Archives For Atlantic

2015-01-18_08-08-35

Water vapour EUMETSAT rgb airmasses: Arctic red, tropical blue

Update Tues pm: system not bringing any significant snow to Reigate, Midlands north might see accumulations but even this is not likely to be disruptive..  The satellite image above shows water vapour and airmasses.  Notice how tropical (blue!) and Arctic (red/mauvey) airmasses are intruding across latitudes – plunging respectively north and south of their source regions.  Cold Arctic air is reaching right the way down to Spain.  Here’s a look at what is going on and how things might develop for Reigate and SE UK especially this week.  Overall things look cold and wintry and there is a slim possibility of snow for us but it’s worth watching the forecasts and twitter updates because it’s very marginal and things can change locally quite quickly.

The Arctic air arriving in London Monday left the Kara Sea a week ago. This Arctic air is arriving over the UK courtesy of a high pressure ridge over the Atlantic and Greenland / Iceland which blocks mild maritime air in the Atlantic from reaching the UK. A low (the remnants of storm #Rachel) over Scandinavia is dragging down cold northerlies assisted by a northerly jetstream aloft.  This set-up makes this week the coldest since March 2013 and, as some light snow fell in Reigate this morning (Sat 17) then that was the first snow fall here for 2 years.

The result of the pressure pattern is airmass temperatures lowering at 850hPa to -7C or lower over the course of the next 48 hours due to the steady invasion of Arctic air. Overnight tonight into Sunday morning a front in the Channel could bring light snow showers to southern England early Sunday morning.  The situation is “marginal” as the truly cold air has yet to arrive in the south and dew points are hovering around or above freezing tonight which can make a difference between whether it snows or rains.

Over Sunday the Arctic air will arrive and the upper air mass temperature at 850hPa (1500m up) will fall from -4C to -7C by Monday.  In such a cold airmass the surface air temperatures on Monday will struggle above freezing during the day to about 3C and wind chill will make it feel colder.

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An important threshold for snow is an upper air temp at 850hPa of -5C or lower.  So from Monday any precipitation might fall as snow, so long as other factors are in place.  The chart below shows the movement of the Shetland low into the N Sea and eventually further south which ushers in northerly winds.

LOW sinks south bringing Arctic air

LOW sinks south bringing Arctic air

There is reasonable model agreement as to what will happen into mid-week but uncertainty thereafter as to how long any cold will last.  High pressure is set to build over Scandinavia blocking the NE track of a low S Greenland.  A trough disruption is set to occur when the LOW near Iceland splits from the main trough and slides down the edge of the Atlantic high and sinks SE over the UK (called a slider low).  Trough disruptions are notorious for causing models problems with accurate forecasts!

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trough disruption

The slider low will bring attendant fronts with milder air mixed in, thus complicating chances of widespread snow and making forecasts tricky.  Tuesday is the first chance of any snow this week for the SE as a front moves in from the west to reach the SE around the afternoon, though details this far out cannot be certain.  As the front moves into colder air the rain could turn to snow, especially on the back edge as overnight temperatures fall.  It is very likely to be snow across the middle of the country and certainly over high ground but snow for the SE is less certain, it could be just sleety or rain depending on the mix and location of mild air from the south in the occluded front. In any case Tuesday looks light precipitation as the front weakens to the east.  This doesn’t help snow formation in marginal situations because less cold air is dragged down from aloft in light rain and there is less evaporative cooling in light rain.

Charts currently show that Wednesday has a better chance of snow action for the SE as another front, this time with heavier rain, clears east later on Wednesday or overnight into Thursday.  Exact timing is uncertain and indeed the development of this might all change despite there being sound model agreement as to the overall synoptic situation into the mid week period.

The latest UKMET chart for Thursday shows the SE in a COL between HIGH pressure SW and NE and LOWS NW and SE. This looks like a cold wet day with sleet for Reigate, but snow is again possible, especially as the LOW drifts south and introduces a cold NE continental flow for a time on the northerly edge. This is most likely to restrict to inland areas or those higher up locally but it needs watching carefully as heavy rain might tip over into sleet then snow due to evaporative cooling.

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After mid week things look like turning somewhat milder for a while as westerly winds eventually break through properly, possibly by late week or the weekend.  This means snow chance reduces to nothing as milder Atlantic winds return.  Nevertheless, the long range models still show some propensity towards building further chilly Arctic incursions later too.

Quick update #2 02/09/2014

Several models (GFS, GEM, ECM) are getting wobbly with the high pressure this weekend and bringing in a trough or upper low pressure down the North Sea over the weekend or up the English Channel early next week.  Either way such scenarios will bring less settled weather to the SE this weekend and early next week: cloudy, some showers, but probably light mostly.  This is not a collapse of HP (high pressure) because pressure remains generally high and the jetstream well to the north of the UK.  Most models also suggest a building back of pressure next week, probably further to the north leaving the south vulnerable to unsettled conditions at times.  Best of the weather by Week 2 of September is likely to be Scotland. Point to note is that UKMet models do not see such an unsettled weekend, so things not certain… but seems to hint at vulnerable nature of the anticylcone.  HIGH building back stronger than ever second week sept.  

