Archives For Reigate

2016-07-22_15-53-56

Reigate June 2016 weather statistics

  • T average 16C
  • Tmax 25.6C
  • Tmin 8.5C
  • Total rainfall 100mm
  • Max gust 28mph
  • total sunshine 114 hours

June 2016 was wet in Surrey with over 246% of normal rainfall.  SE England as a whole received 113mm of rain, 208% of long term average.  At 118.4mm, Surrey was the wettest county in the UK regarding anomalously high precipitation. Despite this, both 1971 and 2012 were wetter months.

The rain was delivered in a number of distinct heavy showers and convective thunderstorm events, notably on 23 June when parts of South London were especially badly flooded in flash flooding during torrential rainfall of more than 50mm in one storm.

http://www.huffingtonpost.co.uk/entry/uk-weather-flood-warnings-as-torrential-rain-falls-on-london_uk_576b9225e4b0b1f1704feabb?7liv7jj9ylkz3q5mi&

The heaviest rainfall was characteristically patchy so not every part of Surrey experienced the same soaking, some parts were a lot wetter than others. For example, while Caterham received over 40mm in one thunderstorm, Reigate received only 10mm from the same storm.  In total, Reigate received 100mm of rain in June which compares with 15mm for June 2015 and 30mm for 2014.

“Surrey with 118.4mm of rain endured the worst rainfall of any county, relative to its average. The June average rainfall for Surrey between 1981 and 2010 is just 50.7mm. This month’s total was 246% of normal.” MetOffice

A thunderstorm on 23 June brought an amber warning and local flooding and lightning damage to some places in the SE. Below are some news reports from the time.

A shelf cloud was spotted over Reigate, one of the first observed over the town.

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Horsham experienced intense lightning and one bolt struck a conifer at 2am which was destroyed in a spectacular explosion.  Fortunately no one was hurt, but cars and property was damaged.

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convective June 2016 Priory Park

June started well but pressure fell from 10 June and the month remained mostly unsettled thereafter.

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Due to the unsettled conditions and rainfall and associated cloud, June was less sunny than usual, with only 114 hours of sunshine recorded in Reigate. June 2015 had 192 hours of sunshine in comparison.

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June was just about 1C above the long term 1981-2010 UK average at 13.9C.  Reigate recorded a monthly average of 16C which is considerably higher than the UK average being located in the warmer SE.

Globally June 2016 was the warmest June since records began in 1880 and was the 14th month in a row to beat global long term average temperature. The charts below show NOAA, NASA and UAH global temp anomaly maps for June. All agree that June was hot.

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With one month after another breaking heat records, 2016 is easily on target to be the hottest year on record despite a waning El Nino.

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This continues the inexorable rise in global temperature in recent years.

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2016-06-12_06-35-12

Reigate May 2016 summary statistics

  • Tmax 27.4C
  • Tmin 0.3C
  • Tav 13.8C (UK 11.3C)
  • total rainfall 42mm (town) 45mm (Hartswood)
  • max gust 36mph
  • average wind direction NNE
  • sunshine 181.7 hours (May 2015 161 hours)

Whilst there were fortunately no severe weather events in Reigate and few across the UK in May, the weather we experienced more widely could be linked tenuously to climate change.  Of course, caution is required with such speculative statements but attribution studies on the May floods in Paris, not so far away, have concluded that they were made 90% more likely due to climate change. The same stalled low pressure system delivered our easterly winds so we were influenced, albeit on the edges, by the same blocked weather pattern.

People attempting to climb Snowdon in North Wales in May were lucky to experience sunnier-than-usual conditions for much of the month (south wales had more thunderstorms which reduced the sunshine totals there).  Meanwhile, in Surrey, we experienced occasionally warm conditions with an unusual mean monthly wind direction from the NNE.

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Reigate pressure rose hesitantly mid-month

Pressure fell across the UK to start May but then rose mid-month, especially to the North, bringing a relatively unusual easterly flow into Reigate and the south.  Whilst there were few severe weather events during the month, this post briefly explores some of the wider factors that may have contributed to this Easterly flow and the possibility of it being linked to climate change.

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Whilst mostly dry for the UK as a whole, occasional showers, some thundery, brought Reigate rainfall totals to just above average at around 42-45mm as recorded from our two weather stations respectively in and out of town.  SE England as a whole recorded rainfall at 111% of normal rainfall, mostly falling in thundery showers, more common in SE wind regimes.

