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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!

Update for storm prospects for Reigate: an intense low pressure is forming out in the far west of the Atlantic this evening where frigid air from Canada is meeting humid warmer sub-tropical air circulating from the south round the Azores high pressure.  This confluence of winds causes lift to occur at the polar front but a fast jetstream blowing at 180mph directly overhead will cause additional lift of the air lowering pressure extremely rapidly.

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rapid cyclogenesis under jetstreak, then further deepening on left exit of jetstream

This process is called rapid cyclogenesis and gives birth to a deep low pressure with tropical air circulating into the storm from the SW and polar air sweeping round the head of the storm to follow in its wake.

The low will deepen and race 1300 miles across the Atlantic to arrive off the NW UK coast by midnight Thursday.  For inland Reigate and SE England the impacts will be lower than in the west and NW but the English Channel will experience significant gales overnight in the warm sector of the depression with gusts possible of 70mph.  NW Britain and especially NW Irish coast might see the biggest gusts in the wrap around winds on the south side of the low core where stingjet winds are possible even as high as 100mph.

arrival on Thurs 00hrs

arrival on Thurs 00hrs

Winds for SE and Reigate will build through Wednesday from late afternoon and through the evening and are likely to peak at possibly 50mph gusts as the squally cold front passes sometime 4-6am.  Exposed places on the Downs could experience stronger gusts. Winds will ease after the cold front moves through by breakfast time but Thursday will stay blustery with showers and feel cooler.  There is a chance that our record wind gust recorded in Reigate of 52mph will be broken in this storm but the town is sheltered from southerly / SW winds which will be the dominant wind throughout the event so this might keep wind gusts lower.

For Reigate the rain is likely to arrive mid evening on Wednesday.  Rainfall will be persistent throughout the event and heavy at times and possibly with isolated thunderstorms as the active cold front moves across early Thursday morning.  Rainfall totals could amount to over 20mm, most of it falling in a short period probably at the cold front.

The weather on Wednesday will be interesting: starting with cold temperatures and scattered snow showers courtesy of frigid air originating from Canada and Greenland on a brisk NW wind and then warming through the day as the storm arrives with a deluge of rain arriving in sub-tropical air from the deep south Atlantic.

Finally, the storm is due to usher in cold polar winds which eventually swing to the north as pressure builds in the Atlantic to bring an Arctic flow across the UK during the weekend.  Various troughs and any low pressures sliding down the edge of the developing high pressure could cause a more significant snow event any time from Sunday and through the week.  This wintry spell is likely to last into next week so dig out the warm woollies! Any snow that we get in Reigate will be the first since March 2013. Lots going on and very changeable so stay tuned!

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So far this winter Reigate and the SE has been sheltered from much of the weather action, which has focused mainly on the NW and especially Scotland. This coming week might change that somewhat, albeit modestly in comparison with the battering the NW has received.  Charts below show temperatures and pressure are overall on the slide this week while precipitation makes two noticeable spikes.

A moist warm airflow ahead of a slow moving cold front arriving from the north Monday pm could stall over the SE overnight into Tuesday am and bring significant rainfall totals.  Temperatures Monday night could hold up into double figures as a warm moist SW flow funnels up from the subtropics ahead of the sluggish cold front.

slow cold front clearing south

slow cold front clearing south

The wave on the polar front emerges out of the SW through Monday with a characteristic plume of rain and a dip in pressure.  Such waves tend to bring a lot of rain despite hardly showing up on synoptic charts.  A modest kink in the front and isobars (see charts below) is the only hint of potential heavy rain action.  Some wintry precipitation and maybe snow is possible for places on the north side of the front later into Tuesday, in Wales/Midlands for example,  but not for the SE as temperatures remain too high.  The front slips slowly south Mon/Tues bringing some significant rain and then brisk cooler weather from showery westerly winds to follow, some sparse showers could possibly be wintry Tues-Weds but not amounting to much for us, most wintry showers will fall further west.  Temperatures will be cooler at Tmax 5-6 on Tuesday and even cooler on Weds, so cold but frost unlikely as too breezy.