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High pressure… getting the wobbles

 

 

Quick update #1 31/08/2014

More agreement amongst models as to firmness of the High pressure lasting into second week, to around 9/10 Sept. ECM latest run shows nice firm blob of HIGH NW of UK.  Only snag with this is IF the subglacial Bardarbunga caldera in Iceland decides to erupt it would be when upper air flow is northerly, direct to UK and Europe.  However, this is all speculative and of course the most unlucky scenario. Check this website for volcano updates http://www.ruv.is/ and the Icelandic Met Office http://en.vedur.is/earthquakes-and-volcanism/earthquakes/

Forecast:

Anticyclonic conditions are building over the UK in the coming week or so courtesy of ex-hurricane Cristobal which is tracking NE across Iceland this weekend. The ex-hurricane has morphed into a deep low pressure that is dragging warm tropical air behind it into the Mid-Atlantic thanks to the jetstream and this is helping to build high pressure across the Atlantic and over Scandinavia.

Ex-hurricane Cristobal leaves trail of construction

Ex-hurricane Cristobal leaves trail of construction

 

Whilst the black and white UKMet synoptic chart (below left) shows surface pressure, the colourful chart (below right) shows important UPPER air flow as well as surface pressure on a so-called “500hPa” chart.  Upper air charts often represent conditions at at 500hPa (hectopascal) and are commonly used by meteorologists to get a better idea of how the atmospheric conditions will develop over longer periods of time.  This is because upper air is less disturbed by surface features such as oceans and mountains and by the influence of day and night that can upset surface charts and make them awkward to interpret predictions over longer time periods.  Upper air charts most commonly show temperature at 500hPa, or 5km altitude, which is about “half way” up through the troposphere, well away from surface upsets.  Upper air temperatures usually correlate with surface pressure, but not always.  See below to compare 2 charts, one surface and one upper air, for the same time period.

Warm upper air, usually of tropical origin, is shown on the 500hPa charts in orange and red colours.  Cool polar air at height is shown in green and blue. Where the colours are tightly packed together it shows a steep temperature gradient and this is often associated with the jetstream.  Warmer air is less dense and, rather like fluffing-up a thick duvet,  takes up more “space” in the atmospheric column compared to the the thin “blanket” of cold Polar air.  As warm air increases the 500mb “height”, fluffing up the atmospheric duvet, more pressure is exerted on the surface which increases the air pressure.  Cloud formation is inhibited as air sinks, warms and dries out.  The HIGH being built early next week, initially over the Atlantic, is formed by a warm upper flow of Tropical air pumped into the Azores high by an upper SW jet following on the heels of ex-hurricane Cristobal.

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So HIGH pressure will bring settled weather across much of the UK, including here in Reigate and the SE, for about a week with some warm temperatures and light winds and generally pleasant weather with some sunshine.   However, not all models show a uniformly “clean” high i.e. an anticyclone that is free from cloud and blocking all incursions of cool or moist air.  Note the UKMET chart (up) shows some weak fronts wriggling across the HIGH and this could mean cloudy skies for some and even odd showers at times.  Rainfall charts also do not show completely dry weather throughout next week (see below).  The ECMWF model (above right) shows a weak trough / cut off low feature to the west of the UK mid-end of next week and eventually forming a low feature in Biscay.  If this scenario comes off it could temporarily spoil the HIGH and possibly send showers into the South as it drags in warmer humid air from a southerly direction later next week.  So the exact duration and nature of the high is still a little uncertain so check back and also check UKMO of course for updates.

 

 

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By later in the second week of September and beyond an active Atlantic with the possibility of more extra-tropical cyclones will probably start to breakdown any remaining high pressure. The chart above shows the chances of tropical cyclone formation (TCFP) in the Atlantic and, whilst % figures are low, this still represents a fair chance of cyclones forming and ultimately entering the mid-latitudes later in September.  So this might not be a durable blocking high or one that lasts.   Nevertheless, it is good enough to keep active Atlantic weather systems at bay for a week.

The positioning of the HIGH is also critical to how “warm” we will get in the UK.  This time of year the continent is still warm so high pressure over Scandinavia to the NE or East of the UK brings in some warm continental air on an easterly flow.  This is how the GFS and UKMO sees things developing initially  with a  high positioned over Scandinavia and Europe through mid-week.  This flow could yield temps by Thursday in the mid-20’s, up to 25c is possible for SE.  With sea surfaces at their warmest this time of year it could be a good time to head to the beach.  By next weekend most models (above) see some movement of the HIGH to the west and NW of the UK where it could introduce a cooler Atlantic flow from Iceland.  So peak temps are probably reached before the end of this week.

Further ahead several models suggest a breakdown mid-September to a more unsettled regime.  The CFS temperature chart shows a rather sudden decline Mid-Sept and then again in October.  Whilst  this is not surprising for the time of year, such steps in the CFS can indicate frontal systems and depressions as different air masses arrive across the UK.  Warmer than average Atlantic sea surface temperatures, in some places nearly 5c above normal, is likely to encourage the formation of active depressions as is usual for Autumn.