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High pressure persists this week over the UK as shown by the chart and the amazingly clear satellite pic showing great swathes of clear skies over a remarkably snow-free Europe today 11 March. It’s worth remembering that on this day last year we saw the lowest wind chill temps of the year in Reigate at -11.9c early on 12 March with -10c at 9:30am on 11 March during an episode of extremely cold NE winds which brought a blizzard to the Channel Islands.

Back to this week… the high pressure has brought delightfully sunny days and Tmax temps to Reigate over 19c last weekend , especially warm and spring like on Sunday. Since then a cooler NE/E breeze around the edge of the high as it slipped north and east has kept us cooler and occasionally more gloomy with anticyclone gloom and stratocumulus blanket on some days.

The outlook this week remains settled but with a distinct drop-off of temps over the weekend and especially into next week.  This is due to the HIGH slipping west and the jetstream dragging down cooler NW winds fresh from Greenland.  A LOW moving across N Scotland and over to Scandinavia this weekend will start the process of dragging down NW winds that will make next week cooler, especially further east where it looks like the N Sea will get a full blast of chilly air at times.

Nevertheless, it is unlikely to become terribly unsettled down here in the SE as the longer term picture looks like high pressure hangs on in the south keeping us mostly dry.  The north of the UK might see more frontal action as LOWS bring fronts across more northerly parts.  Models seem to be keeping the south relatively dry for another week to 10 days.

After the wettest winter on record, groundwater levels in the chalk aquifers are the highest EVER in the SE and the Mole Valley particularly.  There are still 50 flood warnings in the SE and locally flood water continues to pour across the A25 between Reigate and Dorking near Brockham.  This groundwater flooding, though of course nothing like as damaging as the fluvial river flooding during the winter along the R Mole and the Thames, could last for some months yet as the winter rain continues to percolate into the chalk water table and flows out along geological boundaries at inconvenient locations such as normally dry river beds and bournes and across impermeable surfaces such as roads. The map showing soil moisture below shows how wet the soil remains after this extremely wet winter.

Weather is chaotic and numerical weather models are not perfect. The forecast for Reigate today went rather awry, though not completely.  It was forecast to rain heavily, perhaps on and off, but the forecast was for heavy rain more or less throughout the day. Check UKMO forecast from yesterday below.  Some models brought 24 hour totals of 20-30mm to SE at points on the lead up to the event.  The cause of the forecast deluge: a small scale low tracking NW to SE with a tightly wrapped occluded front crossing the area once, then lingering nearby to deposit more rain during the day before drifting off southeast. Once the front had passed through early am, it turned out to be a splendid day with sunshine and bright spells throughout, until rain later.  So what went wrong/right?

The front passed over as forecast during early am dropping 6mm on Reigate before 8am.  It then sat N of London most of the day while further south convection over Sussex caused significant Cb clouds and showers (some thundery) to spark off from midday.  For us in Reigate, we had a splendidly bright day with glorious sunshine by 8am and bubbly cumulus clouds thereafter, the odd spot of rain but nothing significant until early afternoon when the front migrated south east.  So for most daylight hours Reigate was dry, quite the opposite of the forecast.

The photos above and graphics below suggest a possible reason for this.  Reigate sat in a sort of “Goldilocks Gap” between the persistent frontal rain further north and convective rain nearer the LOW further south. It is notable that the convective showers built mostly over the land, showing almost April-shower tendencies to build on warmer land surfaces than the now-cooler sea. The occluded front sat close to Reigate, frontal wave clouds and cirrus were visible above and to the north for most of the day.  This may have helped suppress convection.  As warm tropical air is lofted over an occluded front it spreads out and forms a cirrus veil, this often suggests a broad inversion of warmer air aloft that effectively suppresses uplift of thermals: the cirrus acts like a lid.  So cumulus clouds over Reigate and the N Downs stayed small and harmless.  Not far south, in Sussex, thundery downpours developed as the buoyant air lofted uninhibited by any inversion.  You can see this on the radar image below.

