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2015-03-12_21-10-33

February 2015 Reigate weather summary

 

Weather statistics summary for Reigate during February 2015

Average temperature 3.9C (UK 3.5)

Tmax 10.8C 25/02

Tmin -3C 22/02

Total precipitation 59mm

max wind gust 32mph 06/02

February was around the long term average at 4C in Reigate.  The CET stood February at just 0.3C above the long term average.

After an intially cold start to the month the temperature rose.

Overall February came out around average temperature but note the much warmer continent.

2015-03-12_21-58-44

A notable cirrostratus halo occured around moon on 02 Feb and this accompanied by some brief excitement over a snow band moving south down the eastern side of the country.

This band delivered an insignificant snow flurry on 03 Feb, overnight, with  a cm or so of briefly lying snow that melted rapidly during the morning.

Models threatened easterly winds on occasion but this didn’t arise.  In any case Europe and the continent experienced a warmer than average month so the deep continental cold was not available.  In fact after another brief flirt with snow at the beginning of the month, the temperature climbed through the middle of the month with westerly influences with temperatures exceeding 10C overnight on occasions.

Rainfall 58mm was just a little below average and sunshine hours, at 90 hours, was about average.

 

 

 

MetOffice February summary

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

 

Update: confirmed tornadoes today (see foot!)

Potentially interesting, albeit tricky, weather tomorrow for UK, Wednesday. A deep surface LOW sits to the SW of Ireland and is dragging through complex series of fronts associated with various airmasses.  Occluded fronts are tightly wrapped around the LOW, which is due to migrate NE away from the UK during the weekend and pressure to rise.  For the SE tomorrow it’s not so much the fronts but an unstable mass of warm southerly surface air that will be the main cause of any heavy showers tomorrow and some potentially thundery weather, especially when this warm air is forced up by anything… coast, hills or fronts.

Here’s likely scenario for us in Reigate, Surrey SE: Rain is likely, possibly exceeding 10mm, which is fairly wet for SE: higher is possible.  Most of this is likely to be convective rainfall due to unstable and moist air moving in from the south overnight.  Showers, possibly heavy and thundery, are most likely in the morning as the warm surface air moves into our area, causing lapse rates to increase moderately and this encourages lift and cumuliform clouds.  If the sun comes out then surface heating could spark heavy showers and thunderstorms as warm air rises freely through the atmosphere, encouraged by the jetstream overhead that effectively drags air off the ground.  One of the ingredients for thunderstorms, LIFT, is therefore partly in place tomorrow, although it will depend on sunshine for greatest effect.  If it stays overcast, which is possible, then little exciting weather action beyond just rain is likely.

The above charts also show that the southerly / SW airstream is humid because, as it converges on the coast, the model shows rainfall increasing significantly. This increased rainfall on coasts is often caused by convergence which is due to air arriving onto the coast quicker than it is leaving (check the lower wind speeds inland) so the wind effectively PILES UP on the coast and is forced to rise as it has nowhere else to go except UP!  This is called convergence. It is clear from the charts above that any HILLS also encourage lift as South Wales and even the South and North Downs appear to be pushing rainfall totals up locally: this is orographic or relief rainfall.  The charts below shows another feature of the weather tomorrow: the winds are shown to be VEERING with height (left diag) which allows WARM air to advect (move into) into our region (right diag).  Read on for more about how veering winds and WARM AIR ADVECTION can encourage stormy weather.

 

Another ingredient for potentially unstable weather is that winds are VEERING tomorrow, albeit not dramatically, which means they are rotating clockwise directionally with height thus allowing warmer air from the south to move into a location: it is like opening the door to warm air: winds move through a southerly direction and therefore allow warm air to “advect” into our area.  A moderate wind veer is taking place overnight and into tomorrow morning.  Warmer air at the surface is overrun by cooler westerlies aloft that increases lapse rates: steepens the temperature difference between surface and air at altitude.  The air at altitude tends to stay the same temperature and is associated more with direction and origin of airmass than it is with surface heating or advection of warmer air at the surface. An increase in lapse rates adds to instability which encourages air parcels to LIFT off the ground, should surface heating occur if the sun comes out.  The chart above right shows how WARM air is ADVECTED into the South of the UK and migrates NORTH during the day.  This has nothing to do with solar heating… it is an 850hPa chart (1500m) and shows the airmass temperature which is largely independent of surface influences.  It’s a good example of WARM AIR ADVECTION with a moist air stream increasing instability causing showers and possible thunderstorms.

Finally, the warm relatively unstable airmass is being overridden by a NE turning jetstream that will encourages lift and wind shear.  Wind shear is the vertical change of direction and/or speed with height: rotation.  Shear is moderate tomorrow which might also add a twist to rising air that could even produce the odd tornado.  Nice 🙂  After writing this Estofex issued a tornado warning Level 1.

Update 8 Oct: confirmed tornados from today 8 October 2014

http://www.bbc.co.uk/news/uk-england-derbyshire-29542740

 

Quick resume of some wet weather approaching on Wednesday for the whole of the UK. The current cool pool is being nudged out of the way East on Tuesday by a vigorous return of an Atlantic jet stream blowing across the UK W-E during Wednesday and ushering a wedge of Tropical air.  As the warm humid Tropical air from the SW meets the cool Polar air the warmer less dense tropical air will be forced to rise, cool and condense over a wide area and produce persistent and occasionally heavy frontal rain. This is a classic active warm front and occluding from the NW.  The warm sector will brush across the SE during the day raising temps from 8c to 15c during the afternoon despite the rain! Rain totals for the SE may approach 10mm for the day but the heaviest rain will fall further NW along this extensive warm occluding front.  

warm front

A point to note is that, despite the classic text-book mid latitiude depression having a warm and cold front, it is usual for one front or the other to be more vigorous.  In this case, it seems the warm front is the more active, although the front is occluding as the cold fronts catches up the warm and lifts the warm sector up.

ana front

On Thursday, a much drier and more pleasant day, any sunshine could raise Tmax to near 20c in Reigate! By that stage despite being a relatively warm W/SW flow, the air mass is probably more accurately described as a modified polar maritime, coming in behind the occlusion, nevertheless, the SE will sit in warmer air than of late.

Here is an animation, courtesy of NMM netweather that shows the transition from cool polar pool to warmer SW flow in the next few days.

Quick update for Friday and Saturday:

The Arctic pool of air brought down by the Northerlies through Thursday is due to develop into a cool pool of LOW pressure over the continent which will linger and flirt with the South East of the UK for this weekend. While the North and West basks under high pressure, the South is due for some cool brisk winds and some persistent rain at times this weekend: wind then rain on Friday, rain on Saturday. This will blow the cobwebs away (and some leaves!) after the balmy start to October. Significant rainfall total over 20mm on places.

se cool wash out

The wind starts first as a brisk cool Northerly on Thursday and Friday morning and then swinging round to a cool North Easterly during Friday with gales possible in the Channel and North Sea with 30mph+ gusts for Reigate. Friday pm will see the rain setting in, possibly quite late into the afternoon for Reigate. The rain is associated with complex fronts wrapped around the LOW pressure moving unusually SW across Northern France from the Netherlands. Once the rain arrives on Friday, it is likely to remain persistent until well into Saturday. Some significant totals are possible with moderate intensities on Saturday morning.  With the wind it will feel distinctly cool and autumnal compared to last weekend when 20c was exceeded in Reigate.

A Nor’easter is actually a storm off the coast of New York / Eastern USA which brings in cold blast of NE winds on the back of a depression circulation. This is like a mini-nor’easter but is tracking the wrong way.