There is a weather warning out for this period especially for the south coast but Reigate totals could reach 20mm+ and exceed 20-30mm for places nearer the south coast in Sussex.  The front is followed by increasingly cool showery westerly winds through Tuesday into Wednesday with the odd wintry shower possible but nothing significant.

There is the possibility of a storm for the UK Wednesday-Thursday bringing gales to our area as a “bomb depression” emerges in the Atlantic this week and probably tracks across Scotland (which would be less windy as a result, for a change).  Details of the storm track remain wobbly at the moment but the strongest winds will be found on the southern flank of the LOW centre and various models suggest that the SE could have some strong to gale force winds, albeit nothing like the scale of Scotland recently.  Models suggest a steep pressure gradient as the low tracks somewhere across the north of the UK.  The precise track will determine the impact on the SE.  Current models suggest the strongest winds for the SE will be associated the warm front approaching Wednesday night / Thursday morning.  Gusts could exceed 50mph inland and more on the coast with some excitable models suggesting more but these usually calm down nearer the time! Nevertheless, wind and rain are likely to be significant and expect updates on weather warnings this week. Colder air follows the storm so showers following cold fronts could turn wintry so watch for updates.

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Our own amateur “Wight-Wash Oscillation (WWO)” is an attempt to quantify the potential impact / severity of storms on the South East of England.  The WWO is simply a crude measure of pressure difference between the Wash and the Isle of Wight ( a mini NAO!).  As a benchmark the St Jude storm (99mph Needles max gust) had a WWO of approx 10mb (976 Wash-986mb Wight) and the modest storm of 12/12/14 was a 9mb on the WWO.

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Current models shown below have a variety of WWO results from 8mb UMET up to 12mb for GFSP.  A difference of 1 or 2mb can make a big difference to the severity of the storm as pressure gradient ultimately drives wind speed. Nevertheless, the WWO is just for fun but interesting to see how it works out in the event. On the basis of our WWO this storm currently looks less severe than St Jude, which is a good thing because that storm caused a reasonable amount of damage.

The unsettled weather this week is being driven by an active jetstream over the Atlantic.  This itself is partly a product of the tremendous temperature gradient over the northern hemisphere and particularly over the North Atlantic with freezing air pouring out of a frigid Canada meeting warm sub-tropical air emerging round the Azores High from the south.  These air masses are set to clash on Monday at the polar front over the Atlantic.

Whatever the storm brings it is looking likely that afterwards a cool northerly wind from the Arctic will bring temperatures further down by the weekend.  Although a long way off, for Reigate and the SE it is unlikely to be snowy and most likely to be the frosty and cool type of cold!  Further ahead some models build a pattern into later January (around 20) that could be conducive to cooler conditions: with possible easterly or NE winds for a time as a low sinks into Europe, though this is not looking either particularly extreme or long-lived.

Meanwhile, the North Atlantic Oscillation is set to remain positive, though decline somewhat to neutral, which means that the LOW over Iceland and HIGH over the Azores synoptic pattern is likely to be maintained, albeit possibly less pronounced and with more anomalous meridional moments of cold incursions possible.  This would mean a continuation of the broadly zonal westerly flow of mild windy and fairly wet unsettled weather for the foreseeable future.  Conflicting signals like this have riddled longer range forecasters this winter: the stratospheric warming was only moderate, much to snow-lovers disappointment, and some supposed good indcators of colder winters (the OPI / early Siberian snow cover in October) have apparently failed to live up to forecasting skill expectations as yet.  A month or so of winter to go and no significant signs of persistent cold … yet!

