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iapennell

AUTUMN 2017 PROGNOSIS: OFTEN WET AND STORMY IN THE NORTH, FINE SPELLS IN THE SOUTH

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Congratulations with passing the Advanced Diploma in Accounting Ian and good to see you back.  Thanks for the autumn forecast, very detailed as always!

Edited by Don
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As always, a detailed and interesting forecast with a detailed discussion of the various factors at play. Thanks again Ian.

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On 9/9/2017 at 20:19, iapennell said:

 

With all this in mind, the remainder of September is likely to be a little warmer than usual with plenty of fine sunny conditions over central and southern England, Yorkshire and South Wales.  Daytime temperatures above 21˚C can be expected from time to time as high-pressure builds north and east over England.  However, at this time of year, clear night skies inevitably mean falling temperatures and minima of 5˚C or below can be expected to occur widely across England on at least a few nights during September with early-morning fog and ground-frost likely in valley-bottoms. For Scotland, North Wales and north-west England along with Northumberland and Northern Ireland the baroclinic set-up and Atlantic hurricane activity is likely to result in a more unsettled flavour to the remainder of this month.  Depressions passing to the north will bring spells of rain, strong winds from the west will be frequent and daytime temperatures will often remain the cool side of 15˚C, even in the lowlands. However, even in Scotland and the North there will be fine spells with ridges of high-pressure between depressions with maxima edging towards 20˚C but with the clear nights becoming cold enough for ground-frost at times.

 

Frontal influences will also extend further south to central/southern England and South Wales to bring some days of rain, stronger winds and maxima below 18˚C.  I do however, expect there to be more sunshine and warmth and less rain than usual for central and southern England on the whole.  For Scotland and the north-west of England, along with Northumberland, rainfall temperature and sunshine totals will be remarkably close to the long-term normal overall, but north-west Scotland will be wetter and windier than normal. Gales are likely to occur here two or three times during the second half of the month.

 

During October the cooling Arctic with the North Atlantic liable to remain warmer than usual, combined with continued hurricane activity is likely to mean a stronger atmospheric temperature gradient between the Arctic and the North Atlantic with continued injections of moist tropical warmth from the south:  This strongly suggests deeper depressions, particularly later in the month.  All things considered, with the rapid cooling of the Greenland Ice-cap and NE Canada whilst the westernmost part of Canada and the North Atlantic remain relatively warm- and with the factors discussed above likely to lead to a Circumpolar Vortex around or slightly stronger than usual for the time of year, I am confident of a so-called “three-wave” pattern with an upper ridge close to the North American Rocky Mountains. This would also place an upper ridge over Western Europe with the result being high-pressure encouraged to occur a little downwind- i.e. over central Europe whilst the upper-air pattern pushes progressively deeper depressions from coastal eastern Canada north-eastwards towards Iceland and then up into the Barents Sea.

 

Thus, during October the prevailing winds over Britain will be south-westerly, and they will increase in strength during the month.  During October these winds will often bring wet and windy conditions throughout the country.  However, the southern half of the country- all of England except Lancashire, Cumbria and Northumberland along with south and central Wales- will have a good deal of dry fine weather during the first ten days of October (there are indications that the weak upper-air Circumpolar Vortex will remain weaker than normal into October and the absence of a “Cold Patch” in the North Atlantic will encourage the jet-stream to blow further north on coming into NW Europe). The fine conditions will result from extensions of high-pressure from Europe across England and Wales:  There will be a run of warm sunny days with maxima near 20˚C with light south or south-west winds; clear nights will again result in temperatures dropping well into single figures and local ground-frost and fog can again be expected.

 

For Scotland, northernmost England and Northern Ireland and North Wales, October 2017 will be unsettled from the outset. Strong winds and rain will be frequent and it will get increasingly stormy. By mid-month the weather will turn unsettled even in the South with some wet and windy spells.  Gales and widespread heavy rain will sweep the entire country at least a couple of times during the second half of October and localised flooding is likely to occur in western Scotland, Cumbria and Lancashire. As a couple of the deepest depressions push eastwards late in October colder strong west or north-west winds will sweep the country to bring cold squally showers (with hail locally) and daytime temperatures below 10˚C as far south as Birmingham and the first snow will fall on high ground above 600 metres in Scotland, Cumbria and Northumberland- to provide a real foretaste of winter. There is likely to be at least one night with a ridge of high-pressure following such cold strong north-westerlies when skies clear and winds fall light. Air temperatures will fall close to freezing across the entire country and ground-frost will be widespread.  However, except in some rural upland areas of Scotland or sheltered frost-hollows further south, I do not expect the first air frost to occur for most of us until November.

