; this is likely to lead to the collapse of cold high-pressure conditions over continental Europe as mild air pushes in and especially deep depressions will begin to move east in the vicinity of Iceland: This will lead to really severe gales across the entire country- not even Kent is likely to escape. These gales will bring bands of heavy rain, most especially to the North and to Scotland. Heavy snow will fall above 600 metresâ€™ elevation in Scotland and in Cumbria rather than rain; hurricane-force westerlies will batter the north-west Coast of Scotland and the Northern Isles (and the mountains where severe blizzard conditions would result) on at least a couple of occasions.
Overall mean daily temperature for December 2014 will be close to 5Â°C in the lowlands of Scotland and in the lowlands of Cumbria and Lancashire and much of southwest England; elsewhere in England the mean daily temperature will be 3 to 4Â°C with marginally colder conditions for the Vale of York and the north Midlands. Most of Wales will have a mean temperature of 5Â°C but for Cornwall and the Isles of Scilly the mean temperature will be nearer 8Â°C. Mountainous areas of Scotland and the far North of England will have average temperatures below freezing point. This means that mean daily temperatures will be over 1Â°C warmer than the long-term December norm in Scotland, Cumbria, Northumberland and North Wales but near or fractionally colder than the long-term December norm across Yorkshire, the English Midlands and southern England.
Rainfall will follow a similar pattern with rainfall amounts being very excessive in the North West of England, in North Wales and much of Scotland but rainfall totals will be some 20% below long-term normal December totals in SE England, East Anglia, the East Midlands, and the Welsh Marches and in lowland Yorkshire. Over ten inches of rainfall are confidently predicted to fall during December over coasts and hills in northernmost and western Scotland and in Cumbria, seven to ten inches is likely for the North Pennines and the uplands of the Cheviots, the Yorkshire Dales and for parts of North Wales and exposed parts of SW England. Decemberâ€™s rainfall total will be four inches or marginally less from lowland eastern Scotland south across lowland NE England, Yorkshire, the English Midlands to southern and SE England.
The upper Westerlies and the jet-stream that encircle the Arctic will remain very strong and somewhat north of their usual seasonal position throughout the month: The upper Westerlies are likely to change to a two-wave pattern with an upper ridge centred on the Rocky Mountains of Canada, the patch of chilly water in the central North Atlantic is likely to extend the upper-trough forming some 90 degrees longitude further east some 20 degrees further east towards NW Europe; this means deep depressions will be forming near 65Â°N in the Davis Strait moving more directly east towards Iceland and then northern Norway. The subtropical high is likely to be re-established strong over the North Atlantic and into the (by this time) cooler Mediterranean centred roughly 35 to 40Â°N.
The prognosis this has for northern Britain is for very strong winds- mainly westerlies- to dominate Britainâ€™s weather; frontal bands associated with deep depressions well to the north will bring bouts of heavy rain to western facing coasts and hills. The colder temperate of eastern Canada by this time and the fact that the North Atlantic will have cooled a couple of degrees compared to early December (the NE Atlantic will still- in all likelihood- be warmer than the seasonal norm though) means the Westerlies pushing across the region will be colder: Typical daytime temperatures in lowland Scotland will be 6 to 8Â°C, though the strength of the wind and cloud cover will keep most nights above freezing point in the lowlands. Severe gales will affect most of Scotland every few days; they will be strongest on west facing coasts and on the mountains, and in the Northern Isles.
In the Scottish mountains above about 900 metresâ€™ elevation both day and night temperatures will be below freezing point during January, but heavy snow is likely to fall and lie on all land above about 700 metres during the month. On the highest summits conditions will be very severe with freezing westerly gales and blizzard conditions much of the time. Colder conditions with snowfall are likely to affect all of Scotland at least once by late January as strong north-westerly winds bring subarctic air following the passage of one of the deeper subarctic depressions into Norway; then vicious snow-squalls and strong winds will spread to all parts bringing daytime temperatures close to freezing as far south as Edinburgh. Clear skies at night following such a cold blast with winds falling light are likely to occur at least once in mid or late January,- leading to temperatures falling to or below -4Â°C throughout most of lowland Scotland: Favoured frost hollows (i.e. the Dee Valley in Aberdeenshire) and locations at elevation (such as the Cairngorms) will get much colder with minima down to around -10Â°C on such an occasion.