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The MetOffice charts above show the formation and life-cycle of a December 2014 “weather bomb”, involving the process more properly called rapid cyclogenesis. There are reasons why meteorologists dislike the term “weather bomb” but perhaps the most obvious is that the sensationalist short-hand use of the term “bomb” detracts from the complex processes and variable scale and location of impacts.  The term “bomb” tends to hype stories in the press that can cause over-reaction and unnecessary concern. On the other hand it gets people reading about the weather, which is a good thing (like this post, ahem!).

Nevertheless, a “weather bomb”, a term borrowed from the US and New Zealand, is short-hand for a potentially extreme event.  Bomb depressions are deep low pressure systems that form by the process of Rapid Cyclogenesis (RaCy for short).  RaCy is the rapid formation of a deep depression when the central pressure falls more than 24mb in 24 hours.  Such RaCy depressions are usually of marine origin. About 12 such RaCy bomb depressions hit the UK in the exceptionally stormy winter last year 2013.  Although by no means the most powerful, the first and most famous RaCy depression of last winter was the St Jude storm that hit Southern England with moderate force in October 2013.  Pictures below are from that event and can be compared to the enormous scale of the more recent Atlantic bomb depression of December 2014.

The “bomb” depression that struck this December 2014 seemed to catch media attention, despite the impressive weather impacts being almost wholly restricted to the less populated NW, especially Scotland, where people are entirely used to coping with such lively weather.

http://www.stornowaygazette.co.uk/news/local-headlines/weather-bomb-cuts-off-power-to-18k-homes-1-3630321

December 2014 rapid cyclogenesis: the weather story

The December 2014 “weather bomb” was a depression (low pressure system) which formed rapidly far out west in the Atlantic between SE Greenland and Iceland.  The formation was associated with a fast moving jetstream and the surface convergence of sub-tropical air from the south west meeting a frigid NW polar airstream from Canada and more local air direct from the Greenland ice cap.  The big temperature differences between these air masses accelerated uplift and the lowering of central pressure.

impressive but not the day after tomorrow

impressive but not the day after tomorrow

Descending dry stratospheric air is another defining feature of RaCy systems.  Cold dry air from aloft turbo-charges the depression as it is injected into the depression.  The cold air aloft increases lapse rates in the surface airmass and causes air to rise more purposefully creating a dramatic fall in central pressure.  Descending cold dry stratospheric air can be spotted on the water vapour satellite images as a dark dry slot ingressing into the depression circulation over time and following hard on the heels of the cold front as it is blasted across the Atlantic.  The water vapour images below show the rapid development of the system during Tuesday 8 December.  In later images it is possible to see the speckly cumulonimbus clouds emerging in the unstable cold sector following the cold front. Such instability was caused by the descending dry air.

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Rather than going through the rather measured development stages of a Norwegian Model cyclone, a RaCy depression usually follows a life cycle more like the Shapiro-Keyser model below (though at the time of writing I am not certain as to whether the December 2014 RaCy depression formally fitted all aspects of this model).  Several key characteristics of the December 8 cyclone fit the S-K model fit and this is the usual model associated with RaCy depressions.

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The Shapiro-Keyser depression life-cycle model often features a cold front that is blasted rapidly ahead.   so rapidly that it “fractures” from the wrapping warm front further north. This is known as a T-bone fracture and experts can identify the moment of fracture using satellite photos. Additionally, cf course, upper air moves faster than the surface wind that suffers frictional drag even across relatively smooth ocean.

satellite features of emerging RaCY depression

satellite features of emerging RaCY depression

This meant that the cold front moved so rapidly that it split vertically into a fast moving upper front and a slower moving surface cold front. The cold front literally had its head ripped off!  The frigid upper cold air travelled over a shallow moist zone of warmer sub-tropical air and it is this that increased lapse rates and caused immense instability in the polar air stream that eventually arrived in Scotland.  Instability can be seen on the visible satellite pics as speckly masses of cumulonimbus clouds shown best in the satpic above.  In the charts and sat pics below note the wind speed associated with this polar air and the tropical air preceding it in the warm sector.

