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  1. 4 likes
    A revised version of the earlier "Winter Snow Setups/Non-Snow Setups" topics, this goes through the range of winter setups we can get. As in summer, the main determining factors in what sort of winter weather we get are the positioning and strength of the jet stream. A strong jet stream means that depressions will frequently move from west to east, giving a "zonal" pattern over the UK. Because the Atlantic is relatively warm and moist, assisted by the warm North Atlantic Drift, zonal types often tend to be mild- but not always. A weak jet means lows track less frequently from west to east and blocking highs can form more readily. Whether we get cold wintry weather depends on the positioning of the high. Zonal, northerly tracking jet When low pressure systems track well to the north of Scotland, we usually end up with high pressure close by to the south, and a mild moist tropical maritime airmass covering the north. It is usually dry and mild in the south with a fair amount of sunshine, but cloudier and wetter in the north and west. Bartlett/Euro High An extension of the northerly tracking jet scenario, this setup sees the Azores High displaced over to Europe, keeping Britain in a persistent tropical maritime south-westerly regime. This setup brings Britain's warmest winter temperatures. This setup is often associated with large rainfall totals in the Scottish Highlands. Broadly speaking it tends to be dry and sunny wherever the high covers, northern and western Scotland tend to be dull and wet, and intervening areas often end up fairly dry but cloudy. Zonal, jet tracking over and to the north of Scotland This is the most common "zonal" winter pattern with low pressure systems regularly moving from west to east, bands of rain moving east at intervals, with brighter showery polar maritime air in between the rain belts. If the lows track from SW to NE then southern and eastern areas often spend a lot of time in "warm sector" mild moist tropical maritime air. It tends to be wet everywhere, sunny in the east and dull in the west. If the lows track more from NW to SE much of Britain spends a lot of time in polar maritime air, giving sunshine and showers. It tends to be wet but sunny in most regions, especially sunny in the east and especially wet in the west. On rare occasions, if there is an influx of Arctic air or cold pools from Canada/Alaska into the mid-Atlantic, we get so-called "cold zonality" with widespread snowfalls, especially in northern and western regions. An extreme case of this occurred in January 1984. Zonal, jet tracking right over Britain This pattern tends to be very wet as the lows track straight over the British Isles. Temperatures tend to be close to normal but with a bias towards milder conditions in the south, and cold polar incursions often reaching the north, giving snow for Scotland and northern England. Zonal, southerly tracking jet This pattern is not very common but when it does happen it can usher in prolonged spells of cold snowy weather for the British Isles. Low pressure stays to the south, sometimes bringing fronts into southern areas which can bring snow as the milder air meets cold polar air to the north. Otherwise, high pressure oscillates between Greenland and Scandinavia bringing repeated bursts of northerly and easterly winds. Blocked, high pressure over Britain When a winter anticyclone settles over Britain the weather tends to be dry, but the weak winter sun is ineffective at burning away low cloud. Thus sunshine amounts can vary considerably depending on how much cloud is trapped within the high- in general an input of moist tropical maritime air, or an easterly drift from the moist North Sea, may result in days on end of "anticyclonic gloom" with low cloud and mist and no chance of any sunshine. Alternatively, a clear anticyclone may bring frosty foggy nights and sunny days, as happened in December 2001 (below). Blocked, high pressure to the east When high pressure is well out to the east, this allows Atlantic lows to come towards the British Isles but they stall to the west, which tends to give rise to mild, rather cloudy southerly regimes. The equivalent of the summertime "tropical continental" southerly type which dominated the month of July 2006 rarely occurs in winter, but it does crop up occasionally. Depending on the amount of cloud circulating around the high's western periphery, a prolonged spell of anticyclonic gloom may ensue (as happened in February 1993 and early December 2004), or it may be warm and sunny by day but with cool nights, as happened in February 2008 (below). Unfortunately, with highs both over and to the east of Britain there is no synoptic way of determining how cloudy the highs will be- satellite imagery, atmospheric profiles etc. are your best bet for guidance. Blocked, strong Azores or mid-Atlantic high This kind of blocked pattern results in mostly mild weather as north-westerly winds suck up mild air from the Azores and around the high's periphery to the UK. Sometimes as a low moves out into Scandinavia it may introduce a brief burst of cold northerly or north-westerly winds with some snow showers, but these blasts usually tend to be short-lived. Blocked, Scandinavian high The Scandinavian High is often regarded as the "holy grail" by many cold/snow lovers, because it directs cold continental air across from the east. However, the Scandinavian High is really more of a building block towards an easterly- if the high is kept too far east the continental air may well stay away to the east. If an easterly does reach Britain then it will pick up moisture over the North Sea, and the resulting weather is largely dependent on the upper air temperatures, and the 850hPa temperature is often used as a guide. If the upper air is relatively mild (typically above -5C), the air will be stable and the moisture will give rise to layers of stratocumulus and persistent dull dry weather. However, if the upper air is cold (typically below -5C, preferably -10C or below) then the air will be unstable, and will give rise to heavy, often prolonged showers, especially but not exclusively for eastern areas. This setup brings much of England and Wales its coldest