The late January cold spell will be short-lived however; within 48 hours of its inception south-westerlies ahead of the next depression to the north will sweep away the cold air and quickly melt away lying snow below around 600 metresâ€™ elevation.
For northern England and North Wales the monthâ€™s weather will be similar to that in Scotland although marginally less severe gales will occur and temperatures will be a degree or so milder: Even so gales will sweep through every few days bringing heavy, squally spells of rain- particularly to Cumbria, NW Wales and the western sides of both the Pennines and Cheviots. Prevailing temperatures will be near 7 to 9Â°C during the day and most nights (which will be windy/cloudy) will be well above freezing point in the lowlands. The highest parts of the Lake District, the Cheviots and North Pennines- land above 700 metres will be cold enough for falling and lying snow throughout most of January, and given the very disturbed nature of the weather at times there will often be heavy snowfalls there (similar conditions are also likely to occur on Snowdonia, North Wales- above about 950 metres- for much of the month).
The lowlands of NE England and Yorkshire, sheltered from the strong, squally westerlies by high ground to the west will have somewhat less rain and the gales will be somewhat muted; there will also be frequent breaks in the cloud in these areas to allow through a good deal of weak winter sunshine.
Northern England will be affected by the one short-lived cold snap that is likely to affect Scotland later in the month: This is likely to bring snow-showers and daytime temperatures around 3Â°C to the lowlands of the North (but below 0Â°C above 450 metresâ€™ elevation): Snow is likely to settle on the ground in Cumbria and Northumberland and on all land above 300 metres across Lancashire and Yorkshire and in strong winds the snow will drift in these areas. A rare night with clear skies and falling winds is likely to follow the cold north-westerlies leading to temperatures dropping to around -3Â°C across much of the North; upland areas and frost-hollow locations like the Vale of York will drop to around the -6Â°C mark.
The English Midlands, South Wales and southern England will, like lowland Yorkshire, escape the worse of the gales: However, even here the upper trough to the northwest and very strong upper-level westerlies will lead to gale-force winds and squally bands of rain every few days. Nevertheless the depression tracks will be further north than last winter so there should be nothing like the widespread torrential rain and flooding of last winter, though it will be just as windy at times. In between the passage of depressions far to the north there will be ridges of high-pressure from the south leading to a good deal of weak but bright winter sunshine at times. Daytime temperatures will be 8 to 10Â°C here for much of the month, cloud and some wind on most nights will mean minima close to 5Â°C in the lowlands- but there will be more clear nights with lighter winds this far from the storm tracks that will allow localised fog and frost to occur. The cold subarctic snap later in January will have encountered warmer lands and seas before reaching this far south so it is likely to bring daytime temperatures near 5Â°C with showers of hail and sleet rather than snow to the lowlands, though snow will fall and lie on the Brecon Beacons of South Wales and on the highest parts of Dartmoor and Exmoor in SW England: Clear night skies with light winds following the subarctic snap will lead to minima around -2Â°C across most of the South of England and South Wales- this is likely to be the coldest it gets in January 2015 in these locations.
For January 2015 as a whole mean daily temperatures are likely to be 1 to 2Â°C warmer than the long-term norm in all parts of the country: Mean monthly temperatures will range from about 7Â°C in SW England and South Wales and 6Â°C across the Midlands and SE England to around 5Â°C across lowland northern England and North Wales and 4Â°C in lowland Scotland. The North Pennies and Cheviots above about 700 metres will have mean temperatures below freezing point and the highest parts of the Cairngorms and the NW Highlands will have a mean January temperature close to -3Â°C!
Rainfall during January will be well above normal across central and western Scotland (where more than ten inches will fall over a wide area), Cumbria and western Northumberland (where seven to ten inches of moisture will fall widely), the Lancashire Moors and Yorkshire Dales (where six to ten inches of moisture will fall), Snowdonia and SW England. There will be four to five inches of rain falling during January 2015 across eastern Scotland, lowland NE England, lowland Yorkshire and Lincolnshire, the English Midlands and southern and SE England- making the monthâ€™s rainfall close to or marginally above normal for January in these regions: January 2015 will be nothing like as wet as January 2014 across any part of England, but it will be just as windy at times- and a little colder.
