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Reigate experienced an impressive deluge today when over 60mm* (tbc) of rain fell in a few hours causing flooding in parts of the town.  This was a “surface water flood” caused by drains being overwhelmed by intense heavy rainfall, rather than a river flood, our local River Mole will react more slowly to this event and is unlikely to cause any problems. (*by 4.30pm manual rain gauge 64mm; radar netweather est 50mm; total 24 hour rainfall 74mm manual rain gauge; RGS AWS 36.6mm; Reigate AWS 50.8mm)

The entrance to Morrisons car park and the lower section of Bell Street were overwhelmed with water bursting from drain covers adding to the water running down the roads.  It wasn’t just the roads that were flooded… such was the intensity of rain that Priory Park was also left with surface water pooling up several inches deep on the pitches, and even more in the sunken garden and around the cafe.

This rain event was accurately forecast by the MetOffice and MeteoGroup and several specialist severe and convective weather met agencies who all issued warnings of heavy thundery rain, some totals suggested over 60-90mm.

Two models, the MetOffice Euro4 and WRF NMM model output, turned out to be especially accurate with both putting exceptionally heavy rain precisely over our area at least 24 hours in advance and this was reported by RGSweather on twitter.  The GFS model was less convinced about such heavy rain reaching beyond the south coast but, as always, these convective summer events are especially hit and miss.  Today was a “hit” for Reigate but unfortunately would have probably caused a nuisance for businesses and commuters.

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The maximum intensity of the rainfall in Reigate was measured at 145mm/hr during the storm between 2-3.00pm.  This intensity of rain is unusual in Surrey (highest rainfall intensity in recent years was 183mm/hr 20 Nov 2013) but we are told to expect more of our rain to fall in intense events like this with global warming.  The last time flooding of this scale took place in town was 24 Dec 2014 but, having seen both events, I think this time was slightly “worse”, although given summer conditions the surface water may have subsided quicker.  There are many areas for further investigation worthwhile here: how much rain, for how long and with what antecedent conditions (soil moisture / season / evaporation etc) and what duration of rainfall intensity causes flooding in Reigate?

2015-08-24_15-59-29

rainfall intesity Reigate

The synoptic weather situation that caused this event was a trough over the Atlantic and a blocking HIGH over Scandinavia causing a warm unstable humid airmass to meet cooler Atlantic air along active fronts.  The resulting LOW transported a lot of moisture over the SE and caused the rain in heavy thundery downpours triggered near the fronts.  This situation is not likely to change much until later this week, so expect more rain but hopefully not as intense.

August SE Reigate flood

August SE Reigate flood

An unstable airmass with backing winds over the SE indicated by a skew t chart from the day.

Technically speaking this was part of a “trough disruption” event (see charts above) that started over the Atlantic during the weekend. A “trough disruption” is when an upper trough “breaks” and a southerly section of the trough proceeds purposefully east or NE leaving the centre of the trough behind as a semi-static feature.  The isolated part of the trough can then behave erratically. In this case an unusually active and southerly dipping jetstream for the time of year also played a part in deepening the LOW at the base of the disrupted trough.  Weather events can be “severe” and unexpected with models sometimes struggling to cope when trough disruptions occur.

The LOW pressure that brought heavy showers over the SE and Reigate formed in Biscay yesterday and travelled NE up the English Channel during the day.  SE England therefore experienced SE / easterly winds in the morning, backing northerly and finally swinging westerly during the course of the day, fairly unusual for our part of the world. The anticlockwise change of direction in wind is called “backing”.  Veering is the clockwise movement of wind over time or with height.

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glowering clouds menace Reigate in the morning

Here are some more pictures from this event on my google pics page: please use them but do credit rgsweather. https://goo.gl/photos/Rz9rpsAWozVaEtvB7

by evening it wasn’t any better:

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RGSweather photos featured on the BBC London news that evening (sorry, no sound)

Finally, we can expect more rain this week as the synoptic situation stays static with the Atlantic trough blocked by the high out East. This situation will keep us in a flow of humid moist air that, when it interacts with cooler Atlantic airmasses along fronts, is likely to cause rain.  Add an unusually active jetstream and we have a decent recipe for a wet week.

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wet week ahead

links to other reports

http://www.kentonline.co.uk/herne-bay/news/tornado-spotted-in-kent-42169/

http://reigate.uk/heavy-downpour-causes-town-centre-flooding/

http://www.surreymirror.co.uk/East-Surrey-Floods-Reigate-church-evacuated-water/story-27667834-detail/story.html

http://www.surreymirror.co.uk/Reigate-floods-Petrol-garage-closed-buildings/story-27667710-detail/story.html

http://www.surreymirror.co.uk/East-Surrey-Floods-Deep-water-A25-Reigate/story-27667942-detail/story.html

http://www.surreymirror.co.uk/East-Surrey-Floods-Reigate-homes-evacuated/story-27667812-detail/story.html

http://www.surreymirror.co.uk/Morrisons-car-park-flooded-wettest-day-years/story-27667152-detail/story.html

the system went on to cause significant tornados in Netherlands http://www.dumpert.nl/mediabase/6677432/ac9ba4f6/poldernado_des_doods.html

2014-02-10_22-24-13

Four days of UKMO weather warnings and this hasn’t been unusual this winter!

Unsettled is an understatement, the weather is still on over-drive! Here’s a quick update on rain and storm events upcoming this week for Reigate, as the weekend summary didn’t quite do it justice!  The jetstream is still blasting across the Atlantic and giving birth to storm after storm.  Causes of this extraordinary jetstream are complex, but most “local” to the UK is the extreme contrast in temperature across North America with a buckling jetstream dragging frigid conditions across much of that continent.  This creates a steep temperature and pressure gradient that kick-starts a powerful jet across the Atlantic.  Storm force lows are being created amazingly quickly and most of them are being directed at the UK.