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In the S-K model the cold front is sometimes weakened during the formation process while the warm front remains active, wrapping itself in knots around the central “eye” of the storm.  The 850mb chart below shows temperatures of this cold upper air at 1500m above Scotland. The bomb depression this December seems to have matched this because, while the cold front was relatively weak (narrow squall line) the exceptionally unstable polar air behind it was arguably the defining characteristic of this system, bringing persistent convective storms and an outstanding 5000 lightning strikes and thunder-snow blizzards across higher ground in Scotland during the advection of this exceptionally cold and unstable air for an Atlantic NW airstream.

In the S-K model depression life-cycle the warm sub-tropical air is eventually left “sequestered” as a warm pool trapped in the middle of the mature depression which is called a “warm seclusion”.  The usual process of occlusion is bypassed as the centre of the low fills with warm air.  Meanwhile, the rapidly overshooting upper cold front causes S-K cyclones to often elongate in appearance on surface pressure charts, a feature associated with the rapid forward acceleration of the cold front in relation to the tightly wrapped, almost stationary, wrapped warm front. It is this tightly wrapped warm front (sometimes shown as occluded on weather charts) that shows another defining feature of S-K depressions.

As our initial bomb LOW pressure moved due east and filled and decayed offshore near Norway, a wave depression further south on the Polar Front also “bombed-out” to the SW of the UK and swept across Southern England on Thursday-Friday 11-12 Dec.

This was a separate small scale system but technically another rapid cyclogenesis as central pressure fell more than 24mb in 24 hours, but only just.  This illustrates the varying scale of bomb cyclones: some cover vast areas, some a small.  The 11-12 Dec RaCy depression was much smaller in size and intensity, max wind speeds were much more restricted and the whole system several magnitudes smaller in scale than the “mother” cyclone further north. Charts below show the evolution of this storm.

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Finally, the North Atlantic Oscialltion is a measure used to describe and forecast the mean pressure pattern over the Atlantic. A positive NAO indicates “normal” conditions with low pressure over iceland and high over the Azores. This is associated with a zonal west to east flowing jetstream and fast moving cyclones moving rapidly west to east bringing generally mild conditions to the UK in winter. Note the recent positive pattern matching the westerly flow and active zonal jetstream causing the RaCy depressions.  When the NAO turns negative the jetstream is often more wiggly and flows between latitudes in a more meridional flow potentially bringing cold air from the north when pressure patterns are more slow moving and even “blocked”.  A negative pattern is often associated with cold winter weather for the UK. The NAO is not a driver of weather, merely an indicator of pressure patterns.

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For a bit of fun we invented our own local Wight-Wash Oscillation (WWO) which is a measure of pressure across the south of England between The Wash and the Isle of Wight.  This would give an approximately similar local version of the NAO but just for fun!  We noted a WWO difference in pressure of 10mb during St Jude and only 9mb during the recent bomb wave depression.  The WWO particularly suits the passage of wave depressions across the Midlands which tend to yield the highest wind speeds for the SE.  It would also work in negative conditions which would give cold easterly winds in winter. Note this measure is just for fun!

Positive NAO remains likely on the run-up to Christmas 2014 so chances of a White Christmas is much reduced. Remember that a White Christmas for us in SE England is the rare exception to the rule.  On a brighter note, the earliest sunset has just passed and we can at least look forward to later sunsets from now on!

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Monday 25 Aug

Bank Holiday heavy rainfall wash out!