The Arctic interior, including north eastern Canada will be very cold by this time of year, the northern end of the Davis Strait will have frozen over; persistent cold west and NW winds off NE Canada will have cooled the NW Atlantic down and the patch of cold water in the central north Atlantic near 45N is, if anything, likely to be more pronounced compared with the seasonal norm (bear in mind sea surface temperatures are close to their minimum by February). The NE Atlantic with strong prevailing westerlies having blown through the winter (far less chilly having blown across 2,000 miles of ocean) will still be warmer than the seasonal normal, though not by much.
There will therefore, be a very strong atmospheric temperature and pressure gradient between the very cold Arctic (I am confident that locations north of the Arctic Circle in Greenland and NE Canada will be affected by airmasses with temperatures in the region of -25Â°C at low elevation by February), and the still well-above freezing point waters of the North Atlantic (and North Pacific): This is bound to drive deep depressions and these will move east-NE from eastern Canada to Iceland to northern Norway. In February, the NE Trade Winds are at their strongest because the northern subtropical oceans reach their coolest and continental areas near 35Â°N are very cool compared to the hot, steamy equator (a little hotter than usual this coming winter due to the mild El Nino)- this means stiff NE winds blowing from 35Â°N almost to the equator- consequently there is a corresponding need for strong Westerlies in higher latitudes to counterbalance them.
The prevailing weather in the United Kingdom will therefore be dominated by deep depressions sweeping east near 65Â°N, and very strong westerlies on their southern flank; this will be associated with a two-wave pattern in the upper jet-streams with a ridge anchored to the Canadian Rockies: This places a broad trough in this strong upper jet-stream west and NW of Britain, which means that the very strong westerlies will be accompanied by frequent showers and longer spells of rain- and in much of Scotland and the uplands of northern England and North Wales this will be heavier and often in the form of hail, sleet and (at times) snow. The depressions passing north of the UK will often have central pressures below 950 millibars, whilst the subtropical high (centred at 35 to 40Â°N in the Med) will often have surface pressure over 1030 millibars; consequently severe gales will continue to batter much of Britain but the North West, the Pennines, NW Wales and Scotland will be especially vulnerable.
Typical daytime temperatures will be around 6Â°C in lowland Scotland, falling close to freezing at night; for lowland northern England typical maxima will be 7 to 9Â°C with minima around 2Â°C: Upland parts of both Scotland and northern England above 750 metres will spend most of February 2015 below freezing point and frequent snow-storms will result in substantial accumulations.
The South and English Midlands will escape the worst of the storms- and the wet- being closest to the high-pressure over the Med and furthest from the subarctic storm tracks: There will consequently be less rain and a good deal more sunshine. During February the sun will be getting more than 25 degrees above the horizon at midday from the Midlands southwards so it will feel quite pleasant out of the wind when the sun shines. Typical daytime temperatures in the Midlands and South during February will be 8 to 10Â°C. By the same token the chillier air from the west (compared to south-westerlies) will often bring clear night skies between storm passages and this will lead to frost in locations sheltered from the wind. The strength of the wind and variable cloud cover will keep minimum air temperatures a few degrees above freezing point on most nights in most areas of the south; however clear night skies with lighter winds will occur between the frontal bands of cloud and rain leading to much more widespread frost at times.
Strong mild south-westerlies ahead of the passage of some deep sub-arctic depressions will bring the mildest days in February 2015: Temperatures will then reach 10 to 12Â°C in lowland Scotland and northern England and 12 to 14Â°C in the South, a thaw will envelop even the highest mountains in Scotland leading to avalanche conditions. These mild conditions will occur roughly three or four times during the month; they will however be accompanied by cloud and rain (heavy rain in western Scotland and Cumbria) across most of Britain. A few favoured locations in eastern Scotland and lowland NE England and Yorkshire- along with parts of SE England will probably experience a little brightness; with the warming effect of air pushed over higher ground and forced to descend in the lee of mountains this will lead to temperatures locally as high as 15Â°C in these locations.
