Archives For wind

Imogen is the ninth named MetOffice storm this winter.  She formed in the Atlantic in an area of steep temperature gradients under control from an active jetstream.

 

Storm Imogen is deepening rapidly today to 953mb, though on arrival in the UK she will be occluding and filling gradually to above 960mb on her track over N Scotland into the North Sea on Monday. The exact track makes a big difference to where the strongest winds are.  Current trends are for the storm to pull wind fields further north so impacts could be less than expected. Keep an eye on the MetOffice forecast as things are likely to change. Below is an outline of Imogen’s likely activity:

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Unlike the previous eight named storms, Imogen has a more southerly track, guided by a more southerly tracking jetstream, and the field of strongest winds and heavy rain are possibly set to impact the densely populated southern part of the UK, including the SE. High waves are also expected on the Channel coast.

 

Strong winds on Sunday night will be associated with Imogen’s fronts running ahead of the depression.  The cold front is an active kata-front, associated with descending cold dry air from the stratosphere running ahead of the surface front and enhancing lift and potentially generating heavier rain and gusty conditions (image and info courtesy UKweatherworld).

On Monday gusts up to 80mph on the Channel coast are possible, while inland the MetOffice consider 60mph possible in exposed places.  Around Reigate and sheltered parts of Surrey, 40-50mph gusts are more likely.  The North Downs could see gusts approaching 60mph. The strongest winds for the SE are likely to be through midday and in the afternoon.

Yellow warnings apply to inland parts of Surrey and SE England while the entire Channel coast has an Amber MetOffice warning. The first impact will be frontal rain tonight.  Fronts passing through overnight into Monday could drop over 20mm of rain in places particularly linked to the occluding “triple point” forecast to cross the SE overnight.

 

Monday is likely to see showers, some heavy, appearing through the day.  Warm sea surface temps in the Channel are likely to cause more on the coast but the brisk winds could bring them inland as the day progresses.

The cause of the strong winds behind the cold front on Monday is a steep pressure gradient.  On Monday tightening isobars show the steep pressure gradient bringing gusty showery conditions in unstable Polar Maritime air behind the cold front later on Monday.

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The Wight-Wash Oscillation (WWO) measures the pressure difference between The Wash and the Isle of Wight and is designed as a guide to wind speed in the SE corner of the UK. The WWO on Monday shows a significant 16mb gradient between The Wash and the Isle of Wight on the WRF model.  The Euro4 model has a more modest 12mb WWO.  16mb would be the largest WWO pressure gradient recorded and greater than St Jude, which was 12mb.

On Tuesday models show a wave depression bringing more rain to the SE, some even show fleeting wintry precipitation on the back end of this low as colder air ingresses from the north.  This is unlikely to be significant, at least on Tuesday, as upper air temps remain mostly too high for snow in the SE.

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ECM colder flow mid-week

Colder conditions are preferred by the ECM as northerly winds bring cool polar air further into the country through mid-week.  The Arctic Oscillation is again going negative which shows pressure rising over the Poles trying to push Arctic air south into mid-latitudes.  However, the NAO remains positive so Atlantic depressions will continue to bring frontal depressions for this week.

The 8-10 day mean shows a deep trough over the UK meaning low pressure and unsettled conditions remain likely into half term.

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The ECM builds heights over southern Greenland which links with higher pressure over the Atlantic, a more northerly feed of cold polar air is likely in this scenario into half term . The ECM has been outperforming the GFS so the more Atlantic driven GFS chart would be the less favoured option.

The Sudden Stratospheric Warming going on over the Pole is another astonishing feature of weather at the moment.  Today (Sunday) temperatures in the stratosphere over Siberia has got up to an amazing +12C from a more usual -70C.  SSW events often build pressure over the Polar troposphere a few weeks later which can cause cold incursions into mid-latitudes.  This is by no means certain but is perhaps our last chance of any sustained cold this winter… if it were to happen it would be late Feb/March. One to watch!

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak

 

This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

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The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.

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The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.

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Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.

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Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.

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link to accuweather take on this system

http://www.accuweather.com/en/weather-news/winds-to-whip-uk-north-sea-coa/43063930

 

Three King storms?