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With relatively dry Easterly winds, sunshine totals for the UK were accordingly above average given the relatively high pressure overall.  Reigate experienced 182 hours of sunshine in total for the month.

Unusually, the sunniest places in the UK were in the North and West given the easterly winds bringing occasionally cloudier conditions off the North Sea to the south and east.  May 24 shows a typical scenario with the higher pressure to the North dragging in E/NE winds across the southern part of the UK with cloud across eastern areas and clearer conditions to the west.

Some great sunny days were recorded in the mountains of Wales, Cumbria and Scotland!

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Sunny Snowdonia with kelvin-helmholtz type wave clouds trying to form over Ogwen Valley

The higher than average rainfall patches shown below in the south were associated with showers on occasionally unstable warm and humid SE winds.  Reigate reached a Tmax of over 27C in this warm flow.

The wider pressure pattern across the northern hemisphere was characterized by anomalously high heights over the Arctic and LOWER than normal pressure in mid-latitudes including Europe. This situation is called “northern blocking” and in winter could cause cold conditions in mid-latitudes.  In Spring, as the continent rapidly warms up in stronger sunshine, easterly winds can be warm or even hot for the UK.

northern blocking over Pole

northern blocking over Pole

Northern Hemisphere pressure patterns are measured by the Arctic Oscillation which, as can be seen below, remained unusually negative through much of April and May showing high pressure persisting over the Arctic relative to low pressure in the mid-latitudes.  This pressure pattern turned winds from the usual westerlies into easterlies in the UK and Europe.

negative arctic oscillation Spring 2016

negative arctic oscillation Spring 2016

The causes of this reversal of the usual mid-latitude zonal westerly wind set-up have been linked to low sea ice extent in the Arctic, especially the Kara and Arctic Gateway seas. Warmer influxes of air into the Arctic builds air pressure which then links to higher chances of Easterly winds in mid-latitudes.

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low Arctic sea ice cover March 2016

The very low sea ice extent this year was brought about by much warmer-than-usual conditions during the Polar winter, where monthly average temperatures in the Arctic (>60N) were at times 3.5C or more above average during the cold season of 2015-16. This Arctic amplification is widely accepted as being caused by human induced climate change.

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It turns out that Spring Arctic sea ice extent is some of the lowest recorded in the 38 year satellite series.

 

So, unusual sunshine in North Wales, a warm NNE mean wind direction in Reigate and cloudy conditions on the east coast can be linked to the above tele-connecting weather patterns which, in turn, can be linked to climate change in the far flung Arctic.

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Meanwhile, the strong 2015-16 El Niño declined rapidly through May and ENSO conditions were neutral by early June. Models suggest the chance of La Niña (cool Pacific) conditions by Autumn 2016 are as high as 60%.  Some forecasters bring La Nina through the summer.  La Nina, and the warmer SSTs of the tropical Atlantic, are associated with more frequent hurricanes in the Atlantic basin.  In turn, high hurricane accumulated energy transfered to the North Pole during such seasons can build Polar heights in Northern Hemisphere winters, warming the Arctic and further melting sea ice.  Whilst this is just outrageous long term amateur speculation, it is nevertheless interesting to ponder the potential for feedbacks to accelerate further climate change in the near future.

The turning down of the vast heat engine of the El Nino might be linked to the slightly lower May global average temperature, though confirmation from expert sources has not verified this as yet.

 

Local data for May and all months stretching back to 2012 can be found on our data page here

http://www.metoffice.gov.uk/climate/uk/summaries/2016/may

 

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How can there possibly be a link between a modestly cool month in Reigate and the earliest start to the melt-season in Greenland, the devastating wild fires in Canada and the seventh hottest-ever global month in succession?

April summary weather statistics for Reigate

  • Average Temp 8.2C
  • Tmax 17.7C
  • Tmin 0.1C
  • precipitation 43.4mm (local Reigate) SE PPT 55mm
  • sunshine 140.4 hours
  • Max wind gust 30mph
  • average wind bearing 199 degrees

Reigate, like the UK as a whole, had a cooler than average April at 8.2C. The town even experienced some unusual snow showers on 26 April in a cool northerly air flow.

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The cool month for the UK is in stark contrast to the bulk of the planet which experienced a much much warmer month than average, at over 1.1C warmer than any previously measured April.

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Astonishingly, this is the seventh month straight that has brought record breaking global temperature anomalies.  This continuing succession of warm months globally should be of concern to everyone.  More on this below.

Back to the UK… The Central England Temperature came out at 7.5C,  0.4C below average, and the UK mean was even lower at 6.5C, 0.9C below the long term average.