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Nasty LOW

Like much of the rest of England and Wales, Reigate and Surrey and the SE will see an August Bank Holiday wash-out tomorrow with lots of rain through the day, likely to be heaviest in the afternoon.  This has the potential to be a moderately severe wet weather event with 48 hour rainfall accumulation possibly topping 20mm in our region with some models taking this even higher to 30-40mm in places but this is probably over-egging things.  The WRF and NMM models push totals to 40-50mm in places while Hirlam and Euro4 (UKMET) keep a lid on rain at 20-30mm.  Anything approaching 40mm in 48 hours would be highly unusual for us in SE in Atlantic frontal depressions but it’s by no means impossible.  Remember that a “wet day” for us in the SE usually adds up to only around 10mm, so any totals over 20mm tomorrow will seem a significant deluge. Check the rogues gallery of models below for the worst rainfall offenders.  By the way, always Check the UKMET forecast for a professional look at the situation, this site is for a local perspective on weather and to generate enthusiasm and interest and share some more understanding of the air around us, especially in these times of such uncertainty regarding weather and climate.  As we approach another autumn and winter please share any of your weather stories and photos here or follow us @RGSweather on twitter.  Twitter is an amazing portal for sharing, learning and discussing the amazing fast moving world of weather… join it if you can.

Either way, surface water local flooding is likely to be a feature on roads tomorrow and the UKMET has issued a yellow warning for heavy rain.  You can expect a cloudy, dull, blustery, cool and wet day: so maybe snuggle up with the TV guide and check out some bank holiday films 🙂  and watch @RGSweather on twitter for exciting weather updates of course!

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The bank holiday soaking is courtesy of a low pressure in the Atlantic set to deepen to 981mb overnight as an unseasonably fast jetstream blasting through the Channel draws very moist air from the surface and lowers the surface pressure throwing all types of fronts our way and tightening isobars cross the SE during the course of the day. 

Today, the system lurked in the Atlantic as a significant band of cloud, as shown here by the VIIRS satellite at 13:00hrs.

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The system will bring rain first thing tomorrow to Reigate.  Initially rain looks to remain fairly light but will become progressively heavier through the day with even a low risk of heavy thundery showers as the cold front passes later pm and into Tuesday night (updated) before things gradually calm down Tues am. Whilst breezy through the day, max wind gusts are likely to arrive later in the afternoon but only 20mph.  This is certainly more a wet event than a wind event!

Into Monday morning: update:

Satellite pics show a rapidly developing occluded low pressure with tightly wrapped fronts.  The dark are on the sat pics represents high altitude air that is subsiding (sinking) into the depression.  This dry intrusion can cause increased instability around the cold front where it is forced to rise once more nearer the surface, increasing cloud formation.  Sometimes is can cause thundery doinwpours and some of the models are picking up on this by increasing rainfall totals even further for the SE as the frontal system passes over.  Interestingly this does not count as a “bomb” cyclogenesis which requires a 24mb fall in pressure over 24 hours (1mb per hour).  This has managed about half that pressure fall so far. 

IMPORTANT UPDATE 11AM Fri: UKMET extended AMBER warning to include all of SE inland including Reigate, Surrey, London: gusts of 60mph possible overnight. 

Latest update: (pm Thurs): one HiRes model just HALVED rainfall totals for storm tomorrow: if others follow that’ll be good news for flooding.  This storm is not resolved perfectly yet, some models flip and flop before the event.  Nevertheless, one to watch for signs that, perhaps, the rain is being over-done. In fact, rainfall totals have rarely matched model runs in recent events… fast moving fronts have simply zipped by so fast that rain totals have been rather low.  example: 8.8mm on 12 Feb front yesterday.

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Now you see it…?! Rain totals just halved on latest HiRes model run (left). GOOD NEWS for floods!

Update for Reigate and SE England on UKstorm Fri 14 – Sat 15 Feb:  the storm tomorrow is yet another serious weather event, this time focusing most intensely on SW, Southern coasts and SE England and Wales overnight Fri-Sat.  Below are some satellite pictures from today showing the rapidly intensifying storm in the Atlantic to the SW of the UK.