December 21, 2013 — 1 Comment

Prepare for more stormy weather over the festive holiday… Reigate and Surrey is usually sheltered from the most extreme weather action but Mon/Tues could see significant weather even here, so watch forecasts if you are travelling. Check UKMO warnings for details. http://www.metoffice.gov.uk/public/weather/forecast/reigate-surrey#?tab=fiveDay&fcTime=1387584000

A powerful jetstream, blowing at up to 275mph across the Atlantic, is continuing to drag a train of storms to the UK through next week, though Christmas Day itself looks like a relatively quiet cool respite for us in Reigate.  Inland across the SE is usually sheltered from deep low pressure systems that track across the NW of Scotland: so far we have escaped the worst of these storms.  This week, there are 3 major storms that are due to arrive over the UK bringing gales and heavy rain to many parts.  Each storm brings progressively cool airmasses to the UK from an increasingly more polar origin.  So, let’s call these storms The Three Kings: Melchior, Caspar and Balthazar, the biggest of these is likely to be Caspar shown below in stark detail on the recent NOAA Atlantic forecast run. See if you can spot the UK under that mass of isobars and wind feathers (each feather = 10 knots).

hurricane force

hurricane force

First off, Melchior brings gold: heavy rain! Melchior has already arrived and is set to bring a windy and wet Saturday to the SE.  The notable feature of this storm is shown below as a fast moving cold front and remarkable clearance as polar maritime air sweeps in with showers.  Note how the wind direction is essentially from the Atlantic source, running over warm-ish sea surfaces to arrive here, hence our mild-ish temperatures. For the SE it is likely to be most windy around Saturday lunchtime, with gusts around 30-40mph and lots of rain adding up to 10-20mm during the course of the weekend. Local flooding possible.

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Reigate Priory Park 21 Dec around midday: wet!

Next up, Caspar: bearer of high winds and heavy rain! On Monday-Tuesday a LOW is set to rapidly intensify on the left-exit region of the jetstream.  A remarkably low central pressure of 928mb is forecast on current GFS and ECM runs, while the UKMO brings it down to 940mb (still v low). This storm exhibits all the hallmarks of rapid cyclogenesis and a “bomb” style depression: pressure falling very quickly (20mb in 3 hours) producing exceptionally high winds: knowing where these winds will occur is the tricky bit!  Caspar is set to track close to NW Scotland, with a jetstreak feature bringing unusually powerful winds across the SE on Monday night: so we may not escape the worst of this storm.  Model runs are currently bringing 70mph+ gusts to some parts of the SE.  So wind speeds could be higher than StJude for some locations: remember StJude was tightly located along the South Coast, Casper is a much bigger storm potentially covering the entire country in stormy weather at times.  These extreme wind forecasts may moderate (as the GFS usually does exaggerate things!) nearer the time but it is best to assume that Monday and Tuesday will be inclement weather for Reigate, the SE and the whole country: all models agree on this.  At 928mb, Caspar may not quite be a record breaker for low pressure but it is still a storm to watch, with a central pressure equivalent to a Cat4 hurricane!  (In 1884 a storm reputedly had a central pressure of 925.6mb, the lowest central pressure measured OVERLAND in UK; see below and strongest modern storm since 1993 Jan storm 914mb) Caspar unlikely to beat either of these records.  Caspar could bring snow to the north of the UK, especially to high ground.

Balthazar: the mysterious one as yet: after a quieter Christmas Day another storm is looking likely to emerge later next week that could drag down somewhat cooler air from the Poles.  This is way-off so details are sketchy but a gradual cooling with more direct influence of polar air might be a feature of late December. Throughout next week enjoy the festive period but if you are travelling it will be wise to check the UKMO forecast.

pics above from weatheronline, weatherweb.net and netweather

The 2013 December 5-7 North Sea storm caused “the biggest UK storm surge for 60 years” (UK Environment Agency).  With associated gales across Scotland, coastal flooding in North Wales, Merseyside and the UK East coast, tidal river flooding in Hamburg, the closure of all major North Sea coastal surge barriers and disruptive snow further south in Europe, this storm system was arguably more powerful than StJude back in October.  Thankfully, this storm only killed 7 people across Northern Europe (http://www.bbc.co.uk/news/world-europe-25243460).

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Essentially a storm surge is a higher-than-normal sea surface caused by low air pressure coinciding with high tides which, when thrown into shallow coastlines by winds, can produce exceptional coastal flooding.  A surge can also include associated lower-than-normal water levels with off-shore winds pushing water away from the coast at low tide.

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This post outlines the factors that makes the North Sea so vulnerable to storm surges and, further down, there is a summary of some impacts and a quick resume of the successful responses to this hazard event with some useful links.  Finally, before we get too smug and chill out entirely about future storm surge hazard…will development land lapped-up on exposed coasts, for example in the Thames Gateway, increase our future vulnerability in the face of sea level rise and climate change?  Is it sensible to build in these locations?

