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Reigate Grammar School weather station started recording weather data from August 2012.  Data is updated to our own website pages every 15 minutes and “live” wind readings are updated every 5 seconds.  We also feed data to the UK Met Office weather observations website and to US based Weather Underground (WU). Best viewed on Chrome! Follow @RGSweather

Reigate: cooler, wetter, windy-er!

nasty North Sea LOW

swirl of wind around LOW

gusty Friday

Air direct from the Arctic: cool

rain wrapped N Sea LOW

freezing level drop

The “something and nothing” weather in the South East of the past week and the uncertainties in the forecast are set to continue for a while. Some met-people call these conditions “unforecast-able”. Models seem to be unreliable beyond a few days and even hours. Rainfall has been especially hit and miss to forecast in the SE: predictions have been varying wildly for specific days between torrential, heavy, some and then no rain arrives at all! The reason is possibly the lack of the usual “zonal flow” in the jet stream: i.e. west to east flowing jet.  The jetstream is meandering north-south and weather systems are more or less STATIC: the UK has been stuck in a low pressure trough for over a week.  The normal procession of low pressure systems (depressions) and brief sunny HIGH pressure ridges seems a distant memory: it simply hasn’t been a feature of our weather for ages. Forecast models seem to struggle with this.

The overall synoptic weather situation remains the same. That is: a big blocking HIGH over the Atlantic and very weak westerlies with the jetstream in a North-South pattern (meridional) bringing down cool northerly winds direct from the Arctic which “pool-up” across Northern France and Southern UK creating a LOW pressure trough.  LOW pressure in Spring with a stronger sun can mean pleasant warm sunny spells but showers: it is the showers which, fortunately, have barely troubled Reigate.  However, (and this does look more certain!), a significant little LOW is set to spiral down the N Sea Thursday – Friday, deepen along the way and strengthen Northerly winds and bring rain, especially to the SE: some frontal rain and showers are predicted to accompany this LOW but again – it could be rather hit and miss depending on how close the LOW gets to the SE England and the strength of accompanying fronts.

There is a 30% chance of thunderstorms over Reigate area through Thurs and Friday afternoons (as the sun heats the surface which creates bubbles of warm air through the day which convect upwards through the cool Polar airmass creating tall cumulonimbus clouds).  With upper air temperatures at 5000 feet as low as -12ºC later this week any vigorous showers may fall as hail.  Frontal rain attached to the LOW will certainly feel chilly in the wind. Night time temperatures could fall as low as 4ºC and any wind will make it feel distinctly cool.  A weak ridge over Sat and Sun may bring pleasantly warmer and drier weather but some models are showing a return to LOW pressure in a storm arriving from the NW on Monday bank 

Meanwhile, sincere sympathy and thoughts to those caught by the terrible EF5 tornado which caused such terrible damage in Moore, Oklahoma yesterday.  Following events on Twitter and news reports on destroyed schools was very sad and upsetting indeed.  The tension between following exciting weather and the potential for witnessing terrible disasters unfold in front of their eyes was palpable for the storm chasers and met-enthusiasts involved.  Unfortunately, the weather in MidWest continues to threaten areas with tornadic conditions: lately in New York state too.  Take care out there.  Our UK weather is usually mercifully benign in comparison.  

Oklahoma tornado May 21 2013

Devastating tornadoes hit Oklahoma suburbs schools and towns today.

News at http://kfor.com/on-air/live-streaming/

http://youtu.be/dCbhnPfEvF8

https://www.facebook.com/photo.php?v=10201105488460628&set=vb.1271447672&type=2&theater

http://www.youtube.com/watch?v=reHEcKBvsrs&feature=youtu.be&a

Info @RGsweather

More on this later, thoughts to everyone out there.

Weekend weather is upside-down!

warm from North, cool from South

the surface set up for the weekend

upper air mass shows cut off low

rain on front

sunday temperature gradient

saturday temperature gradient

Reigate weekend in brief: Saturday dry (but v wet North England: low risk of it extending our way); Sunday wet; Monday showers NE wind.

Update: forecast for weekend proved v tricky: Model rainfall forecasts struggled with the easterly cut off low set-up.  Saturday was pleasant and Sunday turned out dry and warm. Note: GFS rainfall seems not to cope well with meridional jet stream with cut-off low over UK scenario!

