Archives For high pressure

 

The very cold upper atmospheric pool of air which put Reigate in the freezer this half term (-11ºC at 1500m, -35ºC at 5000m!) is moving off South… to be replaced by warmer air aloft moving in from the North (-1°C at 1500, -25ºC at 5000m).  Watch the video below to see the warm air moving in from the north and pushing the cold air south.  This is rather upside-down to what we would normally expect and is being caused by warm air being squeezed through by an active jetstream in the Atlantic: this is feeding warmer air into the HIGH pressure sitting atop the UK currently.  Reigate weather will be dominated by the HIGH pressure during this week making things mostly dry but, as we are located on the edge of the high pressure weak fronts will be able to influence our weather as they nudge across the Channel early in the week and bring light snow turning to drizzle through Monday and maybe more light rain on Tuesday. Temperatures remain uninspiring at 5°C.


This week for Reigate: dry and increasingly cold!  Watch the video which shows cold air (at 1500 metres) flooding in from the east.

Wind direction is so important in controlling our winter and early spring weather: many of the “source regions” (where our winds originate from) are at their most extreme at this time of year: the continental interiors, for example, are at their coldest and driest in late winter, so any air mass coming from the continent will be very cold indeed, despite Spring being just round the corner.  These cold and dry easterly winds contrast with our usual warm and wet SW Tropical winds that arrive from the Atlantic.  This week it is the turn of Easterly winds and HIGH pressure to dominate our weather: this means cold and dry.
UK unusual dry patchThe HIGH to the north east and the LOW to the south is a perfect set-up for dry and icy Easterly winds to flood across the South East, arriving from a very cold Russia on Tuesday and strengthening through to the weekend.  It should stay dry with a low risk of snow flurries reaching Reigate.   Actual max temperatures will be +1 or 2°C during the day but the 15 mph wind will make it feel very chilly at -3°C in the daytime and, during the night, as low as -8°C wind chill.  Cloud cover will control how low air temperatures actually fall at night: clear skies could see temperatures drop to -4°C with frosty mornings.  Want to warm up?  Get to Reykjavik in Iceland which will be the warmest city in Northern Europe this week with unseasonably high temperatures of +8°C with a “warm” and wet southerly Tropical wind.  Next week looks warmer and staying dry for Reigate with temperatures up to 10°C. An early taste of Spring perhaps?

&nbsp

The weather during half term looks drier and more settled for Reigate but temperatures could still bite back with COLD returning from mid-week onwards.  A HIGH pressure ridge is forecast to build near to the south of the country this weekend and then develop over Europe to the east where it should more or less block Atlantic westerlies which bring our usual wet weather.  The air flow over Reigate will be more from the south early in the week and then swing south easterly and even easterly from mid-week.  Milder temperatures are due to arrive in the coming days and reach 8 or 9ºC during the day, which might even feel spring-like early next week!  Temperatures will drop quite sharply at night and low enough for frost on any clear nights.  However, the position of the HIGH becomes important from mid-week onwards and here the models vary a lot: one model moves it north, with a LOW forming over Europe; if this happens then be prepared for much colder weather later in the week, possibly even snow showers for the east coast (see post below and map ECMWF) as cold easterlies make a return.

Happy New year! 2013 dawned bright and beautiful over Reigate for January 1st 2013 as the low pressure of recent weeks gradually gives way to an anticyclone (High pressure) building from the south west which will bring calmer and drier weather for us in Reigate for a week or so, after a final shower or two tomorrow.  It may not be that sunny though: anticyclonic gloom is common in winter.  Also, check the chart below as temperatures are due to take a slide (predictably for January!) from the weekend.

London weather January 2013

satellite photo UK new years day January 2013

A snap-shot of weather today reveals how diverse and extreme and fascinating our European weather actually is! The east – west split is obvious: freezing temperatures on the continent with everywhere east of Germany below freezing. To the west we are bathed in warm balmy Atlantic air which is over 10°C. Northern Italy is enjoying almost spring-like weather in the Po Basin, possibly due to air sinking and warming from the Alps.

Meanwhile, Iceland is due to be hammered by hurricane force winds over 76mph (>34m/s)with an intensifying LOW pressure bringing extremely high winds to the NW of the country on Saturday. This LOW pressure will have a central pressure of 942mb: which is extraordinarily low.  Winds blow from HIGH to LOW pressure (albeit deflected to the right by the coriolis force – more later on this).  The bigger the difference between HIGH and LOW pressure, the higher the winds.  See the weather map below and spot the isobars placed close together over NW Iceland: this indicates ferocious Force 12 winds.  The UK will be breezy, located on the edge of this LOW, but no where near as windy as Iceland, fortunately. Heads-up for a “heat-wave” in the UK next week :-)!!

intensifying LOW near Iceland

This blocking High pressure out to the West of the UK will be developing next week – certainly by Tuesday Nov 27: it is called an Omega Block because the High pressure takes on the shape of the Greek letter (outlined in red) and is bounded by deep LOWS on both sides – notably one to the South of the UK.  This will open the door to cold northerly and north easterly winds across the UK – snow watchers be aware – a cold snap, but still not a Daily Mail “Arctic freeze”! another view of the powerful northerly jet stream bringing colder air to the UK next week:

http://cdn.nwstatic.co.uk/gfsimages/gfs.20121120/18/162/hgt300.png