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2015-05-11_10-22-47

April 2015 Reigate weather chart

April in Reigate was a mostly dry, very sunny, rather calm and relatively warm month.  However, the settled weather came at a price with a significant air pollution event on 10 April. Also, April showers were conspicuous by their almost complete absence so convective weather fans were left disappointed.

Here are some April details for Reigate:

  • Tmax 14.4C
  • Tmin -1.9C
  • Average Temperature 6.8C
  • Total rainfall 23mm
  • Max gust 46mph
  • sunshine total 136 hours (sunniest April on MetOffice records!)

April was 43% drier than average across SE England and this shows up in the diminutive 23mm of rain in Reigate.  Pressure rose early in the month and so April turned out 150% sunnier and about 1C warmer than average too. The central England temperature for April came out at 1.1C above long term average.

2015-05-11_10-50-48

It was the SUNNIEST April on record for the UK according to the MetOffice.  In Reigate we enjoyed 136 hours of sunshine in April.

sunny April 2015

sunny April 2015

The significant downside of this calm weather was high AIR POLLUTION. There was a high pollution warning for a time when easterly / south easterly winds brought high levels of PM2 particles into the SE and across London from Benelux countries.

Here are some photos from Reigate during April showing what a mostly very pleasant month it was.

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

There is some interesting weather potential this week, but indications that the SE will see least of any drama which will be mostly further west. The well-established warm plume of S/SE wind from Africa/ Mediterranean and Spain has brought temps up to 19.4c in Reigate this weekend and 20c in London.  A warm sunny Saturday was especially pleasant.  The breezy S/SE wind is bringing Saharan dust falling over the UK, watch for this in any showers that might come our way on Monday.  Check your car for any dust.

Warm plumes from Spain can also introduce unstable moist air and these produce thunderstorms and showers when moist warm air converges or fronts  undercut the plume with Atlantic air creating lift. There is good potential for heavy convective downpours of rain this week due to these scenarios.  Whilst not necessarily a classic Spanish Plume the synoptic situation is very similar to May 1998 when large super-cell thunderstorms drifted to the North and caused torrential rain and flooding in parts of the North.

2014-03-30_18-37-25

Various indicators are used to establish the potential for heavy convectional rain and thunderstorms.  At times the charts show several of these indicators at unsually high levels for end of March / early April this week.  CAPE (convective available potential energy), LI (lifted index) and ThetaE (potential equivalent temperature) … these are all technical charts that are commonly used to assess how likely thunderstorms will be.  ThetaE rarely goes above 19c in the UK so temps of +12c in April are unusual so early. Unfortunately, the development of convective rainfall and thunderstorms is difficult for forecasters to predict for any one location for a particular time.  Thunderstorms and showers are, by nature, hit and miss affairs: one place might get large hail and a deluge while, a mile down the road might remain sunny and dry.  Nevertheless, watch out for some potentially heavy rain, especially if travelling this week.  Watch out for interesting cumulonimbus clouds too!

Total rainfall remains highest in the west and away from the SE where pressure remains higher and frontal action is more limited.  So Reigate may escape the worst of all this convective rain action but still worth keeping an eye out for rogue storms that may well come our way drifting most likely from storms in the Channel around mid-week.

Finally, models are hinting at high pressure building back in for the school holidays after this wet unsettled week. Whilst not yet represented on charts very convincingly, the models suggest a possibility of some reasonable holiday weather in the UK with pressure rising and temps above the 30 year average on the GFS ensemble mean.

 

 

 

Update: BIG cool off likely from mid-week as high slips west and draws down COOL Northerlies / NE polar winds for E UK: Tmax drop from 19c to 10c by mid week! Matches Northern blocking suggested by NAO (see text below) Likely to bring first snow cover for continental Europe. ( check @RGSweather)

cool off mid week

The movement of this high has changed in latest models – update coming later:

Rainfall, or lack of it, is the theme for next week in / update: cool off is the theme now for Eastern UK, SE and in Reigate. Warm upper air flowing from the South and SW is building a blocking HIGH pressure which will sit to the South of the UK and then migrate WEST keeping rainfall and LOW pressure way out west in the Atlantic for much of the coming week. Tmax temps during the day could top 20c and night time temps could even stay in double figures.  Given these high nighttime figures and low dew points it is unlikely that fog will be much of problem with low humidity in the SE, unless the cloud clears at night and ground temps fall away. Is this an Indian Summer? Read here. http://www.bbc.co.uk/news/magazine-15127159