However it is the certainty of cold snaps from the subarctic, of which at least two or three are likely to occur during February 2015, that will stop most folk thinking an early spring may have arrived: Greenland and NE Canada will (as has been explained above) be extremely cold by February whilst the NW Atlantic will be cold by then- and probably even colder than normal given that there already exists a pool of colder-than-normal water near 45Â°N in the mid North Atlantic; that will encourage a small ridge of high-pressure to extend north over the NW Atlantic from the subtropical high further south. Meanwhile the strong two-wave pattern to the upper westerlies will be ensuring relatively mild west and SW winds push deep into central northern Europe: The scene will therefore be set for frigid outbreaks from southern Greenland and Arctic Canada to rush towards and over the United Kingdom as a few deep subarctic depressions east well into Scandinavia; with high-pressure in the NW Atlantic and the dense, very frigid air over Greenland seeking to rush out over warmer locations such a cold snap is inevitable.
The two or three cold snaps that occur in February will arrive on strong north-westerly winds bringing in much colder air from over NE Canada and southern Greenland; these cold winds will bring frontal troughs associated with the deep depression moving into Norway and the unstable cold airstream that will be much colder at elevation. These cold snaps will ensure February is the coldest month of the winter in Scotland, Cumbria and Northumberland; widespread heavy snow showers will affect all of Scotland and most of northern England. Daytime temperatures will not exceed 0C in lowland Scotland, with maxima around 1 to 3Â°C for northern England (all these areas will have 5 cm or more of snow-cover- except the Lancashire coast which will remain free of snow-cover); the very highest terrain in both Scotland and northern England will have maxima below -5Â°C combined with gale-force raging blizzard conditions.
Further south the icy north-westerlies will have been moderated by the milder waters of the Irish Sea; being further from the storm centres the Midlands, southern England and South Wales will have sunny spells and less intense showers of hail, sleet and snow (this will be unlikely to lie except on Dartmoor and the Brecon Beacons in South Wales). Typical maximum temperatures during the February cold snaps will be around 5Â°C in these regions.
A transient ridge of high-pressure over Britain is likely to follow the cold, strong north-westerlies over Britain; this will bring clear skies and lighter winds across the country: Such conditions at night over snow-cover in Scotland and the North will lead to minima as low as -5Â°C occurring widely at low levels; on the mountains in Scotland and in frost-hollows the air temperature will be dropping below -10Â°C on such occasions. In the South, in South Wales and in the Midlands these, the coldest nights that will be occurring in February 2015, will have minima in the region of -3Â°C.
These two or three cold snaps in February will be short affairs, lasting at most 48 hours before west or SW winds ahead of the next subarctic depression bring milder air off the North Atlantic which will thaw all snow-cover at low levels. However areas above 700 metres elevation in both Scotland and northern England will almost certainly probably remain cold enough to hang onto snow-cover throughout February.
Mean temperatures for February will be near or fractionally above the long-term monthly normal in Scotland and the North (in these regions it will be the coldest month of the winter); for the English Midlands and southern counties of England the month will be 1 to 2Â°C warmer the long-term February norm. At low elevations mean daily temperatures for February 2015 as a whole will range from near 6Â°C in southern counties of England, the South Midlands and South Wales, to 4 to 5Â°C in the northern English Midlands, North Wales, and northern England. Lowland Scotland will have a mean February temperature of 3 to 4Â°C because of the greater exposure to subarctic airstreams following the passage of deep depressions not so far to the north. Upland terrain above about 600 metresâ€™ elevation in both Scotland and northern England will have mean daily temperatures at or below freezing point during February.
Rainfall totals are likely to reflect the northerly position of the prevailing tracks of deep subarctic depressions during February 2015, combined with the influence of strong, prevailing Westerlies: Allowing for the fact that the NE Atlantic will be approaching its coldest (and so less able to furnish moisture to the strong Westerlies), rainfall totals for February 2015 will be between six and 10 inches across western and northern Scotland (with the higher totals over coasts and hills- where the greater proportion of rainfall will be as snow); Cumbria, the western side of the Yorkshire Dales, Snowdonia in North Wales, the Lancashire Pennines and North Pennines will have between six and nine inches of moisture. The highest and most exposed hills and mountains of NW Britain- the Ben Nevis range in Scotland and the top of Snowdonia in North Wales- will have much more moisture falling (over 15 inches of rainfall) falling in February 2015, almost all of which will fall as snow: Rainfall totals will be some 50 to 100% above normal in all these exposed locations of western Scotland and NW England- plus upland northern England and North Wales.