weather, and can produce significant snowfalls as happened in February 1991 (below). Northerly type Northerlies are another major source of snow events, brought about by high pressure to the west, and low pressure over Scandinavia or the North Sea. However, northerlies too have a major "stumbling block" if it's widespread snow you're after. Unless there is a southerly tracking jet stream, or a strong anticyclone over Greenland (preferably both), we tend to get brief "topplers" with just 36-48 hours of northerly winds, a few wintry showers for exposed coasts, and then milder weather pushes in. However, if a block can hold to our north-west for long enough for the northerly to sustain for upwards of a few days, then we will often see troughs form in the airflow bringing snow showers well inland. A large area of high pressure over Greenland, extending towards Iceland, will usually keep the British Isles affected by repeated bursts of polar air from the north. The "polar low", a low that forms in cold northerly airstreams and tracks south, is a particularly prominent source of snowfalls in a northerly regime. Although it is usually northern and eastern areas that see the most snow in a northerly regime, western areas can see the largest amount when pressure is low to the north, resulting in the Arctic air being sent south through the east Atlantic and around to Britain from the west or north-west (similar to the "cold zonality" described earlier, but via a northerly regime). Continuing the Christmas theme, this brought many western areas a white Christmas in 2004. Frontal battlegrounds Finally, when pressure is high to the north or east bringing cold polar and/or continental air towards Britain, and this cold air meets Atlantic systems coming in from the south-west, causing the systems to stall, this can lead to prolonged outbreaks of snow. For example many western areas were heavily hit during early February 1996 from this kind of setup.
  2. 3 likes
    The MJO is a major contributor to the global weather patterns, so for those who want to understand a little bit more about it here is a brief overview of my current understanding of the MJO and Rossby and Kelvin Waves. First lets talk a little bit about waves or more specifically Rossby and Kelvin waves. These can occur both in the Atmosphere and in the Ocean and it is important to be clear about the difference between the two. Oceanic Rossby waves take the form of slight height changes in the sea and more apparent changes in the depth of the thermocline. These can take months or years to cross an oceanic basin and have there orrigin in anomalous atmospheric pressure patterns.In the North Pacific, for instance, a Rossby wave, after the 10 years or so that it takes to cross the basin, can push the Kuroshio Current northwards and affect weather on the North America continent. This might have happened already in 1993, the culprit Rossby wave being an effect of the 1982-83 El Niño.The important thing about oceanic rossby waves are that they are slow and westward moving. Kelvin waves move faster and eastwards taking about 70 days to cross the Pacific. See in the link below how an easterly wind anomaly at the equator can produce these waves and in the subsequent link how they are reflected to ultimately produce a pattern which has similarities to the el nino,la nina pattern. Oceanic Rossby and Kelvin wave Thory The Evolution of Oceanic Kelvin and Rossby waves The point here for me is that strong MJO events have large impacts of weather patterns and probably contribute to la nina and el nino events. We should note however that el nino and la nina tend to closely follow the volumes of warm water (20C+) at the equator so the MJO does not have it all its own way. Warm Water Volume and ENSO We recognise Rossby waves in the atmosphere as the long waves in the jetstream but there are also Kelvin waves in the atmosphere which travel eastwards around the world typically taking 40-50 days which show up as a pressure anomally. These lesser known waves may actaully play a most important role is triggering the MJO cycle. Perhaps we should just note that gravity waves are a different phenomenon and although Atmospheric Rossby waves are thought of as planetary waves I prefer to use this term for those waves in the Stratosphere, Mesosphere and Ionosphere. So onto the MJO events which have there orrigins in enhanced convection over the tropical western Pacific which create a low pressure which radiates rapidly eastward as a dry equatorial Kelvin wave over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the sea level pressure anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle. So we have a trigger which circulates the world over a set period (typically 50days) with one event triggering the next.Here we should note that the MJO Phases do not coincide with this circulation but reflect the eastward migration of convection once convection has been triggered. Once convection fires at the start of the cycle you will get a Rossby wave response with pressure troughs to the north and south of the area of convection. The low pressure will bring colder air in to the west of the convection killing of convection while eastward moving warm air spreads the convection eastwards. Eastwards of the low pressure systems will be strong anticyclones (high pressure) which will give strongly easterly winds at the equator. These pressure systems affect the mid latitude jetstream and hence the pattern across the north Pacific, the US and to some extent the North Atlantic and the UK. MJO Phase 2 or 3 weather pattern response MJO Phase 6 or 7 weather pattern response At the moment cool waters in the central pacific due to la nina are tending to damp down the eastward movement of convection while anomalous highs ahead of the convection will be acting to enhance la nina and slowly move it Westwards. Phase 5 through 8 of the MJO can result in a high pressure anomaly towards Alaska and a deep trough down into the central US. There are some suggestions that this high pressure towards Alaska ridges into the arctic region causing a displacement of the stratospheric vortex forcing the arctic oscillation to trend negatively. Perhaps I will revisit this when I know a bit more.
