Archives For May

2015-06-07_11-16-50

May weather summary Reigate Surrey SE England

May 2015 summary data for Reigate

  • Average temp 12.1C
  • Tmax 20.7C
  • Tmin 4.1C
  • Total rainfall 62.7mm (CoCoRahs manual rain guage)
  • average wind speed 21mph
  • Max gust 38mph
  • total sunshine 161.2 hours

At 12C the May average temperature in Reigate was a tad cooler than long term average, as it was for the whole of the UK at a mere 9.6C, (0.4C below the long term average for the SE).  May was the first month since August 2014 to be below CET long term average. The chart below shows the mean monthly 500mb geopotential height for May and shows a big LOW to the NW of the UK.  This pattern would generally cause a cooler than average W/NW flow across the UK, especially the northern part of the country.

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May mean monthly 500mb geopotential height reanalysis

The culprit causing the slightly cool / unsettled May was a LOW pressure in the Atlantic to the N/NW of the UK that caused a NW airflow which also reduced sunshine hours below average, at 161 hours.

May rainfall in Reigate was 53mm.  Frontal rainfall from occluding Atlantic LOWS brought rain early in the month when it was quite windy too, with max gusts over 35mph on May 6-7.  Sunshine totals were low during the first half of the month but picked up mid month and later.

A trough disruption May 13-15 caused a particularly wet 24 hours 13-14 May.  Trough disruptions are notorious for causing unpredictable weather events, including causing poor model forecasts for rainfall.   The charts below show the trough disruption progress.

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trough disruption May 14 2015

The satellite picture below shows the LOW resulting from the disrupted trough and the band of cloud associated with the fronts.  In Reigate this single event caused the highest daily rainfall total for May, at over 15mm (CoCoRahs manual rain gauge).

occluding LOW May 2015

occluding LOW from trough disruption 14 May 2015

A thundery event occurred on 19 May and brought heavy showers across the SE.  Here is the synoptic chart for that occasion.

Some impressive mammatus clouds were spotted locally as the cumulonimbus clouds moved away.

More details here from MetOffice

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

Reigate weather in May 2014: summary

  • Tmax 26.3c
  • Tmin 1.5c
  • Total Precipitation 69.2mm
  • Total sunshine hours 150.5 hours

May was a month with some convective interest but no explosive thunderstorms for Reigate despite one or two possible moments with CAPE values exceeding 1000 kj.  The rapid in-situ development of an impressive Cumulonimbus over the town on a southerly flow on 22 May proved to be interesting for cloud watchers and delivered a well structured gust front but only one crack of thunder and some fleeting cg lightning.

Elsewhere in Europe record breaking rainfall over the Balkans caused tremendous floods and there were numerous funnel clouds and spouts from France through to Italy in unusually convective conditions.

Here are some thundery pics of the synoptic situation in late May.

More pics showing the synoptic situation during thundery epsiodes across the UK and Europe.

After a good start with temperatures climbing to 22ºC in the first week, May 2013 was dominated by the UK being stuck in a TROUGH (low pressure) of relatively cool air producing occasionally convective heavy showers and unsettled weather. We called it a “cold rinse cycle” and, nationally, it was the coldest May since 1996. Whilst Reigate escaped the worst of this, some parts of North and West and NE of the UK suffered downpours and thundery showers.

meridional flow jetstream

A meridional jetstream flowing north-south fed cool upper air down to Europe from Polar regions making the UK and France rather cool while Eastern Europe (on the northward limb of the jet) was bathed in warm air from the Med: heat wave! The other effect of a meridional jetstream is that weather systems get stuck and don’t move. So it turned out that the trough over Europe did indeed not budge far for the whole of May. With HIGH pressure shifting north this week, the Euro-trough continues to linger to the south of the UK for the start of June. The persistent LOW pressure over Eastern Europe has caused storms and extensive flooding in Czech republic and Germany, while the warm northward limb of the jetstream has caused a heatwave further north in the Arctic (Finland reached >30C!).

 

may air pressure 2013

The trough of low pressure over Europe was persistent and air pressure in the SE of the UK stayed pretty low throughout mid-May.  The northerly jet stream pegged back temperatures too, allowing strong May sunshine to kick off showers.As you’d expect, there was a good correlation between lowest pressure and episodes of heaviest rainfall. Nevertheless, periods of showery activity were very tricky to forecast and the GFS model seemed to over-do the rainfall over SE England on several occasions, seemingly not at ease with an easterly flow. Nationally, Spring 2013 has been the coldest for 50 years.

Reigate summary stats:

  • Highest temp 22.7ºC on 7 May
  • Lowest temp 1.7ºC on 3 May
  • Av temp 10.8ºC
  • Rainy days 16
  • Total rainfall 65mm of which 14.5mm fell on 28 May

Interestingly, June seems to be starting in a similar way with HIGH pressure drifting to the North and a trough sticking over Europe… time will tell as to how unsettled it gets after this first week of super June weather!

Find full May stats on data page

Also, visit our page on MetOffice for some interactive graphs

A dry start to May but how long will it hang on?!

Before Saturday’s shower, it last rained in Reigate 5 days ago but rain is forecast this week.  Nevertheless, the first 10 days of May are due to be below average rainfall (see map below), despite wet weather mid-week.  Much of the rainfall in Reigate during early May is likely to be showery, some heavy but rather short duration so not amounting to much.

europe wet and dry may

May Bank Holiday in Reigate is likely to be a pleasant 20ºC, but not quite the sizzling 26ºC claimed in some newspapers! The warm weather we have been enjoying has been courtesy of an upper level ridge to the south pumping a warm air mass from a warm source region to the south west around the Azores. This warm upper air has built a surface HIGH pressure to the south of the UK which has kept us largely dry, warm and cloud free.  The tropical airmass this week has had temperatures at 1500m (850hPA) of +10C over Reigate; (quick reminder that in March we had uppers of -12ºC!). This week could see upper air mass temperatures drop to nearer 2ºC over Reigate by mid-week. Surface temperatures will still be OK, reaching mid-teens in the stronger May sunhsine but night time temperatures could be quite chilly and even risk a touch of frost if skies clear later in the week when a HIGH is due to build through again at the end of the week. High pressure will build back in the south and, though a long way off to be certain, next weekend looks staying dry with temperatures probably recovering nicely in any sunshine.

The cause of the forecast #slightly# cooler weather arriving in Reigate this week is a possible significant switch in the airflow from mostly southerly winds to a more westerly/north westerly wind direction: a cooler direction across a cool Atlantic ocean and from a cooler source region around Iceland. The cause of the change in the wind direction is an Atlantic LOW forecast to cross the UK mid-week. This LOW will bring Atlantic fronts and rain, possibly heavy at times mid-week, before clearing to showers.  A HIGH is forecast to build back into the south by next weekend bringing dry weather back and temperatures back to possibly hit 20ºC again. As usual, this is an early forecast and things will change so do keep in touch on twitter @RGSweather.

high and low pressure cross section

Why is HIGH pressure usually dry and cloud free?  Air sinks in HIGH pressure: as it sinks it warms and dries out. Warm air can contain more water vapour than cool air, so any moisture tends to evaporate in HIGH pressure (anticyclones).

In LOW pressure (cyclones / depressions) air is LIFTED and expands, cools and any water vapour will condense, forming clouds and eventually rain.

Air flows from HIGH to LOW (but not straight! it is deflected to the right in the northern hemisphere by the coriolis force… more on this later).  This gives us the familiar surface pressure charts with winds apparently spiraling round HIGHS and LOWS.

air flows from high to low!