Rainfall was about average in Reigate with around 40mm of rainfall.  The MetOffice SE figure came out at 55mm.

April was sunnier than usual with a total of 140 hours of sunshine.

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This continues the trend of drier and sunnier Aprils in the UK in recent years.

The first half of April was unsettled with most of the rain falling associated with low pressure systems and fronts. The second half of April saw an unusual cool period as northern blocking over the Arctic sent cool northerly winds south with attendant sunshine and showers.

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Globally April was the warmest ever April on record.  An anomaly of 1.1C sent the Paris target of keeping global temperatures below 1.5C into grave doubt as this is the 7th month in succession to yield much higher temperatures than ever. This is now being dubbed a “Climate Emergency” because of the sudden and rapid increase in global temperature to levels not expected to occur so soon.

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The UK / NW Europe was about the only part of the planet, with NE Canada, to record below average temperatures.

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The cool spot over the UK  was due to northern blocking (high pressure) over the Arctic. As pressure rose over the Arctic, cold air pushed out into mid-latitudes.

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It is a matter of chance where high pressure and low pressure set up that determines where cold polar air penetrates in these northern blocking scenarios.  This time the pattern sent the cold air to the UK and N Europe.  The Northern Hemisphere as a whole saw anomalously low snow cover as a result of incredibly high temperatures elsewhere.

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Arctic Amplification, where the northern latitudes experience highest rates of warming, is well documented and of increasing concern to climate change.  It is acting as both a response and a further driving force behind rapid climate change.

Temperatures rocketed over the Arctic this cold season with temperature departures over 3C widely across the Polar regions.  The Greenland ice sheet experienced one of the earliest starts to the ice melt season on record.

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Arctic Sea cover also recorded another record low maximum winter extent.

“On March 24, Arctic sea ice extent peaked at 5.607 million square miles (14.52 million square kilometers), a new record low winter maximum extent in the satellite record that started in 1979. It is slightly smaller than the previous record low maximum extent of 5.612 million square miles (14.54 million square kilometers) that occurred last year. The 13 smallest maximum extents on the satellite record have happened in the last 13 years.” NASA 

This is both a response and a further catastrophe for climate change.  As snow and ice melt in the Polar regions there are connections with further warming as darker sea and land surfaces heat up more readily.

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This Polar warming itself is connected with a weaker jetstream as latitudinal temperature gradients in the atmosphere decline.  It is temperature gradient, especially in Mid-Latitudes, that generates the driving force behind the jetstream.  A weaker jetstream is said to cause more blocked atmospheric conditions as it meanders with greater amplitude in a meridional pattern that locks in swoops of northerly and southerly winds. More extreme weather is caused as these pressure patterns persist for longer.  Sweeps of warmer air penetrate into the Arctic, melting more ice over Greenland and, for mid-latitudes, cooler dry Polar air leaks out causing damaging late frosts and wild fires.

So, whilst it seems tenuous to connect these far-off events to our own rather benignly cool April, it is still important to think globally when considering how our own weather links to increasingly extreme weather elsewhere.

http://www.theguardian.com/environment/2016/may/16/april-third-month-in-row-to-break-global-temperature-records?CMP=share_btn_tw

http://www.bbc.co.uk/news/science-environment-36212145

2016-04-23_10-19-23

Reigate weather summary statistics for March 2016:

  • Tmax 14.8C
  • Tmin -2.8C
  • Taverage 6.3C
  • Total rainfall 80mm (66.4 aws)
  • Max gust 51mph
  • Total sunshine 118.6 hours

March in Reigate started and finished unsettled with Storm Jake (no impact locally) on 2 March and Storm Katie (moderate impact, see previous post) on 27-28 March.  High pressure located to the north of the UK mid-month kept things more settled and mostly dry here.

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Here’s a time-lapse video from Reigate of showers and a defined cold front passing through associated with Storm Jake on 2 March.  Note the distinct drop in temperature as the cold front arrives (windows fog up) and then the change (veer) in wind direction as the front passes and skies clear.  Spot the wind shear too during the cold front passage.

March was sunnier than average for Reigate with 118.6 hours of sunshine.

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UK-wise the month was sunnier but wetter in the south, mostly due to heavy rain associated with a deep low on 9 March and Katie later in the month.

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deep LOW brought most of March rain to the South

March was also slightly below average temperature in the south.  The CET came out 0.1C above average for March at 5.8C.