Reigate was therefore dry for most of the day perhaps because of our location in a sort of Goldilocks Gap (our word) that was just far enough from the occluded front to avoid persistent rain and just near enough to benefit from the inversion to prevent convective showers. Met-Magic!  The graphics and photos try to explain this further.

This is just one possible reason why slight changes in the tack of a LOW will render a forecast completely wrong, even in the middle of a LOW pressure when all hope of a nice day might be thought lost.  Further ideas are most welcome to extend this.

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.

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.

Last Day of Summer 2013… Reigate and SE England enjoyed cloudless blue skies throughout the last day of 2013 meteorological summer today. Reigate reached Tmax 22ºC, was exceptionally sunny with no cloud all day. A light NW breeze meant shadows felt a little on the cool side, but this is just being picky. A very dry NW airmass aloft circling round the Azores High kept the sky cloud free. Relative humidity at 700hPa was 10% and at the surface Reigate had RH less than 40%.  

Is this the end of warm weather, no not at all!  Even warmer conditions are expected to build across the South through the coming week with Tmax perhaps reaching 28ºC by mid-week.  Thereafter, a cold front attached to a LOW could bring a fairly rapidly conclusion to our warmest weather with maybe some heavy showers and thundery rain before the end of the week.  Nevertheless, Autumn is typically characterised by fluctuating waves of alternating warm Tropical air battling it out with cool Polar air, so we can expect periodic flows of warm air at times through early Autumn.  The first big Autumn storm has yet to appear on the horizon, they are stuck across the Atlantic giving E Canada a soaking!  Our time will come, though 🙂

Meteorological (weather) Summer differs from astronomical Summer in that the weather seasons are simply broken into three month chunks: Summer is June, July and August; Autumn is September, October and November; Winter is December, January and February and Spring is March, April and May.

More on this here

Reigate should see 30ºC Tmax for the first day of August and high 20’s on Friday. The cause is a warm tropical plume of air flooding over the south of the UK from Iberia (Spain) and Africa brought in as the jetstream moves north and a LOW to our west and HIGH to the east builds a brisk Southerly airstream. Thursday will be dry and bright and the night will feel very muggy with high dew points and lows of over 20ºC at midnight: warm and damp, the air will feel close! Friday will be warm too but slightly less so for Reigate, dawning bright but with cloud spilling in associated with a weakening cold front pushing east across the country. Little frontal rain is expected but the cooler upper temps could release the huge heat energy built up in the lower atmosphere and kick off heavy / thundery showers for a while, especially Friday morning. Confidence is low currently but this might occur, the potential is there with all that heat.  

Every thunderstorm indicator is giving green lights to thunderstorms in the SE sometime in the next few days BUT the airstream is capped with warm uppers: this appears to be the only factor stopping convective action. This means heat building in the lower atmosphere will stay put until forced to rise into the unstable layers above (see sounding graph above). The approaching cold front might be sufficient to produce something of convective interest on Friday, morning especially… before the warm Euro air retreats back to east. So far Reigate has seen little of the impressive thundery activity elsewhere in the UK this summer. You might be forgiven for thinking it must be our turn soon! It’s unlikely to be in the next few days as the risk is small for any major thundery outbreak.
The synoptic situation remains similar over the next few days with HIGH pressure over the continent and LOWS edging close to the NW of the UK bringing rain and wind to the NW. The SE should escape with any trailing fronts weakening and Reigate should remain largely dry but for any threat of isolated thunderstorms in building heat.

RGSweather is closing for a few weeks until August. Off to a log cabin in the Tatra Mts of S Poland. I think the weather there is edgy at the moment… perhaps some nice thunderstorms! Back posting and tweeting very soon.  Meanwhile, enjoy the beautiful July weather and remember…the best view is always up (but wear sunglasses this time of year!) Happy Hols, enjoy 🙂

poland hol

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.