Animation shows storm surge rolling round the coast and into the North Sea.

http://www.bbc.co.uk/news/uk-25229885

Causes

The North Sea is particularly vulnerable to storm surges because of an unlucky combination of factors that come together to occasionally make the “perfect storm”.  Fortunately, not every North Sea storm produces a surge!  Remember that Tacloban in the Philippines was hit by an even bigger storm surge generated by Typhoon Haiyan due to similar forces and a funnel shaped bay.  Compare videos on this blog to see the difference between Tacloban and North Sea surges. So what comes together to produce the most significant storm surge hazards in the North Sea? There are at least 6 factors that combine to produce the biggest storm surges: here they are:

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1. Sea shape and low lying coastlines: The North Sea is particularly prone to dramatic storm surges because it is open to the North Atlantic and then tapers towards the south in a funnel shape. This funnel shape has the effect of allowing strong northerly winds to direct storm surges towards cities like London, Amsterdam and Hamburg and surrounding vulnerable low lying coastal areas including Norfolk, Lincolnshire, Essex, Kent and the Netherlands. Some of these areas are at or below sea level and require sea walls and dykes and barriers to protect them during storm surge events otherwise they will be flooded. The 1953 storm surge broke the rather primitive sea walls of the time and flooded large areas of Essex and even more of the Netherlands causing the worst European peace-time disaster since the war and killing 307 people in the UK and thousands in the Netherlands (see you tube documentaries below)

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2. Sea depth/ bathymetry: the North Sea gets shallower towards the bays and wetlands towards the south.  These shallows have the effect of increasing the height of tides and surges as they are forced up over submerged shelves into narrowing bays.  This is possibly why Boston, Lincs and Hamburg suffered some of the worst flooding because surges were forced up bays and rivers.

3. Intense low air pressure: A 1 millibar reduction in air pressure allows sea level to rise by 10mm.  This effect can be replicated by sucking water up through a straw. The storm that crossed to the north of Scotland on 5 December had a central pressure of 976mb that deepened to 968mb over the North Sea. This is a similar central pressure to the storm that caused the 1953 storm surge that killed 307 people in the UK and 1800 people in the Netherlands.

4. Storm track: the LOW pressure has to track east over north of Scotland, which will drive a surge of water into the North Sea that is then pushed south by vigorous onshore Northerly winds into the low lying east coast of UK.  Ideally, the storm should deepen on its’ track across the North Sea, thus allowing northerly winds to gain in strength driving the surge and associated wind waves south.

LOW track

LOW track

5. High tides: high spring tides are the final requirement for the biggest surges.  Tides migrate as a bulge of water around the coast and, for the worst impacts, any surge travelling south down the North Sea must match the dome of the highest tide to produce the highest water levels in any one place. Since high tides occur twice a day it is quite likely that high elements of the surge will match a high tide level somewhere down the east coast.

6. Wind driven waves: Finally, surge and tide heights can be increased yet further by strong on-shore winds producing locally high wind driven waves that can over-top sea walls.

Warnings and impacts

The impacts of the 2013 storm surge included flooding in coastal towns on the east coast of the UK with perhaps worst hit being Boston in Lincolnshire. Houses on some vulnerable stretches of coasts such as Hemsby were washed into the sea as waves eroded sand dunes.  There was also significant flooding in Rhyll, North Wales and along the Merseyside coast at New Brighton (note: not Brighton!) where a Morrisons supermarket was flooded. The worst impacts on major populations and cities were avoided by the raising of the Thames Barrier to defend London and the closure of the flood gates on the Delta Scheme in the Netherlands.

The storm was modeled over a week prior to impact.  Initially GFS and UKMO models were seeing a cold surge as the main factor bringing possible snow across the UK but from about 6-7 days out it became increasingly obvious that the exact track and orientation of the LOW meant that powerful northerly winds and a possible storm surge were the greatest risk.  The UK Met Office, with Environment Agency, then started preparations for warning those at risk from flooding.  Most news channels were airing significant coverage from 24 hours out.

http://www.hulldailymail.co.uk/Yorkshire-Wildlife-Trust-blamed-flock-sheep/story-20301801-detail/story.html

Responses

Significant flooding did occur along the East coast, notably in Scarborough in Yorks, Boston in Lincs and Hemsby in Norfolk. In Hemsby some vulnerable houses located on the sand dunes were washed into the sea. Bridges near the sea were shut for a time, like the Humber Bridge; and rail services in some eatern counties were disrupted for a time.  Power was cut to homes in Scotland due to high winds.  Hundreds of residents were evacuated prior to the floods in various locations but some claimed to have little warning.