  3. 2 likes
    There was already a guide written by me about UK thunderstorm set-ups, but it was done some 10+ years ago now and I've felt for a while that it needed a re-vamp and updating to make a more comprehensive guide to the processes that produce the various types of thunderstorms we see in the UK. So here it is ... the Netweather guide to thunderstorms in the British Isles .... 15 pages long: Thunderstorms in the British Isles.pdf
  4. 2 likes
    This is my revised version of the summer synoptics guide. The standard summer synoptic setup Traditionally, in summer, we have a strong Azores High out to the south-west, low pressure systems moving from west to east to the north of Britain, and westerly winds dominating, bringing cool cloudy weather and rain at times. Southern areas see the warmest and sunniest weather as they are closest to the influence of ridges from the Azores High. Low pressure dominated scenarios Cool, cloudy, unsettled summer weather is often associated with a conveyor belt of strong westerly winds, a flattened Azores High well to the south-west, and low pressure systems and fronts bringing bands of rain west to east at regular intervals. The Julys of 1992, 1993, 1998, 2002 and 2004 all had this pattern. When lows track further south than usual (over northern Britain, say, rather than to the north of Scotland) it can be especially wet- July 2007 was a good example. However, if we get a significant gap in between fronts, the result tends to be a mix of sunshine and showers- the most common pattern being a sunny start, a build up of cloud towards the afternoon and then sharp showers. When we have slow moving low pressure close by and no frontal activity, such "sunshine and showers" weather can persist for days on end- this situation generally arises when the jetstream is weak. However, if the low pressure is also associated with slow moving fronts, then instead of being bright and showery it tends to be cloudy and drizzly. High pressure setups For spells of warm dry sunny weather, many look out for ridges from the Azores High extending over towards the British Isles, quietening the weather down. Sometimes this can indeed herald the start of a fine spell, if high pressure can establish over the British Isles for upwards of a few days, but more often, the ridge brings just a day or two of fine weather before the next Atlantic system comes in and the high retreats to the south-west. Often the ridge just covers southern areas, giving warm dry sunny weather in the south, and dull damp weather in the north. If a ridge from the Azores High connects with high pressure over and/or to the east of Britain, however, we may get a prolonged spell of warm dry sunny weather, for instance the famous summer of 1976 was dominated by this setup. Eastern blocking Blocking over and/or to the east of Britain can sometimes bring hot sunny spells on its own, without the need for ridging from the Azores High- such a pattern typically has low pressure to the west, and the Azores High displaced to the west of its usual position. Persistence of this pattern resulted in the hot summer of 1995 and the exceptional July of 2006. It can also give rise to significant thunderstorms when Atlantic systems push against the block, bringing a "Spanish plume" event with southerlies bringing storms up from the near Continent. However, if the jetstream strengthens, such a pattern is usually temporary, as the Atlantic systems push through, the block retreats eastwards and we get a thundery breakdown followed by westerlies. Northern blocking Sometimes high pressure prevails to the north of Britain (this type of setup is far more common during June than July or August, as the westerlies are traditionally weaker). This brings a pattern of easterly winds, it is often warm, dry and sunny in the north-west, cool, dull and misty near the east coast, while central and southern areas tend to be warm and humid with thundery rain periodically moving up from the south. The mid-Atlantic high Finally, it is also possible for the Azores High to be displaced northwards into the mid-Atlantic, giving northerlies over the British Isles. This setup tends to be cool and cloudy, especially in eastern areas, as frontal systems move southwards around the periphery of the high, though western areas are often sunny. However, if we pick up an unmodified draw of air from the Arctic with a significant gap between fronts, the result tends to be sunshine and showers- similar to what I described under the low pressure setups- this setup tends to be cool but can also provide very dramatic weather with hail and thunder, particularly for eastern England.