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The wind and rain forecast on NWP models for inland SE areas are severe: often the forecast winds and rain do not transpire, coming in at a good deal less than the modeled projections especially for sheltered towns where most people live.  However, the magnitude of figures consistently being predicted for gusts and total rain associated with this storm in our area are consistent enough to be of some concern, especially for places exposed to southerly winds (on hills like North Downs, for example) and at risk of flooding, either surface water flooding or in the flood plain for the River Mole or tributaries.

Rainfall will be heavy throughout Friday and could total over 20-30mm in 24 hours in places.  If this exceptional rainfall does occur then extensive local flooding is likely to result rapidly, especially along the River Mole catchment which has a flashy response to rain within 10hours.  Winds are due to possibly exceed those experienced this winter so far, forecast to peak at over 60mph gusts inland early Sat am.  Heavy rain is also likely to add to the problems of flooding.

UKMO warnings stand at Yellow alerts for Reigate and Surrey from Fri through to Sat 15 Feb and AMBER alerts for all places nearer the coast.  The storm is rapidly intensifying and will bring heavy rain all day to Reigate and Surrey and the SE as a whole through Friday, totals could be over 30mm, which will cause the River Mole to flood in places and plenty of surface water on the roads. Strong winds build gradually through the day and will peak overnight in the small hours of Saturday morning.  The winds, even inland, could even gust at 60-70mph and for southern coasts 75-80mph.

These are exceptionally strong gales for inland and, if these NWP modeled magnitudes transpire, it would be exceptionally bad news for the region already suffering from flooding. Remember our strongest gust for YEARS in Reigate was 52mph in January.  So any wind speeds over 60 mph will be exceptional. The UKMO also forecast wind gusts over 60mph around midnight.  ESTOFEX have issued a Level 1 tornado warning and for isolated gusts from afternoon onwards due to warm air advection and presence of sub-tropical air wrapped in the system.  This will add lift and energy to frontal rain bands which might cause thunderstorm activity, the odd clap of thunder embedded in fronts might be a sign of tornadic potential activity so keep an eye our for funnel clouds or drops in cloud base or tornados.  If you see any of this please tweet or email a picture!

The saving grace is that it passes over relatively quickly and winds will abate on Saturday leaving a bright possibly showery and blustery day. The remainder of half term week looks less dramatic and with a ridge of high pressure flirting with the South it should be more settled, albeit with another LOW early in the week, but this is much less intense.

RGS weather is on holiday for the remaining week, inspecting the other side of the jetstream in the Atlas Mountains of Morocco. Intending to fly out from LGW at 4am on Sat morning! Close up and personal with this storm but sadly no tweet updates are likely.

By the way, press details can be hopelessly inaccurate, especially certain papers!  The pic below is wrong: indicating several storms that were not a special problem for the UK… handle with care!

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

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

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

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

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

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

02-01-2014 22-10-19

rain totals fri – sat

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

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

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

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Severe weather warnings out: Gales and heavy rain across the South…

UKMO warnings page: http://www.metoffice.gov.uk/public/weather/warnings/#?tab=map&map=Warnings&zoom=5&lon=-3.50&lat=55.50&fcTime=1387756800

Flooding map: http://www.environment-agency.gov.uk/homeandleisure/floods/142151.aspx

Brief update on storm “Caspar” (our own name, middle one of Three Kings ): possibly worst in 130 years for UK (BBC source).

WIND: Reigate can expect possible gusts of 50-60mph, though the town itself is sheltered from southerly and SW winds by Priory Park hill and woodland so that we can shave off 20% from max wind gusts, usually.  The max quoted gust from UKMO is 66mph in small hours of Tues am as cold front goes through. StJude, the last big storm in October, had a max gust in town of 48mph during high winds that lasted only a few hours.  Caspar is set to be a longer duration, across a wider part of the country and is a bigger beast, the storm being centred off the NW of Scotland, rather than running through the Midlands like StJude.