Wondering if the weather is a bit odd recently? Well, you may have it right… the weather is literally upside-down from what we would normally expect in the UK. We would normally expect LOW pressure systems to whizz over the country from west to east. Temperature normally decreases further north and we usually expect warm air to arrive from the south. This weekend turns this normal expectation upside-down! A WARM air mass is pressing down from the NE, having arrived on a long journey from the Mediterranean and Eastern Europe (where there is a heatwave). This warm air is due to meet the COOL polar air hovering over the UK and cause torrential rain over Saturday up North. Reigate will see none of this and will stay mostly dry on Saturday.  On Sunday, however, cooler air will push up from the South and cause heavy rain for the SE and Reigate. Warm air from the North?  Cool air from the South? Slightly unusual. 

Where air masses of different temperatures meet the warmer air is forced to rise up and over the cool: a front.  The greater the difference in temperature between the air masses and the steeper the temperature gradient, the greater the uplift.  This causes condensation, clouds and rain.  Fronts usually approach the UK from the West.  Currently a huge HIGH pressure is blocking all westerly winds.  A warm front moving south from across the North Sea is most unusual!

The cause of all this is the development of a cut-off LOW over Europe. A very looping jetsream (meridional) has been drawing polar air down across the UK this week and this cold air has pooled up over Western Europe, hence all the convection and showers. Meanwhile, the northward moving limb of the jetstream has been dragging warm air up over Eastern Europe (which has a heatwave).  This warm air is due to meet the cool air over the UK this weekend.
The good news is that Reigate is likely to have a reasonable Saturday out of all this. Sunday could be pretty wet though.
The cut-off LOW will hang around for early next week and the unsettled theme continues for us in Reigate until the end of next week when things could improve for Bank Holiday as HIGH pressure builds in from the Atlantic. This means the west and north are likely to see better weather than the SE for Bank Holiday, but that’s a long way off to be certain. Keep tuned!

UK stuck in COLD RINSE CYCLE

Going nowhere fast: trapped by the jet

small but packs a punch

worst of it Tuesday night

totally nasty rainfall

risk of thunderstorms

more to come with cool NE winds

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.

kink in the polar front

moves over the UK

lower pressure

fills over the North Sea

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

#Reigate: if you haven’t got an umbrella, get one!

frontal wave depression

warm front moves up from S

more rain this week

cut off LOW

Last week our total rainfall in Reigate was 14mm, some models are predicting 40mm+ this week! Much of this will fall in heavy frontal rain mid-week and in thundery showers Weds/Thurs thereafter.  The jetstream is developing a meridional flow at the moment, which means it is meandering wildly and developing slow moving weather systems which sit around for long periods; the more usual zonal flow is when the jetstream flows west to east and drags weather systems briskly across the UK and keeps weather moving. Meridional flow gives rise to interesting weather: LOW pressure with heavy showers is the basic scenario this week: great for growing awesome cumulus clouds!

LOW pressure sits right over the UK all this week bringing sunshine and showers but with an added twist mid-week when a deepening short-wave trough is predicted to cross southern regions bringing potentially heavy frontal rain across the whole of the south, including Reigate. Thereafter, on Wednesday heavy thundery showers with hail could develop as winds slacken off.  Watch out for tall cumulonimbus clouds and rainbows! Any showers could build into single cell thunderstorms which will be slow moving in the slack winds and persistent and cause localised flooding. Around Reigate these showers will probably be quite isolated and some places will miss them completely but if you get caught in one you’ll know it! Temperatures remain uninspiring at 15C, although climbing slightly through the week. The end of the week remains very uncertain but could develop more heavy rain for us in the SE as the cut-off LOW over Europe feeds a miserably cool NE wind our way.
As always, this is early days and the end of the week looks especially variable and interesting!
Please check @RGSweather for updates!

Cut-Off LOW to bring “ox-bow lake” of awful weather next week!

1# meandering jetstream

2# ox-bow lake in the jetstream

3# cool pool

4# run for the hills!

5# cut-offs: not much to look at

6# convective rainfall peaks

The jetstream meanders like a river of air in the high atmosphere. Much of the time the jetstream blasts pretty straight west to east, at other times it loops wildly north to south. This week the jetstream is due to loop wildly, like a meandering river and threatens to form a special weather feature called a “cut-off low”.

How do cut-off lows form and why are they special?

1# A loop of the jetstream descending over the UK is forecast to form a TROUGH of low pressure over Europe this week. The southward limb of the jet directs cool polar air towards Europe and LOW pressure.

2# The loop becomes so sinuous (bendy) that, like a meandering river, the neck is cut-off as the jetstream re-forms to the north.  The LOW to the south becomes “cut-off” as HIGH pressure builds to the north.