Air pressure was lower across the south of Europe than the north during March, hence the drier conditions to the north.

The Atlantic cool blob sea surface temperature anomaly continued to develop during March and this may have had some influence on moderating temperatures regionally.

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Globally, March was the warmest ever recorded March by a considerable margin at over 1.29C above the long term average for the month.

 

Whilst Europe was about average temperature, there were notably extreme temperatures across the Tropics, in particular Indonesia, parts of India and Australia, as well as tropical North and South America and parts of Africa.

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This continues the trend of warming shown in the graph below. Note the correlation of anomalously warm years with El Nino events.

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The current record breaking 2015-16 El Nino is fading fast and is forecast to be replaced by cooler than average Pacific equatorial sea surface temperatures known as a La Nina by the autumn.  This might have implications for a more active Atlantic hurricane season.

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Met Office March summary

refs

http://www.metoffice.gov.uk/climate/uk/summaries/2016/march

 

2016-03-27_08-43-08

Before we get to Katie, it’s worth mentioning recent and current weather for Easter Sunday: the first convective day of 2016.  A cold pool of polar upper air followed the impressive cold front yesterday which developed a very long squall line.

Thundery conditions followed the cold front in an unstable Polar airmass with notable CAPE (convective available energy) and negative 2 Lifted Index for the time of year – both indicators of potential thundery heavy shower activity.  The limiting factor on multicell thunderstorms locally was lack of wind shear and an anticyclonic jetstream, both subduing anything truly impressive or sustained. Nevertheless, some hail and thunderstorm activity is rife across the UK today in this unstable polar maritime airmass.

Storm Katie is riding a powerful jetstream and deepening rapidly over the Atlantic through today.  She is some 600miles away to the SW but approaching and deepening rapidly as she undergoes rapid cyclogenesis (RaCy) falling from 999mb to 975mb:  24mb in 24 hours, qualifying as a “bomb depression”.

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Katie emerged out of Canada earlier this week as cold continental air met humid sub-tropical maritime air.  She engaged with an active jetstream to be launched across the Atlantic.  Storms tend not to move fast and deepen rapidly at the same time, so her rapid progress across the Atlantic will now slow as she now expends more energy lowering pressure on her approach to the UK.

The evolution and track of this storm is similar to St Jude October 2013, but Katie is forecast to be less powerful, due in part to the slacker pressure rise on her departure into the North Sea.** (update: in the event Katie pipped StJude, see below!)

Katie also has some similarities to the October 1987 but again is significantly less powerful than that rare 200 year event.

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The water vapour sat pics below show the evolution of Katie and cirrus cloud associated with the jetstream.

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Baroclinic leaf in the Atlantic, jetsream cirrus to the UK

 

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dark slot shows rapid intensification stage as strat air descends into LOW, showers over UK

Storm Katie is forecast to deepen through the SW approaches to below 980mb on an expected track overnight Sunday-Monday from Biscay into the Bristol Channel through to exit around the Wash.

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The pressure gradient between the Isle of Wight and the Wash could be as much as 12mb… the highest of our Wight-Wash Oscillations recorded, and greater than the 10mb WWO recorded in StJude, if this comes off.

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However, the pressure rise behind Katie is expected to be less rapid and sustained than St Jude, so this critically reduces the potential for highest gusts, sting jets aside.

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The highest wind speeds for Storm Katie will be south of the low core in the occluding warm sector in the small hours of Monday morning.

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Max gusts of over 80mph are possible in the Channel, 70mph along Channel coasts and 60mph further inland.  For Reigate this means a brief episode of gusts possibly exceeding 50mph and more over exposed hills.  This could wake people up momentarily as gusts roar through trees and round houses.

Rainfall will be significant too and some hi res models put down >40mm over parts of the North Downs, showing the significance of small hills in generating orographic rainfall.

Rainfall more widely is more likely to be around 20-30mm, enough for local flooding. The MetOffice have issued a yellow warning for Storm Katie.  Check the weather impact matrix showing moderate likelihood of moderate impact (low yellow).

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Update: Amber warning issued for Storm Katie.

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The shipping forecast is less equivocal and shows storm warnings right around the UK associated with Katie and other low pressure systems.  Storm Force 10 is possible in the Channel for a while. http://bbc.in/1RtwvyS

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Furthermore, evolution so far matching StJude and the Wight-Wash pressure gradient forecast seems to be matching or beating St Jude.

