#Reigate this week: Atlantic block holds the balance of shower

February 1, 2015 — Leave a comment

Is this it for winter? Read on…

Reigate stays cool for the first week of February after a couple of modest wet snow non-events overnight this weekend.  Snow thawed in the morning each day as temperatures climbed above freezing.  Whilst the air temperature was comparatively high at 5.8C Tmax at midday, the stiff northerly wind in Reigate was gusting to 30mph in town which brought wind chill as low as -5C at times.

The cause of the current cold weather is a northerly wind set up due to HIGH pressure ridging north in the Atlantic and a LOW over Scandinavia. This sets up a northerly flow, called an Arctic airmass, albeit modified by its significant journey over relatively mild seas.  Also, this particular Arctic airmass is not direct from the Pole, if you follow the isobars back from the UK you can see the air originating from southerly areas in SE Europe, so not truly polar.  In any case, it’s usual for Arctic airmasses to bring dry weather to Reigate and the SE: the long transit over land means it lacks moisture, usually dumping any significant snow over NE facing coasts well before it gets here.  More locally, our sheltered location beneath the North Downs, facing south, also affords good protection from Arctic airmasses and N or NE winds.  So, either way, Reigate rarely gets lots of snow from this airmass.

2015-02-01_17-28-01

arctic airmass

This week is likely to see further cold weather continuing as the HIGH slowly nudges in from the west by mid-week.  With HIGH pressure not far away and a relatively dry northerly airmass, a major snow event or indeed much precipitation at all is unlikely.

2015-02-01_17-27-26

So as pressure rises we can expect a cool mostly dry week with frosty nights and possible fog on occasions in lighter winds (fog not really ’til end of week though as wind remains significant running round high). In this set up a big snow event for Reigate seems most unlikely.  Nevertheless, an easterly / NE’rly wind for a time is a strong possibility, initially Tues-Weds as a front moves south, and so modestly disruptive snow showers reaching us cannot be ruled out.  Also, don’t forget icy roads and fog are arguably the most risky of all winter hazards so this kind of high pressure wintry weather should be handled with care if travelling.

At the moment  GFS, Ensemble and ECM models are agreeing that the HIGH pressure is likely to land up sitting somewhere to the north / NW of the UK by the end of the week.  With unusually LOW pressure in the south of Europe, this could set up a cold easterly wind especially across the south east, albeit this might yet not come off as other runs show the HIGH right over the top of the UK cutting off any easterlies very quickly.

The exact position of the HIGH will therefore make a big difference to whether we get much or any precipitation.  NE’rly or easterly winds, depending on their strength and track, can bring snow off the North Sea and inland into Kent and Surrey as well.  There is already a MetOffice yellow weather warning for the possibility of such an event mid-week, although these often don’t come to much they should not be ignored as the potential for perky snow showers causing local traffic problems has already been experienced with one minor brief thundery shower wintry episode last week.

The time-averaged charts show the overall story for the next week as being dry and colder than normal.  Throughout this episode the jetstream is wrapping round to the north of the HIGH, actually building it with milder upper air from the SW, and, eventually, it could help to deliver our easterlies at the end of the week.  As usual, for exact details check the twitter feed and consult official sites like metoffice for decision making.

refs

http://www.metlink.org/wp-content/uploads/2015/01/arctic_maritime_jan15.pdf

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