So, here’s a summary of storms on offer this week for Reigate and Surrey…

Tuesday: active front with cold air behind sweeps across the UK and arrives in Reigate am for heavy rain at lunchtime with gusty winds for a time. 10mm is possible in a short space of time which could lead to local FLOODING.  The front will whizz through quickly by the afternoon and leave a brighter but cooler feel pm even with possible wintry showers on a brisk cool W/NW wind.

Wednesday: looks grim… another tight storm approaches from the SW.  This one brings more gales to the SW, Wales but also pushes them further inland across the Midlands.  The south coast and the NE coast (on departure) will also see gales.  Wind for the SE will be pushing in off the Channel at possibly 50-60mph with 40mph gusts inland for inland places Reigate, peaking pm.  Rain is due to be heavy, especially pm and early evening and it will be cloudy all day.

Thursday… looks like a brighter showery day with possible heavy thundery showers at times. Friday / Saturday brings in the next storm with high rainfall totals adding to already saturated ground.  The likelihood of more flooding seems high at this stage which is terrible news for those places along the Thames that have been so badly hit today.

No, not a record low pressure this one but the stormy weather continues for the UK with another deep area of low pressure sweeping up past the NW of Scotland tomorrow, Friday.  This one, called Anne, has violent storm force westerly winds out in the Atlantic building high waves and matching high tides, causing unusual storm surge conditions for the west coast. It is the orientation and track of this storm that appears to be causing most trouble: spot those isobars directed straight at the UK and building high seas with those high tides.  The distance which wind travels over the sea is called FETCH and the longer the fetch the greater the possible wave height.  Note also the waves and wind that build in the Channel.

There are currently 17 severe flood warnings from the Environment Agency for the west coast of Wales and SW England.  The River Severn estuary is also at risk as as it faces the SW winds, funnels tides and has high river flows all to contend with.

Envt Agency are even warning people to evacuate if possible from vulnerable locations on West coast.  N Ireland also adopting this too.

http://www.bbc.co.uk/news/uk-northern-ireland-25580692#TWEET1001676

http://www.bbc.co.uk/news/uk-25572390

For Reigate and the SE (inland) heavy overnight rain will pause for a while in the morning only to resume as winds increase to gusts possibly 30-40mph, gusting higher on hills, and with possible heavy showery rain.  Rain totals for Fri-Sat 48 hours could amount to 30-40mm, but more likely 10-20mm for Reigate and Mole Valley.

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rain totals fri – sat

Is there any sign of improvement? Met experts watch the high atmosphere for some long range forecasting. In particular, the stratospheric Polar Vortex is a a possible indicator of how the tropospheric jetstream might be acting in weeks to come.  The stormy weather we have been experiencing has been “caused” by an extremely powerful polar vortex: a great contrast in temperature difference between high and low latitudes sets up high winds in the stratosphere which act like a belt to hold in cold air to the Pole.  The North Pole stratosphere is extremely cold this winter and this has encouraged a powerful polar vortex.

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You can see from the chart below that the vortex is forecast to change shape: split or squeeze, meteorologists are watching for this to tell them if the jetstream will weaken, and there are signs that it will in mid-Jan.  This may also have the effect of allowing polar leaks of cool air to reach the UK: so maybe less stormy but a tad cooler. Check these guys for further details: http://www.weatherweb.net/wxwebchartsecmopmsl.php

01-01-2014 13-59-58

Finally, the Express is unusually conservative with this article: the wind speeds are too low!

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The UK is trapped in a “cold washing cycle” with no end in sight this week.  The meandering north-south flow of the jetstream mentioned earlier is partly to blame for fixing LOW pressure over the UK which simply is not budging. Our LOW is sandwiched between a HIGH to the west (Atlantic) and the east (Baltic) and is going nowhere for a while. Hence the heavy showers are set to continue and there is worse to come… starting tomorrow!
A deepening wave depression is set to form on the polar front and spin out from the Atlantic across SW, Central and Southern UK during Tuesday and Wednesday. Heavy rain, gusty winds, cool temperatures and even wet snow are predicted. The worst of this will be in the south west, Wales and parts of central southern England but Reigate and the SE will get continuous rain for 24 hours peaking in intensity overnight Tuesday into Wednesday. Once the fronts move away during Wednesday morning cool unstable north westerly winds and warming surface temperatures will create unstable conditions with a risk of heavy showers and thunderstorms through Wednesday and Thursday pm.
Rain is likely to arrive Reigate Tuesday during the morning and it won’t stop until sometime Wednesday am, to be replaced by heavy showers and a risk thunderstorms. Reigate could see >30mm of rain before the end of the week.
The weekend looks utterly dreadful, especially for the SE: LOW pressure swings close to the SE from the continent and this could bring heavy thundery showers close to Reigate for the weekend.

Throughout the convective weather later this week: points to watch out for are tremendous cumulonimbus clouds and even tornadic conditions with mamatus clouds and perhaps the odd funnel cloud?
Some models show a ridge of HIGH pressure building from the west next week – this is forecast to bring a decent end to May.

Classic short wave depression: the following synoptic charts show how the wave depression forms out of the polar front jet stream.  An innocent kink in the front is the first indication of lower pressure.  Thereafter the fronts become more pronounced, central pressure falls and the whole circulation moves across the UK. Note that, throughout this episode, the “mother low” to the north of Scotland barely moves.

p.s. “cold washing cycle” is not a meteorological term!