  5. 1 like
    Noctilucent clouds are visible in June but rare enough to cause excitement. These high ice crystal clouds can only be seen in the midsummer twilight. Once the sun comes out the light is too strong for these delicate, pale threads to be seen, they need the semi-darkness. Usually, cloud viewed in high latitudes, they were seen in mid-June 2018 in Norfolk and the Netherlands with brilliant displays over northern Britan and Northern Ireland. Read the full article here: https://www.netweather.tv/weather-forecasts/news/8977-noctilucent-clouds---night-shining-polar-mesospheric-clouds
  6. 1 like
    A quickie on the abbreviations... WZ- Wetterzentrale ( German website for viewing charts) NAO- North atlantic oscillation PNA- Pacific North american SOI./ENSO- El-Nino Southern oscillation WAA- Warm air advection CAA- Cold air advection 528 DAM- is the line drawn on the maps that equated to the temperature ( MAX) that snow can be often observed at GFS- Global forecasting sytem METO- Met office model UKMO- United kingdom Met office ECM( Or ECMWF) European centre of medium range weather forecasts.. ASL- Above sea level- PPN- Precipitation Ensembles- 10 GFS model runs- Control run is the one seen on the models SST'S sea surface temperatures PM- Polar maritime air MT- Maritime Tropical air PC- Polar Continental air LRF's - Long range forecasts MRF's- meduim range forecasts Trough- Upper level equivalent to a surface Low pressure Ridge- Upper level equivalent to a surface High pressure Blocking- The jet stream being moved AROUND CLOCKWISE a large area of high pressure. Regards Steve
  7. 1 like
    I hope the article below will help to explain what the term means and how its value is arrived at. The term DAM is used at times but its correct term is 'thickness' between the two levels in the atmosphere. Remember although its often referred to at the 1000-500mb level it can be used between any two levels. For snow forecasting the other most often used is the 1000-850mb values. DAM heights or total thickness between two levels, usually the 1000mb and 500mb I hope this may help (!) to show how complex is the relationship but also how relatively easy it is, knowing the two heights, to calculate the ‘thickness’. This can be done for any two heights. The two most referred to, usually on Net Wx to do with the will it or won’t it snow, are the 1000-500mb and the 1000-850mb heights for ‘thicknesses. Fortunately this has all been done for us by Paul and Karl with the charts shown below! DAM is what refers to the 1000-500mb thickness chart. Its rather complex but there are several ways to work out its value. Below are some of the methods which might help = height (500 hPa surface) - height (1000 hPa surface) [ for those of you, like me, too old to catch up with all the changes the world brings, millibars = hPa!, so 500 hPa is exactly the same as 500 mb. ] h(500) = h(1000)+h'(thickness). Or from that h'(thickness)=h(500)-h(1000) Thickness can be calculated from the heights reported on a radio-sonde ascent, or a thermodynamic diagram can be used to add up the partial thicknesses over successive layers to achieve the net (total) thickness. An example of the former would be 500 hPa height = 5407 m 1000 hPa height = 23 m Thickness = 5407-23 = 5384 m (or 538 dam) Careful note must be made when the height of the 1000 hPa surface is below msl thus: 500 hPa height = 5524 m 1000 hPa height = - 13 m Thickness = 5524 -(-13) = 5537 m (or 554 dam) Note the example above when surface pressure is BELOW 1000mb. Roughly it is taken that 8mb is equivalent to 6DM when forecasters are manually drawing the various upper and surface charts. If we take the actual msl and 500mb chart from GFS/Extra for 06Z this morning, see below On the left is the surface isobar chart with the 500mb height; to its right is the ‘thickness’ chart Notice the differences in values between the left and right charts-obviously the surface values are identical but NOT the ‘thickness’ and 500mb values. Or to look at how the 00z ascent for Herstmanceux differs in its 500mb height and its 500mb ‘thickness’ In the basic data format the 500mb height was given as 500.0 5490 -22.9 -50.9; i.e. 5490DM; that of the 1000mb height was 1000.0 87 8.2 5.6 The ‘thickness’ is 1000 hPa to 500 hPa thickness: 5403.00 How is that arrived at, see the formula above 100mb height is 87 500mb height is 5490 Therefore 500mb ‘thickness’=5490-87=5403DM Additional information on atmospheric thickness and it's use is available on the NOAA National Weather Service website: https://www.weather.gov/source/zhu/ZHU_Training_Page/Miscellaneous/Heights_Thicknesses/thickness_temperature.htm John Holmes
  8. 1 like
