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Amazing clear blue skies this weekend in Reigate! Subtle changes at first this week, then more significant deterioration by the end… probably 😉

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HIGH pressure stays in charge through the first half of this week but with more cloud building and a few scattered showers possible.  We are on the cool side of the HIGH with an airflow from N/NE and occasionally brisk cool winds on the coast circulating round the HIGH.

HIGH pressure UK early June

HIGH pressure UK early June

Easterlies / NE winds are due to build on occasions this week especially on the south coast as super-geostrophic wind circulating round the HIGH draws cool air from the North East.  Cool nights are likely in a generally cool airmass when skies are clear of cloud and heat escapes.

cool nights.. frost up north?

cool nights.. frost up north?

Could even be a touch of frost up north on first few nights early this week.

The thickness chart above shows how the east side of the HIGH has a less “thick” airmass which is cooler as measured between 1000-500mb height). This means that, despite the overall high pressure, surface warming during the day can increase lapse rates lifting thermals into the cool air which can spark scattered showers as land warms through.  There have been big thunderstorms in Europe along the front separating this cooler airmass from the building heat of the Med.

Later in the week warmer thicker airmass is due to fold round the top of the HIGH but this might bring more cloud.  Things are on the change by the end of the week as a LOW forms in Biscay and threatens to edge north / NE to bring possibly wet weather to the SE by Friday.  The MetOffice fax chart shows a triple point crossing into the South by Friday and these can yield a lot of rain. Spanish Plume potential brewing … (updated Tues)

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metoffice fax chart plume

Whilst this is unlikely to be a full-on classic plume plume (update Tues… yes it could be best for some time!) it could sweep unstable air into the south and possibly cause thunderstorms. Heavy rain is possible too.

Thereafter the scene into the weekend looks more unsettled as a trough over the UK replaces the HIGH as it regresses (moves west) into the Atlantic. An Atlantic LOW from the NW looks on the cards for the weekend while pressure remains low. June has started cool (especially due to cool nights) and will continue to be overall cooler than average for the next 10 days. Note the change visible on the charts below.

There is general agreement for this deterioration amongst models.  So this week is likely to see a change to less agreeable conditions: lower pressure and a bit more rain at times and staying rather cool for summer!

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model agreement

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March 2015 Reigate summary

Reigate March 2015 weather summary

  • Average Temperature 6.9C
  • Tmax 14.4C
  • Tmin -1.9C
  • total rainfall 23.8mm
  • max wind gust 46mph
  • sunshine 143.2 hours

Anticyclonic conditions controlled a good chunk of March bringing a lot of dry weather to Reigate.  Total Reigate March rainfall of 23.8mm is around half of that expected from the long term average for March recorded since 1873.  Whilst March rainfall this year was low it was not outside the “normal range” with mean monthly rainfall for March in South East England being 49.8mm and the mode (most frequently occurring) at a relatively dry 38.7mm.  March 2015 turns out to be the 36th driest year since 1873, the lowest being 1929 with a paltry 2mm of rainfall.  So March 2015 was dry but not super-dry!

With high pressure around March was sunny and not terribly windy, especially through the middle month.  Mid-Month the temperature dipped notably (see top chart) as a slack easterly set up with the high moving over Scandinavia.  It was not a beast from the east because the continent was warm and there was no really cold air available.  The slack conditions finished at the end of March, however, as a significant NW gale blew across the UK.  This broke our daily wind run record at 305.4 miles.  Wind run is the “fetch” or distance that the wind has travelled passing a point during the day.  Imagine a balloon drifting in the wind during the day… how far would it travel? This is the wind run.  The NW wind also turned lorries over on motorways further north, gusting over 70mph in places.  In Reigate the max gust was 46mph, pretty strong but with no leaves on the trees it did not cause damage.

143.2 hours of sunshine in Reigate exceeded the long term average.

March will be remembered for the “deep partial” solar eclipse that crossed much of the UK on 20 March.  Sadly for us in Reigate and much of the SE we saw nothing of it except a gathering gloom and slight dip in temperature.  More on our eclipse observations here: https://rgsweather.com/2015/03/21/reigate-some-eclipse-effects-on-weather/

2015 overall is above the long term CET average but not by much. Nevertheless, as a moderate El Nino is set to start properly this summer it is likely that 2015 will be break more temperature records globally as a hot year overall for the planet.

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All our weather data can be downloaded from the data page here.

MetOffice March summary 

http://www.metoffice.gov.uk/climate/uk/summaries

http://www.metoffice.gov.uk/climate/uk/summaries/2015/march

Despite the gloomy stratocumulus which didn’t shift much this weekend, it has been warmer than usual.  Saturday night was especially balmy with Tmin of 16c and today saw Tmax a shade off 19c.  This week warm Tropical upper air will continue to waft up from the South being generated by a deepening TROUGH of LOW pressure spinning up just NE of the Azores. This will continue to pump a warmer than usual plume of upper air at +13c 850hPa (5000ft, the height air mass characteristics are usually measured) which flirts off and on with the south of the UK almost all week. The result is warmer than average September temps by some +4 ot 5c.  It will, however, gradually get edge away as the week progresses.