3# The cut-off LOW over Europe is left as an “ox-bow” of cool unsettled weather, especially near the centre of the low pressure.

4# The surface warms and CONVECTION occurs through the cool air: air rises forming big convective storms.  These are forecast to migrate from the continent and effect especially the South and SE of England.  Higher pressure should keep the north more settled. 

5# Cut-offs fill gradually as warm air convects aloft, reducing instability.

6# The rest of May looks to have more rain and cool unsettled conditions as LOW pressure dominates.

(pics courtesy of netweather)

Thursday: 100mph+ winds for Reigate…at 20,000 feet!

jetstream creates surface winds

strongest over the sea and hills

cool air wrapped up in this LOW

Local winds

Windy at altitude

Thursday morning will see winds in excess of 100mph for Reigate… fortunately well above the town at 20,000 feet or so! This is the jetstream. These winds are pivotal in producing strong surface winds which will “touch-down” later in the day in Reigate. Surface gusts of over 40mph are possible with +30mph average winds. These winds are due to a deepening area of LOW pressure crossing the north of the UK over the next few days. The Irish Sea, English Channel, south coast and hilly areas like the South and North Downs will see the highest gusts. With trees in leaf there may be the odd branch falling. Batten down the hatches and secure loose bins! (Video: you can see the LOW crossing Northern England.  Reds 60mph; yellows 40mph; greens 30mph).

Quick update for NEXT week: LOW pressure dominates as a cyclone sinks down from the NW and an upper TROUGH sits over the UK while HIGH pressure builds to the NORTH: this will bring a distinctly AUTUMNAL feel to Reigate next week: temperatures struggling to get into the mid-teens, with periods of showery and breezy weather: so, NOT hot at all: cool, unsettled and rather wet for the time being.  More details later! Check @RGSweather for regular updates.

Wetter, windy and cooler but main rain skirts SE

deep low changes the air

most rain will miss Reigate

unsettled May

windy!

Update Weds 20:25hrs: Gale warning for South coast and exposed hills: SW wind 30mph+ GUSTING 40-50mph Thursday pm. Channel will experience v strong winds.

A deepening LOW pressure is winding up off the west coast of the British Isles and will cross the UK this week bringing cloud, rain and strong winds and cooler temperatures.  Reigate and the SE, however, will remain comparatively sheltered from the worst of the rainfall, probably only amounting to 10mm over the next 10 days or so, which isn’t much compared with the deluge elsewhere.  The first fronts make an entrance tomorrow with moderately heavy rain for Wednesday, slowly clearing off in the afternoon. The most interesting bit of weather looks like Thursday overnight into Friday when strong south westerly winds circulating around the LOW cross our region, gusts around 50mph are possible in exposed areas like the North Downs around Reigate.  The BIG map shows wind arrows circulating around the centre of the LOW as it moves across Northern England and Southern Scotland (wind speed in mph). Friday looks pretty wet all day and Saturday could bring a threat of thundery showers.  The unsettled regime continues with blustery westerly winds throughout the weekend keeping temperatures around the mid teens. Weekend looks blustery and unsettled with showers Saturday, some thundery possible.
The longer term May forecast looks rather unsettled especially for Reigate with LOW pressure likely to sit over Europe bringing rain to the SE, while higher pressure sits over the North and West of the UK bringing better conditions to those regions. The graph shows more rain through the rest of May showing this sort of set up. Lovely!

“May” be a tad cooler mid-week: a switch from Tropical to Polar air

May be less balmy

rain crossing from west

cooler wind direction

polar air behind the cold front

newspapers…exaggerate a bit!

brilliant start to May

rainfall in bursts

switch in the jet

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.

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.

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.

Reigate April weather summary

mamatus cloud over Brixton on 12 April

thundery showers and hail on the 12-13

April ended with ideal cricket weather

April 2013 Reigate summary

After a cold March, April jump-started Spring 2013. The key moment when spring started was thundery showers on 12 and 13 April: a cold front wrapped round a depression broke the Arctic block. This allowed much warmer upper air temperatures to stream in from the south and west pulled in as the jetstream meandered north of the UK. Temperatures leapt from around 10ºC to 18ºC daytime max over the 12 and 13 April. April was a dry month with only 38.6mm of rain compared to 68mm in March and 114mm in December 2012. Nearly half of the monthly total rainfall fell on the 12&13 April during thundery weather along a cold front which swept away the cold and brought in warmer and more humid tropical airmasses. April rainfall after that wet spell amounted to less than 6mm with sunny and dry weather dominating through to the start of May.  

Here is a movie of clouds seen over Reigate in April!