Lowland north-eastern areas of Britain from eastern Scotland through NE England to Yorkshire and Lincolnshire will be sheltered by hills and mountains to the west and so will be much drier; four inches of rain or somewhat less is likely in all these regions even through the westerly winds will blow very strongly at times. This means rainfall totals will be close to the February norm in all these locations; although Lincolnshire and Aberdeenshire may be a little wetter than normal due to the absence of hills to the west of much of Lincolnshire and the proximity of Aberdeenshire being closer to deep depressions in the subarctic.
The English Midlands and southern counties of England will have between three and five inches of rainfall during February due to the unsettled regimÃ© prevailing throughout the month; Devon, Cornwall and South Wales will get five to eight inches of rainfall due to greater exposure and proximity to the North Atlantic: The southern regions of Britain will-overall- be less wet than further north because of the distance from the depression tracks but even across the south mean monthly rainfall will be some 20 to 50% above normal for the month in most areas. That said, February 2015 will be nothing like the wash-out of February 2014 anywhere,- so there should not be a repeat of the persistent flooding that plagued parts of Somerset, Devon and the Thames Valley last winter.
Well here we are again folks, that special time of year when the anticipation of Christmas is matched only by the joy of the now annual tradition of the LomondSnowstorm winter forecast. Last winter promised much and delivered some, although on the ground it was generally more of a slushy hinterland than a winter wonderland. So can we continue the trend of generally below average winters which have developed in tandem with a more southerly tracking jet, or will the glorious summer of 2013 usher in another trend bucking warmer than average season? In this forecast Iâ€™m going to look at a number of factors, including the El Nino Southern Oscillation, Global Angular Momentum, the Quasi Biennial Oscillation, Arctic Sea Ice, Eurasian snow extent/autumnal gain and Solar Activity to put together a forecast for what the winter of 2013/14 has in store for us. Once again, the usual disclaimer goes out about the low confidence nature of the more detailed parts of the forecast, but itâ€™s good fun and paints a picture more readily for those who donâ€™t have a particularly strong meteorological background.
The El Nino/La Nina state, an index of the Sea Surface Temperature anomaly in the Eastern Pacific, has been shown to be critical to the global circulation pattern, so Iâ€™ll address if first. At present, as it has been for about a year now, weâ€™re in a neutral phase (between 0.5 and -0.5 amplitude), and the model outlook suggests that it will stay that way throughout the winter:
Neutral ENSO winters have been shown to have SSWs less frequently than in those with either a clear La Nina or El Nino state (Butler and Polvani, 2011) which would indicate a lower chance of Polar vortex disruption and therefore a higher chance of more â€˜normalâ€™ winter conditions than we saw last year. However, neutral/weak ENSO values are also associated with a greater frequency of â€“ve NAO conditions, which is strongly correlated with below average temperatures.
The Quasi Biennial Oscillation is another key factor to consider, with easterly (negative)QBOs encouraging a weakening of the Polar Vortex and westerly QBOs a strengthening. However, it is more complex than that, and a combination of a Westerly QBO and a peak in Solar activity has been shown to increase the probability of a Sudden Stratospheric Warming Event. Currently, the QBO is in a Westerly state and will remain so (given that itâ€™s a two year cycle) for the rest of the winter:
This is one of the hottest (if youâ€™ll pardon the pun) areas of meteorological research at the moment, and along with the Stratospheric developments this has contributed a huge amount to the advancement of medium range forecasting in recent years. The interest began when the historically cold winter of 2009/10 occurred during a deeper and more prolonged sunspot minimum than weâ€™ve seen in 100 years:
The inevitable upturn in the 11 year cycle has taken place since then which takes us up to near the peak of the cycle this winter, although itâ€™s a peak which is far more akin to those of the freezing Dalton minimum years than those of more recent years, . Nonetheless, remember the linkage between Solar Activity, West QBO and an increased prevalence in SSWs, because it may be critical to the coming winter.