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Below is a Meteosat picture (courtesy Dundee sat receiving station) showing Katie moving into the North Sea, deepening again slightly to 972mb,

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Meteosat visible photo of Storm Katie moving into North Sea on Monday 28 March

Post-Katie we enter a broadly zonal westerly flow but turning cooler as slack winds swing northerly through the week as LOW pressure moves into North Sea/Scandinavia. Unsettled and showery activity continues through the week but with pressure slowly rising from the west by Thursday. No further significant storms are expected for the time being. Into the start of April, pressure over the Atlantic is expected to rise and progress over the south of the UK into Europe to give a settled period for the start of April.

RGSweather is off to East Iceland, so expect occasional tweets and updates from cooler climes.

Storm Katie: In the Event:

various meteorological impacts show Katie exceed StJude in power but both fall well short of October 1987 Great Storm.

  • Max wind gusts Needles 105mph (St Jude 99mph)
  • Reigate max wind gust 51mph, widely 50-60mph and 70mph on North Downs (Kenley) Gatwick 59mph
  • Redhill aerodrome recorded 70mph gust at 04:50
  • Lowest pressure: 971mb (St Jude 2013 976mb; Oct’87 958mb)
  • Reigate lowest pressure 976mb (St Jude 980mb; Oct ’87 est
  • Reigate rainfall 20mm
  • Wight-Wash Oscillation 14mb (St Jude 12mb, Oct ’87 storm 20mb!)
  • Rapid rise in River Mole catchment – no significant flooding but overtopped bankfull stage.

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Note, compared with the Oct ’87 storm Katie and St Jude are found wanting.  The Wight-Wash Oscillation for the 200 year storm of October 1987 was a whopping 20mb! See the chart below for the exit of the Oct 87 storm through the Wash on 16 October 1987.

Finally, which of these big SE storms are most powerful? Play SE storms TRUMPS to find out..

2016-03-28_12-13-54

http://www.bbc.co.uk/news/in-pictures-35910151

http://www.bbc.co.uk/news/uk-24708614

http://www.chichester.co.uk/news/update-more-than-600-incident-in-sussex-as-storm-katie-batters-the-south-1-7299324?utm_medium=twitter&utm_source=dlvr.it

http://www.telegraph.co.uk/news/weather/12205613/Storm-Katie.html?frame=3601967

http://www.bbc.co.uk/news/uk-england-35909651

http://www.dailymail.co.uk/news/article-3512119/Storm-Katie-batters-Britain-Gales-100mph-dozens-flights-cancelled-UK-wakes-Bank-Holiday-washout.html

http://www.theguardian.com/uk-news/2016/mar/28/storm-katie-flight-diversions-wind-rain-batter-southern-england-and-wales

http://www.mirror.co.uk/news/uk-news/see-full-carnage-storm-katie-7644830

local coverage

http://www.dorkingandleatherheadadvertiser.co.uk/Storm-Katie-Trees-power-Surrey-storm-hits-PHOTOS/story-29006658-detail/story.html

2016-03-20_09-26-16

High pressure has dominated the last week of our weather but it has turned out disappointing here due to cloud cover lingering under a persistent temperature inversion, not unusual for this time of year.  Lingering decaying fronts have caused drab stratocumulus cloud to spread out beneath an anomalously warm upper air mass causing anticyclonic gloom for much of the SE.

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Any convection has been limited to the lowest 1km and been unable to break the inversion, so cloud, unable to rise into cumuliform tufts associated with the stronger sunshine in April, simply spreads out into a boring grey blanket, especially when the flow arrives from the North Sea bringing additional moisture in the lower layers. An inversion is when temperatures increase with height through a part of the atmosphere, usually referring to a lower layer.

So, whilst upper air temperatures have been anomalously warm, the surface temps have been kept disappointingly low.  Somewhat “upside-down” weather.

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This is because the Spring sunshine has been unable to break through the cloud and warm the surface.  The exception has been the north and west of the country, especially the hilly parts of Wales and NW England, which have enjoyed more sunny days.

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anticyclonic gloom

Today it was the turn of the E/NE coast to get the sunshine as weakening fronts shifted south around the high.

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The coming week sees the HIGH slowly deflating, like a sad party balloon, into the sub-tropical Atlantic.  A couple of powerful late winter storms emerging out of the NE US and Newfoundland start the onslaught to break a westerly unsettled flow back across the Atlantic by Easter.

For us in the SE this change to unsettled conditions happens slowly but ensemble runs are showing around 20mm of rain is possible before the end of March. So we might expect some wet and windy days before the end of March, including a risk of a wet bank holiday period.