The NAO (north atlantic oscillation) is a measure of pressure difference between the Azores and Iceland.  If the pressure difference is high then it yields a positive NAO and this usually accelerates westerly winds and the jetstream and brings fast moving wet and warm (winter) Atlantic weather to the UK.  Negative NAO usually indicates a weak pressure difference, weak westerly winds and a blocking pattern with higher pressure than usual and slow blocked weather which sticks around for ages.  The weaker westerlies in mid-winter can allow cold polar air to flow out of the Arctic depending on the location of the HIGH. Slow moving conditions can mean extreme weather if a HIGH causes dry weather for weeks (drought) or a static slow moving LOW accumulates a lot of rainfall.  So negative NAO is not necessarily “good” weather.  For us, at present, we have a very negative NAO, one of the most negative NAO’s of this year but the ridge is bringing in a warm flow from the continent.

The longer range CFS chart shows a major gear change down to cooler conditions later in October. To be expected of course, but it looks rather a harsh step function in the weather.  It’s long range so things can change.
Enjoy the warm dry weather and Autumn sunshine but think of the USA today where Winter storm Atlas is bringing blizzards to S Dakota and Wyoming, Tropical Storm Karen is making landfall this weekend over Louisiana and Mississippi and, in between, there are tornado warnings in Iowa, Kansas and Nebraska. Update: severe tornadoes breaking out near Sioux City, NE Nebraska. Thoughts to people there. Stay safe.

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

 

Hurricane Humberto: a hurricane which left Cape Verde on 8 Sept and has been hovering in the Tropics ever since. Early models suggested a direct hit with the UK sometime next weekend which would have been wet and breezy. Latest charts show HUmberto gliding safely north of the UK. The GOOD thing about this is that Humberto will bring a great deal of Tropical warmth with him, especially on the south side. The heat embedded within the Humberto circulation will push warm upper winds from the Azores area north and build an upper ridge and surface HIGH pressure over the UK from the end of the week and hopefully over the weekend.  Warm air moving north aloft lifts air pressure through the whole warming column of the atmosphere and the result is increased surface pressure: an anticyclone (opposite of this weekend!).

Humberto is currently just a tropical depression having lost energy over a cooler part of the mid-Atlantic.  However, he is due to intensify when he meets the mid-latitude westerly circulation around the end of the week and entering the jetstream will deepen circulation further creating an extra-tropical mid latitude depression.  Charts say this will pass between the UK and Iceland… check back for updates.

humberto track into jet


So, it’s a way off to be sure, but the latest models seem to be pointing towards a nice weekend courtesy of Tropical Cyclone Humberto. Interestingly, Humberto left West Africa on 8 Sept and the first significant landfall he will make might well be Iceland next weekend!

Back to school,  a new term begins and Autumn is knocking on the door… but not quite yet as summer will hang on by its fingernails in Reigate! A flirt with cooler Autumnal conditions, a last blast of summer heat and a possible thundery breakdown are all packed into next week … so read on as the weather gets more interesting.

So, no surprises of course but summer is on the wane as cooler air from Iceland threatens to sweep away our benign August warm calm conditions at the end of next week. This weekend is a rather weak “dry-run” of future Autumn storms with cooler air briefly sweeping a slightly cooler airmass from NW across the UK, but Reigate should stay dry throughout the weekend, albeit a little cooler than recently.

However, Summer is not going quietly and while cooler temps will inevitably win-out, mid-week looks like returning warm temps as high as 27ºC with a plume of warm air from Spain. Unfortunately, this last blast of summer heat (probably!) will be rudely and rapidly swept aside by brisk NW cool air flow by the end of next week associated with a LOW pressure system to the north of Scotland. There could be a brief thundery breakdown associated with this change as fronts lift away any warm air still lingering over the UK, especially the south. Most of the instability bringing any threat of storms is located over the near continent on current models but that could change and the SE might see some storms as cooler air undercuts the warm. One to watch!

spanish plume poss

 

 

Temperatures through this week look increasingly summery with dry weather courtesy of our summer friend the “Azores High”.  Coincidentally, the meteorological summer starts today, June 1st.  Temperatures are due to mostly increase this week in Reigate possibly topping out at 25°C by next Saturday but the building heat comes with a twist!  The HIGH pressure we are enjoying over the UK right now is due to slip North and then West. UPDATE: this will take its time… breakdown looks like Mon/Tues next week, weekend fine!
later next week

This will expose us increasingly to LOW pressure over Europe and a feed of warm air from a LOW in Biscay.  Summer heat combined with LOW pressure can yield thunderstorms as thermals lift into the atmosphere creating big cumulonimbus clouds.  A feed of warm air (warm air advection) will also encourage this thundery activity.  

early june rainfall europeThe continental LOW over Europe has been a breeding ground for thunderstorms and heavy rain which has plagued the continent recently.  With high pressure possibly leaking away from the SE UK by next weekend, models are suggesting that, with a stronger sun and lower pressure, heavy thundery rain could invade from the LOW pressure to the South.  So, enjoy our summer friend this week, the Azores HIGH, because it could break down and allow convective storms and fronts to move across the UK from the continent by next weekend.  