    Ok I will start a new thread for dicussions along this line and perhaps I will draw on some ideas expressed in the stratospheric thread and artic sea ice thread. I guess you would be the best person to explain all this GP but for those who don't know this thread is about global angular momentum and how it oscialltes up and down (Global Wind Oscillation) along the lines discussed by Ed Berry. Angular momentum is of course a measure of the turning force in the winds, so could perhaps be considered a measure of the strength of low pressure systems, but also relates to how much the jetstream undulates and how much blocking we have. The budget of angular momentum goes up and down as energy is lost as weather systems crash into mountains and increases as cold air meets warm. Each phase of increasing and decreasing momentum suggests different types of weather for the UK. Looking at the current GWO plot we see a liklihood of going into phases 3 and 4 based on how it usually cycles round. This implies increasing angular momentum as the various torques including mountain torque diminish (i.e those things which take energy out are not active). This is certainly true of mountain torque. For frictional torque and gravity wave torque then the jury is out. Overall it looks like global angular momentum is on the increase. The tendecy during december has been upwards. Short term I think we are looking at phases 3-4 and more of an Atlantic influence. What I am guessing at though is that low pressure systems crossing the US will increase mountain torque, equally the jet stream across india is not a weak flabby one which might increase asian mountain torque. The strong jet in the western pacific along with OLR charts suggest strong trade winds with a stationary high to the north east of Australia and convectional activity to the north west of Australia. So back to phase 1-2 fairly quickly I think afterwards. All maps are available in the link below. PSD Map room for AAM I am sure GP will tell us what he expects from the MJO and convectional activity in the pacific and how and if he expects rossby wave development as a result. It is those Rossby waves which in part will affect the stratospheric vortex and the low angular momentum could be linked to a more blocked pattern and sea ice build up to our north which I talked about in associated threads. What we should always remember though that this a complex interaction of parts of which the stratosphere plays a large part during the winter. Please note that this post and subsequent comments have been copied from the forum, so the dates/times of the comments are not correct.
  9. 1 like
    The Arctic Oscillation Arctic Oscillation is an important lead on expected winter conditions in the Northern Hemisphere, loosely described as negative ( colder ) positive ( milder). The image below gives you a great contrast of a winter we will all easily recall with an extremely negative AO and a little further back a winter at the opposite end of the scale. When considering the overall forecast for Winter it is important to note any variables which provide clues as to which end of the scale the AO will tip towards, this in turn informs us of potential for blocking episodes and also the behaviour of the jet stream. Throughout the forecast elements indicative of the mean negative AO over winter are noted. Further description The Arctic Oscillation describes simultaneous, geographically 'choreographed' shifts in multiple features of the polar vortex: air pressure, temperature, and the location and strength of the jet stream. They all follow the hemisphere-wide oscillation of atmospheric mass back and forth between the Arctic and the middle latitudes, sort of like water sloshing in a bowl. L : Positive AO R:Negative AO At one extreme of the sloshing, there is lower-than-average air pressure over the Arctic and higher-than-average pressure over the mid-latitudes. The jet stream is farther north than average under these conditions, and it steers storms northward of their usual paths. The mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual. These are all characteristics of a strong, “well-behaved†polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index, which tracks relative pressure anomalies across the N. Hemisphere, will have large, positive values. At the other extreme, the conditions are reversed. Air pressure is higher than average over the Arctic and lower than average over the mid-latitudes. The jet stream shifts southward of its average latitude and can develop waves or “kinks,†with “troughs†that help steer frigid, polar air southward. These are all characteristics of a weak polar vortex. When the atmosphere is in that state, the Arctic Oscillation Index will have large, negative values. Source : Climate.gov C.Kennedy, R Lindsay
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