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A return to more mobile frontal conditions is not all bad news, especially in the SE.  A westerly flow with weakening Atlantic frontal systems will break the gloomy cloud cover and bring sunny intervals and showery episodes to clear the pollution phase we have experienced lately.  The risk for us is any fronts stalling over the SE in front of a European HIGH – this situation can dump fairly large amounts of rain. Chart below shows wind speed and an active cold front for Sunday, too far off to be reliable but worth watching.

 

The multi-model charts below show this change from HIGH to LOW during the next week.

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Note the model agreement below by Saturday for SW winds, bringing temperatures up possibly into the mid-teens in the SE.

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In short, we can expect the weather to turn the “right way up” again and we should enjoy more mobile, fresher, brighter conditions, albeit with breezy episodes of potentially heavy rain at times. The charts below show temperatures rising in the SE but rain returning by Friday.

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Further into April there are hints that pressure rises across the Pole which will bring the potential for further unsettled cooler conditions during the school holiday period. RGSweather is off to Iceland (East fjords) again so this could mean some nice cold conditions for our trip there as the AO is expected to turn negative and the flow northerly, at least for the N Atlantic.  The UK appears to get stuck in an unsettled trough for early-mid April. Worth watching as JMA and CFSv2 both agree on this blocked pattern.

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February 2016 weather statistics for Reigate, Surrey, England.

  • Tmax 13C
  • Tmin -3C
  • T Average 5.3C
  • Rainfall total (CoCoRaHs) 48.2mm
  • Sunshine 99.6hours
  • Average MSLP 1010mb
  • rain days 18

Feb CET 4.9C (anomaly +1.1C)

Mean UK temp 3.9C

Storm Henry opened February weather with inclement wet windy conditions for the far NW but no significant impact here in the SE.

Storm Imogen on the 8 Feb impacted Wales and the SE more with the highest gusts of the month exceeding 41mph here in Reigate, with a max gust of 81mph on the Needles, Isle of Wight.

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February was another mild winter month, though not on the scale of record breaking December 2015.  February UK average was 3.9C, the Central England temperature (CET) for Feb was 4.9C, an anomaly +1.1C above the long term average. Locally our February average was 5.3C showing that the SE was warmer than the rest of the UK.

February was the sunniest winter month with nearly 100 hours of sunshine, compared to 94 hours in January and a dull 80 hours in December.

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The mean 500mb heights for the month (above) shows a trough placed over the UK for much of February and an overall Atlantic flow bringing milder than average temperatures, mainly to the south and SE. The continent was even warmer with some anomalous temperatures exceeding 10C in E Europe and Russia.

Rainfall for February was about average for the month at 48.2mm, similar to the MetOffice official SE England precipitation figure of 49mm.

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Global mean temperatures turned out warm again for February, in fact 0.47C warmer than the previous record.

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stratospheric warming February 2016

A notable atmospheric event that went mostly unnoticed at the surface was a significant stratospheric warming in February (and March).  The sudden warming of temperatures high in the stratosphere, much anticipated by the weather community and long range forecasters as a key tool to forecast winter conditions in the Northern Hemisphere, came too late to cause much cold winter weather in Europe.  El Nino winters are often accompanied by Sudden Stratospheric Warmings (SSW) events in late winter.  Such events can cause cold weather in mid latitudes as pressure eventually rises in the troposphere some 2 weeks after a SSW.  The warming during an SSW is truly remarkable, with temperature rises exceeding 80C in a matter of days at 10hPa (30km), from -80C to +10C in some SSW events.

An SSW is like inflating a balloon in the high atmosphere, pressure is expected to propagate down and eventually rise in the lower atmosphere some weeks after an SSW. Importantly, the pressure rise often disrupts the zonal westerly flow of the jetstream.  This appears as negative anomalies on the zonal wind chart below (blue colours). Unfortunately for this winter, the SSW came too late for much significant winter cold to penetrate into Europe.  You can spot that the anomalous easterly winds only got going in March… too late for deep cold to push into Europe.

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The JMA snow charts below also show how February came out as an anomalously un-snowy period for much of Europe.

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http://www.metoffice.gov.uk/climate/uk/summaries/2016/february

 

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After a pleasant dry and sunny day in the SE, the satellite photo from this evening spells trouble ahead for mid-week with a deep depression over Iceland and an increasingly active Atlantic with a long frontal boundary trailing across the ocean into thick bands of cloud.