[size=6][b]Arctic Sea Ice Extent[/b][/size]
For the unitiated this one might seem a bit strange, but in fact there is a reasonably strong basis for [i]lower [/i]Sea Ice extent being associated with an increase in the prevalence of blocking highs and a subsequent decrease in average temperatures across western Europe. As anyone who pays much attention to environmental news knows, Sea Ice in the Northern Hemisphere has been at historical (at least during the post 1979 satellite age) lows in extent. However, this year has seen something of a comeback for the ice, with the extent so far for the year being the highest since 2006:
Still, it remains below the long term average so this can be tallied up as being a slight mark in favour of increased blocking. October [size=6][b]Eurasian Snow Cover/Snow Gain[/b][/size]
This one makes perhaps more sense, although again perhaps not for the reasons youâ€™d imagine â€“ increased snow cover in October and November across the Eurasian continent causes low ground temperatures which leads to stagnant rising air and hence high pressure systems to form across high latitudes, which leads to a weakening of the vortex from below and the displacement of cold air southwards to our latitudes. After an exceptional end to September for snow gain, the uptake since then has been less spectacular, with the â€˜Snow Advance Indexâ€™ for October being somewhat below the long term average, although weâ€™re still sitting hemispherically at above average once again thanks to recent gains in the Eastern US.
[attachment=191373:Snow cover.jpg][attachment=191372:snow cover 2.jpg]
Overall then this index is neutral, with the gain being far less spectacular than it could have been but partly because of a record high starting point.
[size=6][b]North Atlantic SSTs[/b][/size]
The feedback between Sea Surface Temperatures in the North Atlantic and the North Atlantic Oscillation, or in other words blocking over our side of the Northern Hemisphere, was shown to have been largely responsible for the record cold December of 2010 (Met Office, 2013). The key signature of a negative NAO is a tripole in the North Atlantic, with a cold section around 30-50 degrees latitude just off the coast of Eastern Canada, with warmer sections to the north and south:
The signal is rather muted but a tripole of sorts has been showing in the North Atlantic in recent weeks, which is a fairly promising sign, although we need to watch to see if this sustains and develops further.
[size=6][b]Arctic Oscillation tendency[/b][/size]
Two recently developed indices of snow cover and height anomalies respectively have an exceptional record of predicting the Arctic Oscillation tendency - Cohenâ€™s Snow Advance Index, which as previously discussed is indicative of a moderately positive Arctic Oscillation, and the very recently developed October Pattern Index, which can be thought of as the atmospheric effect of the SAI, which is actually at its highest value since 1991/92. Anyway, given the agreement between the two and the high predictability both have, Iâ€™d put our chances of seeing a +ve Arctic Oscillation for the winter at 90% plus. This doesnâ€™t necessarily preclude a below average winter, and is less reflective of temperatures at our side of the pond than the North Atlantic Oscillation, but hemispherically it points towards high latitude blocking being generally scarce and does reduce our likelihood of a cold winter. However, this does not imply a permanently positive Arctic Oscillation, and we should bear this in mind.
[size=6][b]Long Range Forecast models[/b][/size]
These have been fairly mixed, with their output mid-October on the back of large initial snow gains being very much favourable to cold and snow at mid latitudes, but almost all of them, including the GloSea and last weekâ€™s CFS average, call for a milder than average winter. However, these are generally pretty volatile, donâ€™t handle the Stratosphere particularly well (with the exception of the EC long range forecasts which was impressive last winter) and some also include a positive height anomaly across Greenland, which is of course very much linked to below average temperatures across the British Isles. While their inputs are valuable, Iâ€™d be impressed if any of them were able to pick up on a major pattern change more than a month out given the current and near future strength of the vortex.