The worst impacts were successfully controlled by the massively impressive engineering schemes built since the devastating 1953 floods.London has nearly 200 miles of flood walls and 8 barriers holding back the tidal Thames. The Thames Barrier was opened by the Queen in 1982.

The Eastern Scheldt storm barrier was closed for the first time since the 1970’s.  The Netherlands barriers are built to withstand a 1 in 10,000 year storm surge event so it is perhaps unsurprising that they easily saw off this event.  It is also noteworthy that the Dutch have great faith in their storm surge protection barriers.

These measures, along with warnings and on the ground assistance for places that were flooded, proved extremely effective.

http://www.bbc.co.uk/news/uk-25272050

http://www.bbc.co.uk/news/world-europe-25242991

Further useful links on 1953 and 2013 storm surges:

1953 storm surge: original newsreel and timewatch documentary

http://www.metoffice.gov.uk/news/in-depth/1953-east-coast-flood

The sting in the tale?

London is sinking into clay and, along with the rest of the SE, it is tilting into the sea partly due to an epeirogenic / isostatic adjustment taking place since the glaciation released the north of the country from the burden of millions of tonnes of glacial ice causing positive isostatic rebound in the north and related subsidence in the south.

Flood plains and reclaimed land exposed to storm surges are still being lapped up by hungry developers as places ripe for building, like the Thames Gateway in London.  But is it sensible to concentrate massive new urban development in low lying areas vulnerable to coastal flooding when we have sea level rise and climate change?

http://www.deeestuary.co.uk/news1.htm

 

The potential for a major storm hitting the South of the UK on Monday remains. The exact track and intensity are still uncertain, varying from N France to Scotland and having varying impacts on the south of the UK, from very little to a lot, depending on the final track.  However, best to be forewarned! Several models show a vigorous LOW developing to the SW of the UK due to an extremely lively 180mph jetstreak or potential stingjet over the Channel. This has the effect of lifting air rapidly off the surface and lowering pressure rapidly.  A rapid lowering of pressure to produce a storm is sometimes called a “bomb” and is similar to the propagation of the October 1987 storm (not a hurricane!).
As it looks at the moment… the LOW will cross the UK from SW to NE and develop Force 10 SW gales in the Channel during Monday morning and, as it moves into the North Sea, stronger winds are possible up to Storm Force 11 with potential for extremely rough seas and 70-80mph winds.

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Inland winds will be less powerful but gusts could reach 50-60mph widely in the worst case scenario. All from the S or SW and, later on veering to the NW. Waves of 3-5metres in the Channel are possible and 10 metres in exposed waters.
Overnight Sunday and Monday morning seem the most likely periods of highest winds for the South. The systems moves away rapidly into the north sea and winds ameliorate to sunshine and showers.  Thereafter, the rest of the week continues to look unsettled with wind and rain.  One to watch definitely!

Watch the propagation of wave heights across the Atlantic as the mother LOW moves in during the early part of next week.

 

Update: cool off will be a fall of 10c for much of the UK. Spot the difference below! These are Tmax temps: from high anomalies to October normal.UK cool down

The UK enjoyed being on the warm side of the HIGH this weekend and this will last for a day or two still.  However, a significant shift in the position of the HIGH pressure by models has placed it moving out WEST of the UK through this week leaving the door wide open for Arctic Air to spill out from Svalbard and make a dash for it down the North Sea. This air will be chilly! Later in the week the door closes on the Arctic but the cool air is left skulking very close to the SE in a LOW pressure of swirling showery cool air bringing rain threats to the East and SE. So while Reigate enjoyed 20c this weekend and some warm days Monday/Tues, by next weekend the best we can probably expect is half that, or thereabouts, and with heavy showers to boot.

arctic plunge

The HIGH moving west will have the effect of drawing down some brisk cold Arctic air directly from some islands called Svalbard (check out the web cam from today: this air is setting off our way very soon!) directly down the North Sea arriving in Reigate and the SE sometime mid-week having travelled across 2000 miles of gradually warming ocean surface, picking up heat and becoming UNSTABLE in its lower layers as a result.  Instability means heavy showers, drifting down on the chilly northerly breeze and threatening east coast districts and the SE. The cool pool of air will feed a LOW pressure over the near continent which will be cut-off as the HIGH topples over the North later this week, taking the jetstream and any warmer air, far to the North of the UK. The cool pool cut-off LOW thus produced is likely to bring showers to the East coast and SE England and lurk about for some time.
The HIGH in the Atlantic, meanwhile, is effectively blocking any movement of these systems and the weather is likely to stay put for sometime, gradually warming through but producing convective showers as it does so.