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Reigate and the south of England are set for a potentially very wet day on Wednesday as shown on the Euro4 chart below showing just 12 hours of rainfall during the morning.

Unusually high rainfall totals could mount up, possibly to around 30mm for the day on some models. However, models usually exaggerate rainfall totals but it is likely to be soggy!

A strengthening Atlantic jetstream is causing the convergence of moist sub-tropical and polar air at the Polar Front over the Atlantic Ocean.  The winds can be seen converging on the Atlantic chart below.

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The convergence of Polar and Tropical airmasses can also be seen on this chart showing the trajectory of winds arriving in the UK on Wednesday.  Note the surface tropical airmass circulates round the Azores high and meets the incoming Polar air from Canada.  It’s the less dense moisture laden maritime tropical air which is lifted over the cold, enhancing rain on frontal boundaries.

The 850hPa (15oom) temperature chart below shows the steep temperature gradient between contrasting airmasses across the Atlantic.  The water vapour Meteosat satellite picture shows a broad sweep of moisture laden air crossing the Atlantic from the Sargasso Sea.

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The boundary of the contrasting moist Polar and humid Tropical airmasses causes lift and this is set to rapidly form a depression over the UK later on Tuesday into Wednesday courtesy of a jetstreak to the west of the UK.

Large amounts of Atlantic moisture are set to converge in this low pressure as airmasses meet at frontal boundaries.

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The result over SE England is an unsually steep rise in dew point to 6-7C, indicative of increasingly moist air.

The atmospheric column looks to become exceptionally moist on Wednesday and saturated through to a height of 25,000 feet.  Cloud depth will make it a very dull day.

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Rainfall charts look impressive and, at the moment, show the rain arriving on Wednesday morning. Here is a medley of rainfall charts from 3 different models showing the potential for a deluge, though do note that models tend to exaggerate these totals this far out.

The 6 hourly total chart from GFS shows an extraordinary 26mm over parts of SE on Wednesday morning.  This would lead to local surface flooding on roads.

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Note this is not a convective event so no thunderstorms are likely, which makes such high rainfall even more unusual. It could be the biggest daily rainfall total for quite a while, over 36.6mm of rain in a day was recorded on 24 August 2015. Keep posted on twitter and check MetOffice forecasts for updates if travelling.  Some disruption could occur if this comes off as models suggest.  Milder and settled conditions are expected into the weekend after our mini-monsoon!

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8-10 day mean upper air chart

The most wintry set-up of the “winter” has decided to arrive at the start of Spring! High pressure over the Pole is still keen to push out polar air into mid-latitudes as the meteorological Spring starts tomorrow.  The Arctic Oscillation shows this tendency as it has been dipping negative, showing relatively high pressure over the Pole and lower pressure in mid-latitudes. The belated rise in Polar pressure has been attributed to the stratospheric warming which occurred around a month ago.

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Unfortunately, for most of late winter the jetstream has remained strong enough to push a predominantly Atlantic flow into the UK and breach any blocks attempting to drag in sustained cold air. This is shown by the slight but persistently positive North Atlantic Oscillation below: this set-up spoilt any chance of proper cold this half of the winter.

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However, this week the wind is expected to turn more northerly as pressure lowers first over the North Sea and then the Channel / N France. By the weekend the UK will be in a cold Arctic flow courtesy of the same LOW lingering over Europe and a blocking Atlantic ridge. Between them and the jetstream they will do a good job of pulling down a cold Arctic flow this weekend. Spot the cold anomalies in the chart below. Lots of lying snow is not likely but some wintry precipitation is possible at times here, especially a marginal chance on Friday am.  Things change though, so check weather professionals like the MetOffice to make any weather related decisions.

Here is the story of how we get to some belated cold by this weekend.  Tuesday sees an end to the cool clear HIGH that brought Spring-like sunshine to Reigate and Tmax 9C.  A warm front will sweep in tomorrow morning and bring rain for most of the day for the SE.  It will be breezy, though relatively mild in the warm sector shown below, Tmax 10C.

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warm sector Tuesday

Things cool off into Wednesday as an active blustery cold front ushers in colder polar maritime air through the morning.  Any snow is restricted to the NW of the UK.

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The flow swings increasingly to the north west during the day with the possibility of showers later here, some quite heavy, maybe even with the outside chance of hail and thunder thrown in.  Tmax 6-7C.  Update: The video below shows how that showery trough passed through Reigate during Wednesday:

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Thursday is the crux to building a set-up capable of producing any snowfall at all for the SE.  A LOW is forecast to develop from a system off Greenland and cross the Atlantic smartly from the NW.2016-02-29_19-50-10

This low, with rapidly occluding warm sector, will deepen slightly and meet the cool pool sitting over the UK from Wednesday’s Polar attempt. The LOW is expected to arrive late Thursday and track south east overnight into Friday bringing in a cool NE flow capable of wintry precipitation for a while on Friday morning.