[size=6][b]Global Angular Momentum[/b][/size]
Finally, we have GLAAM, effectively the â€˜turning forceâ€™ due to the Earthâ€™s rotation, frictional torque, Mountain Torque etc. This broadly follows the pattern of the ENSO, although there is a bit of a chicken-egg debate going on regarding which causes which. Anyway, with the slightly negative ENSO values this index has also been tracking negative but it does look likely to increase towards a positive state as the winter goes on, which could be key in disrupting the vortex
Having looked at years from 1958 onwards (prior to that the datasets become sketchier and itâ€™s tougher to pattern match) and basing on the criteria outlined above Iâ€™ve come up with 11 composite winters which broadly match the likely winter ahead. These were all either ENSO neutral or weakly positive/negative (value less than 1), featured positive QBOs and are â€˜weightedâ€™ towards those which are more similar. While some years with negative Arctic Oscillations were counted, years were double weighted based on the Arctic Oscillation value given the likelihood of a positive one this year. The most similar winter was 1990/1991, which is counted four times in the composite charts, with 1971/72, 1980/81, 2001/02 and 2008/09 counted twice and the rest just once. The full list of years are:
2008/09, 2001/02, 1990/91, 1985/86, 1980/81, 1978/79, 1971/72 ,1966/67, 1961/62 and 1959/60.
For December, the Aleutian Ridge and negative height anomalies over the Western side of the Arctic are the most notable features, but otherwise the anomalies are fairly muted, with a positive anomaly out in the mid Atlantic and actually a slight mean trough over Europe.Half of these winters featured either a Canadian Warming or SSW in the first half of winter, with 8/10 featuring a notably cold stratosphere initially, so this may well be the key to our winter once again. The Stratosphere composite looks like this for December:
with a very strong West Greenland centred vortex, but already there are signs of weakness creeping in from the Eurasian side.
[attachment=191364:Dec the strat.jpg]
Onto January and we have a very different looking anomaly â€“ Greenlandic height rises come into play but with a very strong negative height anomaly stretching right across from NE Canada into southern Europe. Again, this is very far from clearcut, with evidence of some vortex disruption particularly in our neck of the woods but still with negative or neutral height anomalies across the pole:
[attachment=191368:Jan the strat.png]
A look at the stratospheric picture once again may help to enlighten us further â€“ the stratospheric vortex looks to end up split over the North American and Siberian sectors with a mean strat. â€˜ridgeâ€™ over Europe and the Atlantic:
Given the predicted height anomalies it does look more like a split than a displacement SSW (and in fact there are a few â€˜hintsâ€™ of the Cohen tropospheric response model in the anomaly) if it comes off, with the timescale being somewhere around mid January. Last winter, bizarrely, may end up being a reasonable analogue for January, although given an almost polar reversal of tropospheric conditions it is likely to play out somewhat differently, with the tropospheric vortex more likely to recover.
With the recovery of the tropospheric vortex over the Arctic, it then remains a question of how long and whereabouts the blocking can hold on for. Perhaps unsurprisingly given the initial split, it appears that the Eurasian-Atlantic sector may well continue to see the effects of the split vortex well into February:
Once again, a mean Euro trough is evident with the main area of blocking from Iceland to Scandinavia while globally we do appear to return towards positive Arctic Oscillation values. At the stratospheric level, things are somewhat different though â€“ the stratospheric vortex does not appear to recover fully even while the tropospheric one rebuilds, which again suggests potential difficulties for snow lovers afoot with propagation. Still, if it turns out similarly to the composites shown we should have a decent crack of the whip before we see a breakdown of the pattern.
Given the SST pattern Iâ€™d suggest that the mean height anomaly for February is centred somewhat further west over Greenland, while low heights south of 60 degrees north are likely to be confined to southeast Europe through December.