A weakening HIGH pressure over the UK will gradually melt away and drift over the continent and allow cooler air into Northern Britain. Reigate will remain in the warmer air most of the week by the looks of things.
Most models agree that the Azores LOW will fill gradually and drift NE over Spain and France during the latter half of the week and bring a nasty rash of showers to those areas. These might encroach on Southern England by the end of the week and weekend bringing potential for thundery outbreaks but this is too far off to be certain.
The distribution of rainfall is somewhat unusual, with a large accumulation of rainfall accumulating in a great spiral to the west of Spain associated with the Azores “Low” and lots of rain in Eastern Europe associated with the large Russian LOW. Note that the SE of England and Reigate in particular might just be one of the driest places in Europe by the end of next weekend as we seem to be sandwiched between these two great weather systems… well, models permitting! *one of driest places in Europe, probably! The GFS shows no rain at all for Reigate this week, but that’s quite a long shot! 

Another observation to match the LOW over the Azores is the negative forecast for the NAO: the North Atlantic Oscillation measure.  The NAO shows the pressure difference between Iceland and the Azores: when NAO is positive there is a big difference / pressure gradient and a strong jetstream and unsettled, fast moving weather across the Atlantic.  When negative, like this week, there is a meridional jetstream, a weak flow west to east and weather systems can get stuck… so not much change is expected.  This matches the forecast this week… not much change and no fast moving weather.

negative NAO sept

The ensemble model below shows that there will be a gradual decline in temperature during the course of next week and certainly beyond. More unsettled weather is also likely to arrive sometime next weekend but models still look rather uncertain so… keep checking back!

gefs 22 sept

The HIGH pressure now building over the UK looks like persisting for at least a week and possibly for longer with a breakdown only hinted at from 15 July or later on current model runs. High pressure brings warm, dry weather with mostly clear skies as air sinks from aloft and warms and dries out as it does so. High pressure has pushed the jetstream (which guides LOW pressure storms at the surface) well north towards Iceland. So GREAT summer weather for Reigate and the UK over the next week and beyond!

Temperatures will mostly be in the high 20’s and could climb to 30ºC locally. Night time temperatures are also going to remain well in double figures for most of this period.
One potential spoiler for east and SE England could be the build up of brisk NE and E winds swinging round the edge of the HIGH as it continues to build in pressure during next week and slips north. The forecast Northerly / NE winds running south down the North Sea and swinging east into the Channel could reach 20mph and inland could be 10-15mph (supergeostrophic = surprisingly breezy in high pressure as winds swing out round the pressure gradient). The North Sea is still only around 13C sea surface temp and this will have a cooling effect on the breeze and east coast areas especially.  This wind might even pick up some low cloud and mist for the east coast. How far inland this effect reaches remains uncertain but most likely the cooling will be restricted to the immediate coast. So, warm everywhere but west is certainly best!

SE heat 7 JulyUpdate: scorchio for Sunday but thereafter in SE and Reigate it will be pleasant and “normally” summery and warm.  The NE breeze will take edge of scorching temps in SE and especially near coasts.  The NE may also bring cloud which will further peg back temps to a more normal feel at mid-20’s rather than high 20’s. 

monday 8 july

HIGH pressure is set to dominate the weather for Reigate most of this week and, quite possibly for much of May, with a few significantly wet interludes caused by a “cut-off” low early next week, possibly. This week, however, we have little or no rain forecast and pleasant spring sunshine raising daytime temperatures to 15ºC or more. Night time minimums could still fall to freezing to produce pockets of frost on some clear nights. North and North Easterly winds will prevent the temperatures reaching too high but winds should remain light. So.. very pleasant weather to come!

An interesting aside to all this nice weather is what is happening elsewhere: a feature called a “cut-off” LOW is causing cold and very wet weather over Spain and parts of France and thunderstorms over the Mediterranean.  This feature was born at the weekend when the cool plunge of showery weather over the UK (an upper trough) reached Spain but was “cut-off” by the jetstream slicing through South East England and cutting the neck off the trough.  This cut-off LOW will yo-yo back towards southern UK later this week (check the rain in France on the map) but is NOT forecast to quite get here… except possibly brush Sussex with rain. It will, however, bring some heavy rain across France. Cut-off LOWS are interesting because they barely feature on surface pressure charts but can cause utterly miserable weather nevertheless. So, whilst we enjoy some wonderful weather think of the Mediterranean and much of continental Europe in this LOW pressure regime courtesy, to some extent, of the action over Surrey with the jetstream last weekend!

Bank Holiday looks better and better: once light rain on Saturday has passed (a weak cold front) the risk of any showers reduces to almost zero through sunday and monday! the rain is staying firmly to the north of the UK and in France.  Bank Hol weather looks very pleasant with temperatures over 20ºC in Reigate.

 

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?

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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