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The latest charts suggest the track into N France will result in NE winds which could bring snow for a time in the SE on Friday morning.

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meteoearth Friday ppt

It is all rather marginal for us in the SE and Reigate though. A rain / sleet event with a possible wintry mix at times is more likely and nothing much is expected to settle.

By Saturday and into the weekend a cold northerly / NE flow sets as the “Greenland” LOW settles over Europe. Wintry showers could develop across the SE, especially in any distrubances in the Arctic flow.  However, pressure is likely to be on the rise as the Atlantic ridge creeps in from the west under an increasingly anticyclonic jetstream.

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How long any early Spring cold lasts is not certain, the Atlantic ridge looks like toppling over to bring in warmer conditions later next week.

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The UKMet analysis chart for today shows a building ridge in the Mid-Atlantic and a complex large LOW over Scandinavia.  This is dragging down a cold Arctic airmass, which will usher in progressively colder upper air each day through to Thursday.  The low NW of the Azores is drifting ESE but filling and can be ignored for now.

Air at 850hPa, roughly 1500m up, is used to judge airmass characteristics because, at this height, the airmass is not affected by diurnal and surface changes like sea, forest, mountain, towns, which can create big temperature variations near the ground.  The 850hPa temperature by Thursday could dip as low as -8C over Reigate.  Whilst this is not extra-ordinary it is about the coldest and most sustained chilly dip we have seen so far this winter.  Skies will be mostly clear and nights will turn frosty with day Tmax struggling to 5-6C. Wind chill will make it feel more like freezing for most days this week.

High pressure nudging in from the Atlantic will keep any precipitation light and restricted to coastal areas.  Friday sees things get interesting and unusual.

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A southerly diving jetstream gets into action on Friday and is set to amplify the 500mb trough and deepen a low off the west coast of the UK and develop it further into Biscay into Saturday and create a cut-off feature by Sunday.

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The Atlantic / Biscay LOW is unlikely to impact us in the SE much directly but, as it passes south of us through the weekend into Europe it will drag in cold easterly and then NE winds.  Our own mini-Nor-Easter!

Nor-Easters are famous powerful winter storms in the USA.  Our own version this weekend is a rather tame feature in comparison but notable because it is relatively unusual and has potential to bring a more sustained wintry feel to the SE through the weekend and maybe into next week.

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This southerly tracking LOW will move NE through the continent and will set up a cool Easterly then NE wind over the SE. Nothing outrageously cold for us because the coldest air is likely to stick further to the north east in Russia and Scandinavia.  Nevertheless, by Sunday a brisk NE wind with upper air of -8C or so will continue to make it feel chilly, after a cool week.

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The winds over the SE will therefore swing from  Northerly mid-week through to Southerly on Friday and thence to Easterly and finally Nor-Easterly / Northerly by Sunday. This anti-clockwise rotation is called backing and often ushers in cold air.  This is despite the wind turning through a seemingly mild southerly direction.  Remember we are sitting well north of the jetstream this weekend, which is somewhere in the Mediterranean, thus all airmasses are relatively cold and polar.

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Pressure stays relatively high throughout this “mini-nor-easter” episode and so this will limit chances of any snow unless the low decides to track further north nearer the UK OR we pick up sea effect snow as winds turn NE over a relatively warm North Sea.

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Snow showers are theoretically possible near the North Sea coast if Sea Effect / Lake Effect snow can be kicked off.  Such sea effect snow occurs when (very) cold air masses cross warm sea surfaces.  This can trigger lines of convective showers that dump lots of snow in places like the Great Lakes in the US.  Sea Effect snow occurs best when there is a temperature contrast of at least 13C between 850hPa (which must be well below freezing of course) and a warm sea surface.

The charts above show the North Sea is anomalously warm at more than 8C across a large area, although cooler near UK shores.  The upper air temps this weekend are around -8C, making a potential 16C contrast in temperature, theoretically sufficient to trigger showers. Unfortunately the airmass seems to be dry and, at this stage at least, rather stable. Looking ahead the cold spell could last into next week. Check our twitter account @rgsweather for local updates but always check professional weather forecast providers for decision making, of course.