 

The large scale upper-air pattern is likely to bring a return to some fine dry weather lasting just a few days during the second-half of October- again this fine weather- brought about by a westwards extension of the European High will be confined to central and southern England, South Wales and Yorkshire.  The fine spell is likely to be associated with cooler European air coming in from the south-east late in October and with weaker late-October sunshine daytime maxima anywhere above 16˚C are unlikely- even in the South.  Clear skies at night would encourage temperatures to drop sharply to within a few degrees of freezing point over a wide area and patchy morning fog will be widespread. However, as I have already hinted above, air-frost is unlikely to occur except in some rural low-lying valleys: Most places in England will not be getting their first air-frost of the season until November. Scotland is liable to miss out on any substantial October fine spells altogether.

 

First fortnight of October has been dry and warm in the south-east but a different story for northern England and Scotland where it has been wet (though still milder than normal overall). September has followed a similar pattern though temperatures have been close to the long-term normal in my part of the World (near nenthead in the North Pennines).  Hurricane Ophelia looks set to mark the transition to stormier conditions across Britain over the next week, but there is a prospect of drier colder conditions affecting central and southern Britain from the end of next week (according to the BBC long-range forecast).  Seem to have got the general prognosis about right, although September in the Midlands and South has proven to be a bit more unsettled than my original assertion of "Plenty of fine sunny conditions".

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I wouldn't say Northern England has been wet, it's the usual NW/SE split.
NE England right into Northumberland as had a dry month so far e.g. only 11mm here.

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38 minutes ago, 4wd said:

I wouldn't say Northern England has been wet, it's the usual NW/SE split.
NE England right into Northumberland as had a dry month so far e.g. only 11mm here.

I do believe I included Yorkshire in with Central and Southern England in my seasonal predictions for September-October as I felt it would be on the drier side of the said NW-SE split owing to prevailing south-westerly winds.  At Nenthead in the North Pennines (420 metres above sea-level) we had 148 mm (5.8 inches) of rain in September and so far this month (October) I have logged 60 mm (2.4 inches).  And we are set to get a lot more rain over the next few days, although being further east and sheltered from south-westerly winds by the Pennines and (further upwind) the Welsh Mountains you will be spared the brunt of it.

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I hate to spoil the party but is that not what you would expect from Autumn !

 

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    • By iapennell
      This coming winter (2017/18) there are global (macroscale) influences that suggest that the UK will have some colder spells of weather. This does not mean a cold winter by any means, just that there is a stronger signal for cold spells (related to blocking over northern Europe), likely to occur during January-February 2018.  There are six significant global-scale weather-patterns that will have an influence on the weather in the coming months:

      1)     The Quasi Biennial Oscillation (QBO) has turned Easterly:

      This is a wind-pattern high up over the Equator, affecting the Equatorial Stratosphere that gravitates from westerly winds to easterlies, then back to westerlies over a (approximately) thirty-month period. After an unusually long period of westerlies winds have become strongly easterly at the 30 mb level (about 38 mph as averaged through October).  At the 50 mb level winds are still Westerly but are now very light (just 3 mph as averaged through October) and these winds are likely to become easterly this month (November 2017).  These strong easterlies high up in the Equatorial Stratosphere are a source of negative Westerly Atmospheric Angular Momentum –or AAM for short (that is, winds blowing from east to west, rather than west to east) and its impact- once the easterlies of the QBO descend and impact the general global circulation- will be to resist (and weaken) the upper westerlies that move polewards aloft over the tropics and subtropics- and then go on to weaken the Westerlies aloft and near the surface in higher latitudes. Weaker Westerlies coming into north-western Europe mean less storms and a higher likelihood of high-pressure being able to form in high latitudes sending icy north or easterly winds towards Britain.