This blocked weather pattern with weak westerly winds and HIGH pressure moving slowly or not at all, is characteristic of the very negative North Atlantic Oscillation showing up at the moment.  Northern blocking and HIGH than normal pressure over the Arctic with lower than normal pressure over the Azores, allows Arctic and Polar air to leak out and flow south over Europe and the UK.  This was what brought our COLD late Spring 2013.  Is this happening again this year?

An interesting development which continues to back up several indicators that are showing the potential for a COLD winter ahead.  Salt and snow shovels at the ready! The chart below is a monthly averaged CFS seasonal prediction for February 2014 anomaly air mass temps: how far above or below temps are from the 30 yr mean.  Europe, on this chart, looks v chilly (but these temps are for 5000feet, not surface!).  This is not, however, a forecast, merely an indication.  These charts also flip-flop quite a bit so we need to check them regularly to get an idea of patterns.  So far, the cool pattern is prevailing.

feb 2014 anomaly

This weekend a big LOOP in the jetstream is set to rip up the summer heat still lingering over the southern part of the UK. In a dramatic weather battle in the upper atmosphere above our heads the southerly blowing limb of the jetstream will bring POLAR air to west of the UK to fight it out with the continental tropical heat lingering over the south and east. Heavy rain and strong winds will be the result of this battle but the exact location of the heaviest downpours is tricky to be precise about. Broadly speaking, the heaviest rain will start in the South of the UK on Friday and move North, rotating over to the NW and falling heaviest over the northern hills as the LOW pressure drifts NW during Saturday and Sunday. The South could have comparatively drier days after any heavy rain on Friday, but stay tuned for details on that.  Friday rainfall for Reigate could exceed 10mm starting in the afternoon – so possibly very heavy rain for a few hours Friday pm, with lighter rain through Saturday and possibly none at all on Sunday as the LOW moves away to the NW.

Very warm air drifting up from the continent over England by a northward blowing jetstream will be forced to rise over the markedly colder polar air invading from the NW through Friday and Saturday. The difference between these air masses is very marked: the warm 28ºC surface air currently over Reigate equates to -12ºC at 5000m while the invading cold Polar air mass is -28ºC at 5000m, which will push down surface temperatures on Friday to a Tmax of only 16ºC! It is this contrast between the air masses which is a hallmark of autumnal weather and the key to creating lots of rain: polar air meets tropical air, forcing it skyward, forming rain with gusty winds spiraling round LOW pressure: typical autumnal scenario (except this weekend’s scenario is actually quite unusual: called a trough disruption with the surface low drifting off in an unusual direction: from SE to NW, unlike usual LOWS which track west to east across the UK along a zonal jetstream).  Wind speeds for Reigate this weekend could gust at 25mph at times on Saturday. No thunder is now forecast this weekend as the polar air is relatively stable, being on a return leg to the Poles.  

Thereafter, as you can see from the ensemble below, next week looks to remain cool and unsettled but with some improvement in the south possible later in the week as pressure could rise by next weekend, but no return to hot conditions is expected.

ensemble sept 4

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.

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.

europe wet and dry may

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.

high and low pressure cross section

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.

air flows from high to low!

A powerful extra-tropical storm is hovering between Iceland and the UK for the next few days. The central pressure of this LOW has fallen to an astonishing 930mb, one of the deepest ever recorded in the Atlantic. Wind speeds of 90mph and 50 foot waves are being recorded. It has been named “Jolle”. It will not directly cross the UK but is forecast to send several fronts and “baby” storms our way: the potent fronts corssing Wales and delivering heavy rain and thunderstorms tonight are part of this new weather pattern associated with a strong jet stream. Warm, wild weather approaching: hang on to your hats!

http://www.wunderground.com/blog/JeffMasters/show.html

http://www.washingtonpost.com/blogs/capital-weather-gang/post/five-mind-boggling-images-of-freak-atlantic-storm/2013/01/28/cd5cf8f8-6982-11e2-af53-7b2b2a7510a8_blog.html