Increasing storm risk

Increasing storm risk

The above graphs show rainfall and CAPE.  CAPE is a measure of convective available potential energy and is a predictor of showery and thundery activity. CAPES so far this year around Reigate have been max of around 300 j/kg: giving a few thundery showery days during May.  Some model runs predict CAPES of 800j/kg later in June which could yield some bigger thunderstorms.  Nevertheless, recent tornadic supercells in Oklahoma have readily built CAPES in excess of 6000 j/kg so ours are small-fry by comparison.  Also, this is along way off and models do get things spectacularly wrong, so storms are certainly not a certainty but worth considering as an idea. 

Update Friday: v unstable atmosphere passing over Reigate Friday after active warm front passes early morning: all ingredients for heavy showers and thunderstorms tomorrow increased risk hail thunder for Reigate, predicted earlier in the week @RGSweather.

Spring warmth is set to arrive big time this weekend, especially from Sunday on wards and particularly in the east of the UK: Reigate could be one of the warmest places so let’s keep our fingers crossed!  The contrast with just a week ago is remarkable with temperatures for Reigate possibly rising to over 20ºC on Sunday and remaining well in the ‘teens during the daytime throughout next week.  There will be a breeze of 15 mph so find a sheltered spot to enjoy the balmy atmosphere or take to the hills with a kite, go sailing, have a BBQ… #getoutside

The initial cause of this long awaited warming is LOW pressure in the Atlantic engaging with a HIGH pressure over Europe which will feed a brisk southerly wind from source regions in North Africa and the Mediterranean.  An upper level ridge building from the south is also critical in feeding warm air aloft (above 5000 m) to the UK.  The jet stream is migrating northwards and helping to feed in this warm upper airmass.  Upper air temperatures at 5000 feet (pic 1 above) are set to rise dramatically from well below freezing last week to +12°C or more through next week: Tropical air has arrived!  The rest of next week looks good with warm and mostly dry weather (but not always) continuing for Reigate and, whilst not always as warm as Sunday, certainly feeling pleasantly springy.  The synoptic configuration is set to remain HIGH pressure in the South and LOW in the North with a warm flow of SW or W winds over Reigate.

However, before all that, Reigate could have some significant rain: while overnight rain should dry up quickly Thursday morning after the front moves North, Friday looks potentially showery and some could be heavy.  April showers are kicked off by heat at the surface causing thermals to rise buoyantly through the atmosphere and condense creating convective clouds called cumulus.  If these grow into shower clouds they are called cumulus congestus or their bigger brother cumulonimbus.  Several of the right conditions for heavy showers are present on Friday such as uplifting air, plenty of humidity and wind shear.  Due to the cold weather thus far, there is currently a lack of heating at the surface to spark off significant thunderstorms but things can change.  Saturday could also have some showers associated with a weak warm front before the much heralded warm and dry weather arrives on Sunday.  Saturday warm front turned out much more perky than first thought… delivered lots of rain all pm! Keep posted on twitter @RGSweather for updates.

Main roads remain clear around Reigate after the evening snow and temperatures are rising above freezing, where they will remain throughout the night and through tomorrow. The main snowfall moved away north west by 10 pm and further light snow showers will peter out overnight. The weather radar shows showers becoming increasingly limited to the south coast. Any further showers overnight are likely to be light and fall as sleet as temperatures rise above freezing. Wednesday and Thursday’s forecast remains cloudy and cold with the risk of any light snow decreasing all the time as a HIGH pressure builds from the north east.  Look to Friday afternoon and through the weekend for the next exciting weather installment for Reigate weather!

Warm December weather

December 23, 2012 — Leave a comment

Temperatures reached 12C in Reigate yesterday and stayed above 10C overnight. This is very warm for December! The reason is shown in the latest NOAA high resolution satellite taken this 10.15am Sunday. The bank of cloud shows the warm funnel of air brought from the SW by a vigorous jetstream. North of this stationary front are blobs of cumulus cloud shown catching the sunshine. South of the front is a big High pressure giving Spain clear conditions. Also, spot the snow on the Pyrenees. The jetstream turns South over Europe and is bringing cold conditions over places like Greece where snow has fallen over Mount Pelion on the Aegean coast.