      However, one must not over-estimate the likely impact of the QBO on weather in Britain, the air at the 30 mb level is just 0.03 times the density of air at sea-level, so the 38 mph easterly would only slow down surface westerlies by just over 1 mph. Also, the easterly QBO air will mix (eventually) with the vast bulk of the atmosphere globally- further reducing its impact. But the impact is greater when the slowed-down upper westerlies reach higher latitudes (because of the reduced distance to the axis of the Earth’s rotation); hence there is likely to be a discernable impact on Britain’s weather. It takes two to three months for the easterly QBO winds at 30 mb to descend, enter the global circulation and then reach higher latitudes in its impact: Thus the signal from the strong easterly winds high up over the Equator will affect Britain as weaker intermittent westerly winds (with icy winds spilling out from northern Europe at times) during late January/ February.

      Other influences on the winter weather-patterns affecting the UK may well have a greater impact, as mentioned the QBO plays out in the rarefied atmosphere (at 10 to 50 mb level) high over the Equator and the proportion of the total global circulation directly involved with the QBO is small.

      2)     El Niño Southern Oscillation (ENSO) has entered a weak La Niña Phase:

      La Niña watch (12th October 2017) predicts a weak La Niña during winter 2017/18, with below normal sea-surface temperatures for the east and central Equatorial Pacific Ocean with sea surface temperatures a little above normal in the far western tropical Pacific Ocean.  The impact of cooler than normal Equatorial waters would weaken the low-pressure zone of hot rising air near the Equator, thus weakening the north-easterly trade winds blowing into it from the Northern Hemisphere: Consequently  the Hadley Cell weakens and westerlies aloft further from the tropics (including the subtropical jetstream and- further north- the Arctic Circumpolar Vortex are weakened in turn). Again, this suggests weaker westerlies reaching northwest Europe, increasing the chance of cold frosty spells associated with blocking highs over Scandinavia.

      By way of contrast, the strong El Niño of 2015/16 contributed towards winter 2015/16 being exceptionally wet, mild and stormy in the UK.

      However, one must not over-estimate the likely impact.  A weak La Niña means conditions are not so far removed from normal.  Furthermore, La Niña implies stronger easterly Trade Winds over the central and eastern Pacific Ocean (to push cool waters westwards into the central Pacific Ocean) along with stronger easterlies aloft coming across the northern (Equatorial) Andes of Ecuador and Colombia: These would result in more Westerly AAM being added to the global atmospheric circulation- resulting in a tendency towards milder stormier winter weather along western continental margins in higher latitudes; the pattern of  cooler than normal Equatorial waters off South America and steamy waters in the far west (near Indonesia) would also tend to encourage stronger easterly winds across the Equatorial Pacific Ocean.  However, it is mostly the south-east Trade winds blowing off northern South America that impacts the ENSO cycle; if these are stronger and causing La Niña the excess Westerly AAM is liable to affect the Southern Hemisphere atmospheric circulation rather than the Northern Hemisphere, leaving the cooler waters to weaken the Hadley Circulation and the north-east Trades that deliver Westerly AAM to the Northern Hemisphere atmospheric circulation. That being so, it means La Niña, even a weak one, would lead ultimately to a small weakening of winter westerlies reaching north-west Europe.

      The available literature on La Niñas suggests a tendency towards cooler drier conditions for north-west Europe in the winter months (see here “La Nina may chill Britain in run-up to Christmas” (http://www.weather.com) and here  El Nino and La Nina- The Weather Outlook (https//www.theweatheroutlook.com/two)): This backs up the prognosis that I have just made. That said, the La Niña this coming winter is expected to remain weak, impacts on the UK will be small.

      3)     The Madden Julian Oscillation (MJO) is weak:

      The Madden Julian Oscillation (MJO) is a large-scale tropical pressure and wind-pattern that affects the tropical oceans; it propagates eastwards at the rate of 10 to 15 mph per day and circulates right around the world in a period of two months on average.  It is, of course, modified by the ENSO Phase, the relatively cool tropical Pacific waters associated with La Niña (as is the case this year) tends to weaken the MJO Cycle- and this is indeed predicted to be the case over the next couple of months.  The MJO is hemispheric in its phases- when one hemisphere of the deep tropics has increased convection and rising air the other hemisphere of the deep tropics has weak convection and even subsidence that would suppress rainfall.