[size=7][b]2013/14 Winter Forecast[/b][/size]
A very zonal period to start the month following a short, sharp Arctic blast at the end of November, with west-northwesterly winds and very limited blocking. Near constant low pressure systems will bring wind and rain throughout, with only brief drier interludes. Temperatures above average in England and Wales but near or even slightly below average for Scotland, where colder upper air temperatures will bring the odd smattering of snow even to lower levels in spite of a lack of frost, with precipitation above average initially everywhere. Perhaps something along the lines of this:
From around the 12[sup]th[/sup] onwards things will quieten down, with high pressure building in from the south, bringing a brief spell of very mild southerlies followed by a dry and eventually frosty spell in the run up to Christmas. Temperatures above average everywhere up to the 20[sup]th[/sup] but cooling down towards average from the south as heights transfer northwards, precipitation generally below average away from the far northwest Highlands where theyâ€™ll be around average. By Christmas, heights will transfer westwards with an initial spell of rain followed by a genuinely cold northerly blast as low heights dive temporary southeastwards, bringing more widespread snowfall and low temperatures, although with accumulation generally confined to the usual spots (which of course vary depending on the exact wind direction) before the dam breaks and the heights sink once more by monthâ€™s end. Temperatures will be generally above average for most of England and Wales, with an initial CET punt of 5.6C, the Scottish mean will sit around average at 3.4C. Precipitation will be marginally above average for all of Scotland and much of northwest England but for southern England and Ireland it will be around or below average. In other words, a fairly typical December.
Iâ€™m anticipating a switch around in January, where transient colder snaps brought about by an excessively strong vortex are replaced by a more wintry pattern. Nonetheless, it wonâ€™t start off that way â€“ more bog standard Atlantic frontal systems will dominate the first half, with temperatures and precipitation widely above average. However, as the month progresses, mid latitude height rises will ridge northwards, with the jet finally being diverted southwards with colder air encroaching from the east. With still a large chunk of the vortex situated over Canada the height rises will take a week or two to become properly established, with a number of transitional snow-rain-snow events interspersed with more settled milder days, and temperatures will generally be around or slightly below average, but eventually, by around the 25[sup]th[/sup], a cold easterly flow will be established, ushering in one of the main cold spells of the winter. With the centre of the high between Iceland and Scandinavia rather than further east and with still a fairly impressive cold pool over the Arctic the UK could tap into some severely cold uppers if the setup works out. This would bring a period of very low maxima, perhaps sub zero, with the potential for significant snowfall right across the British Isles, but particularly for areas exposed to the easterly wind (including the Forth-Clyde streamer area) where showers would merge into longer periods of snow, although with low heights anywhere in the south of England could see some impressive snowfall totals, perhaps upwards of 8 inches quite widely IF we tap into the cold pool before the flow is cut off. (Note: the timing and severity of this event are very much low confidence, so Iâ€™d wait until at least early January before stocking up on tinned goods). Temperatures for the month as a whole will be below average but not massively so â€“ a CET of 3.1C and a Scottish mean of 1.8C are my current bets. Precipitation totals generally around average, with the exception of the Western Isles and Northwest Highlands where it will be somewhat drier than average and southern England where it will be above.
With the easterly flow cut off by the start of the month as heights lower from the north a brief spell of anticyclonic weather will prevail through the first part of February, away from the far south of England where the odd snow shower may remain. Temperatures at the surface would be well below average even without the upper cold pool as the snow fields caused minima to drop like a stone under clear skies. With the southerly arm of the jet still dominant, and the vortex still not really managing to get a foothold east of Canada, a reload of the cold looks likely, with a weak Scandi trough/ weakfish southeast Greenland high providing a possible route, propped up by the strong southerly jet. This would give more snow to eastern parts, with Aberdeenshire in particular taking a bad hit, and temperatures way below average once again. By mid-month the pattern will look to shift westwards, with height rises over Western Greenland, leaving us in a west based â€“NAO state (think February 2010) with a rather messy cold trough, bringing a rather dour mix of snow, sleet and cold rain from a variety of wind vectors before eventually height rises over southeastern Europe build northwestwards and introduce a milder flow to end the month.
Precipitation once again above average, although moreso in eastern parts, with temperatures very much suppressed until the very end. My CET punt is a very cold 0.8C, the coldest February since the sub zero 1986 (one of the â€˜lesserâ€™ composite years incidentally) with the Scottish mean a positively balmy 1C.
I hope I havenâ€™t bored/scared you too much, and Iâ€™ll be looking to update it throughout the winter and give an honest assessment of if/when it goes completely bust and we end up with a heatwave early February. Iâ€™ll be updating this in the next few days too with one or two synoptic charts which maybe give a better representation of my thoughts than the description does.
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