      For the next month the MJO is predicted to be weak, but may strengthen a little as the large active (convective) zone moves (or reforms should it be totally swamped by La Niña) over the tropical Atlantic in late January/February (the position of the convective area dictates the Phase of the MJO). Phases 7 and 8 are when the convective zone is over the tropical Atlantic Ocean and if the MJO is fairly active in these phases it increases the likelihood of blocking patterns over Northern Europe.  Atmospheric waves associated with an active MJO in any phases can penetrate upwards into the Stratosphere- and this can lead ultimately to possible stratospheric subsidence and associated Sudden Stratospheric Warmings over the Arctic: That in turn encourages a sharp weakening (and expansion) of the Circumpolar Vortex and much colder winter conditions affecting north-west Europe (source: http://www.cpc.ncep.noaa.gov>mjoupdate).

      The phase and intensity of the Madden Julian Oscillation may (slightly- in view of how weak it will be) increase the likelihood of some very cold weather from the east or north-east affecting the UK in late January and February- but it is likely to have almost no overall impact on the first half of the season.  On the whole, the MJO Phase may have more of an impact than either the weak La Niña or the easterly QBO from mid-January onwards, but certainly not before then.

      4)     The current Sunspot Cycle is drawing towards an end as the Sun goes Quiet:

      The Sun is entering a quiet phase with few sunspots and solar flares:  In the process the energy output from the Sun is also declining slightly compared to recent years.  It is known that high levels of Sunspot activity combined with solar flares and coronal mass ejections (CMEs) result in high speed plasma crashing into the Sun-facing side as this hurtles westwards at 66,660 miles per hour as the Earth orbits the Sun (65,600 mph taking into account the Earth’s rotation from west to east).  Thus these solar winds crash into the atmosphere from rather westwards of vertical and impart Westerly AAM to the global circulation- leading to a spike of deep depressions associated with stronger Westerlies in higher latitudes, especially in winter (“Clear link between solar activity and winter weather revealed”, October 2011, https://phys.org>Earth>Earth Sciences).  The Sun is, of course, entering a quiet phase and, notwithstanding the large Solar Flare that affected Earth in September, expect little solar activity overall this coming winter: Less Solar Flares and CME’s mean less of the forces that would increase Westerly AAM and (with it) strong Westerlies in higher latitudes.  This is yet another factor that favours a slightly increased chance of colder drier weather from the east this winter.   

      As the Sun enters it’s quiet phase nearing the end of the current Sunspot Cycle, total solar output has been declining slightly:  The Solar Constant was 1362 Watts per square metre on average during 2014, now it averages under 1361 Watts per square metre with dips to 1360 Watts per square metre (Source: Solar Irradiance and Sunspot Numbers, http://www.climate4you.com>Sun). This is a drop in total solar irradiance of 0.1% or more, one that would cause a global temperature drop of 0.1˚C (other things being equal), but nearer 0.2˚C (and greater in high latitudes) once positive feedbacks are taken into account.  The additional greenhouse effect arising from the increased CO2 levels over the last three years (about 10 ppm) does not even half counter such a sharp drop in solar output. Slightly reduced solar output in itself would lead to a weakening of the Hadley Circulation in the tropics and subtropics, with slightly weaker north-easterly Trade Winds at the surface (and weaker westerlies aloft). This weakening of the Hadley Circulation would result from the 0.1% drop in solar irradiance, whether the drop in solar output led to an overall global cooling in the face of rising CO2 levels- or not. The weakening of the Hadley Circulation may be slight, but it would also lead to a weakening of the higher-latitude westerlies.  Again, that means there is a small increase in the likelihood of severe cold reaching the UK from Russia this winter. 

      The impact of the Quiet Sun with a 0.1% reduced Solar Constant has perhaps the most potent impact on the coming winter of the global climatic drivers so far discussed; yet for reasons to be discussed below this does not by any means guarantee a season like 2009/10 or 2010/11.

      5)     Arctic Sea-Ice remains below normal but not by much, whilst the North Atlantic Ocean and North Pacific Ocean are 1 to 2˚C warmer than the seasonal norm in early November

      Arctic Sea-Ice extent was overall below the normal for early November by about 1.4 million square kilometres and the southernmost extent of the pack-ice is some 250 km north of its seasonal position to the north of the Bering Strait and in the Barents Sea/Svalbard area. However, sea-ice extent is close to the seasonal normal extent in the Davis Strait and the Canadian Arctic Archipelago. (Source: Arctic Sea-Ice News and Analysis http://nsidc.org/arcticseaicenews/).  Sea surface temperatures in early November were 1 to 2˚C warmer than the seasonal normal across the North Atlantic (http://weather.unisys.com>surface>sfc_daily/), about 1˚C warmer than usual across the North Pacific.  However sea-surface temperatures are up to 4˚C warmer than normal in the Bering Strait between Alaska and eastern Siberia and up to 5˚C warmer than normal off the East Coast of the USA.  Sea surface temperatures are actually up to 1˚C cooler than normal across much of the Equatorial Indian Ocean, parts of the Equatorial Atlantic as well as 1 to 2˚C cooler than normal for the time of year over the eastern Equatorial Pacific Ocean.  By way of contrast, the subtropical North Atlantic and subtropical North Pacific sea-surface temperatures are above the long-term seasonal norm, again by about 1˚C for early November, sea surface temperatures are also about 1˚C above normal in the Norwegian Sea. The pattern of warmer than usual sea-surface temperatures in both the North Atlantic and North Pacific, with the Arctic-ice limit further north than usual in the European Arctic is supportive of the Circumpolar Vortex- the river of very strong westerly winds in the upper atmosphere over higher latitudes- being contracted and blowing closer to the Arctic.  There is still likely to be ice-cover over the Arctic as this expands in extent going into winter to ensure the Arctic interior gets extremely cold by the time we reach December (i.e. below -25˚C at the surface and colder still aloft); this with warmer than usual oceans in mid-latitudes will intensify the atmospheric temperature and pressure gradients that drive sub-arctic depressions and the strong westerlies that blow on their southern flanks.  Furthermore, unusually warm waters off the USA Eastern Seaboard combined with sea-ice extent and that of severe cold near normal extent around north-east Canada will help foster an intense atmospheric temperature and pressure gradient that could help send some really powerful depressions headed across the far North Atlantic, to bring strong mild south-westerlies to the UK. 

      On the contrary there are cooler than normal sea-surface temperatures over Equatorial waters but slightly warmer than usual sea-surface temperatures across the northern sub-tropics: This would  substantially weaken the crucial temperature gradients that drive the Hadley Circulation; since the normal 35˚N to Equator lower-atmospheric temperature gradient is about 15˚C averaged over the Northern Hemisphere in winter the sea-surface temperature anomalies (such as they are) are likely to lead to a weakening of the Hadley Circulation by up to 5% This means weaker north-easterly Trade Winds, and with the Circumpolar Vortex likely to be north of its seasonally normal position (due to reduced Arctic ice-extent and warmer-than-usual mid-latitude oceans) the subtropical high-pressure belt is likely to be 35 to 40˚N with the slightly weaker north-easterly Trade Winds covering a somewhat greater area of lower/tropical latitudes than usual, westerlies in higher latitudes would not need to be any stronger to provide a sink for the westerly atmospheric angular momentum generated by more extensive (but weaker) north-easterly Trade Winds. 

      Despite all this, the fact that the Westerlies are likely to be blowing in higher latitudes (due to warmer mid-latitude oceans and reduced Arctic ice extent) means these Westerlies will be blowing closer to the axis of the Earth’s rotation; thus they will need to blow stronger to provide a sink for all the westerly Atmospheric Angular Momentum (AAM) caused by the north-easterly Trade Winds and high-latitude (Polar) Easterlies.

      However, snowcover and severe cold over the interior of Asia could help displace the strong Westerlies (at least aloft) to the extent that the Himalayas and Pamirs become a major sink for Westerly AAM; then all bets are off with all factors likely to weaken the mid-latitude Westerlies combining to cause a Sudden Stratospheric Warming over the Arctic: This we will now discuss.
      (continued below)
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