Strongest ever jetstream? Chris Fawkes BBC weather

23-12-2013 05-20-02

Winds will spring up early tomorrow and by 11am could be gusting at 50mph.  They remain strong for the next 12 hours, peaking as the cold front arrives overnight/early morning Tuesday.

The emerging storm is now visible in the Atlantic as a familiar hook shape develops with cold and warm fronts.  The storm is set to explode over the next 24 hours in a process called cyclogenesis, caused by a very powerful 270mph jetstream “sucking” air off the ground as air diverges aloft.  This lowering of pressure by some 50mb in a few hours will cause air to “rush” into the “void” (converge) and this, in a simple way, is the cause of the maelstrom.  The spin of the Earth causes the winds to rotate into the centre of the Low pressure in a process called coriolis effect.

928mb

928mb

The surface pressure is expected to fall to somewhere around 926-928mb, one of the lowest pressures recorded near mainland UK (lowest land surface pressure 925.6mb, 1884, Perthshire). The BBC explains the development here:

http://www.bbc.co.uk/weather/features/25485532

RAIN: Reigate can expect almost continuous rain for 12 hours, starting early Monday morning, around 6am, through to around the same time on Tuesday. Heavy rain is caused as air rises, cools and condenses especially where warm air is forced to rise over cooler polar air.  This will happen especially at two moments as the storm passes…  first the warm front and then a cold front (overnight, probably Tuesday early am).  They have different characteristics.  Of most concern to us and the Environment Agency tomorrow, is that this storm will be causing more or less non-stop rain from 9am Monday through to about the same time Tuesday.  Rainfall totals 20-30mm can be expected during the course of the storm and, on saturated ground, this will cause flooding and plenty of surface water on roads.

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The “good thing” is that cold fronts pass pretty quickly and moderated wind conditions arrive soon thereafter, this means Tuesday is likely to see brightening ameliorating conditions arrive during the morning for any clear up. Flooding will be a problem by then and it’s likely that our River Mole will flood key back-routes and surface water on more major roads could be a problem.  Winds of more than 50mph can also causes trees to be knocked down and other damage is possible.

http://www.bbc.co.uk/news/uk-25484998

Finally, waves in the Atlantic are expected to reach near “phenomenal” heights with significant wave heights of 13m by Christmas Eve (which means waves double this height could occur once an hour).

After a quieter rest over Christmas things sadly don’t get a lot better… another storm is heading our way for Friday and the weekend.

23-12-2013 00-11-59

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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21-12-2013 13-36-34

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 well-advertised cool-off this week arrives in Reigate late Monday and through Tuesday with the coolest days being mid-week: some remarkably cold upper air temps are due to cross the SE this week. While Tuesday will be cold and bright under a brief ridge of high pressure over SE, the coldest feel will be in more windy and damp conditions Weds night – Thurs am.  Whilst this is far from an early winter-Armageddon we are certain to see chilly temps during the day staggering to maybe 5 or 6c Tmax with night temps dropping below freezing: car scraping, road gritting with cold days and chilly nights, if you happen to be out, will be the norm in Reigate from Tuesday on wards, but must stress nothing outrageous, just chilly, especially when wind picks up with arrival of a LOW skirting down from N of Scotland and journeying south down the North Sea mid-week: sticking tightly to the East coast.  This will eventually introduce a COOL POOL low over the near continent which will form the basis of weather in the SE for later this week and into the weekend with cool and, usually gloomy, NE and E winds off N Sea, whilst a frosty HIGH builds over the north of the country. 

A meridional (wiggly) North-South jetstream is responsible for dragging down frigid air from the Poles… the distance from the Greenland Ice sheet to Reigate is some 2000 miles, so this Polar air will warm up over the comparatively warm North Atlantic Ocean courtesy of the warm ocean current, North Atlantic Drift. Nevertheless, a series of cold fronts Monday and then again Weds will introduce successively colder air from N-S down the country, with accompanying rain (snow for north and northern hills).  These fronts will also bring down rain, occasionally “icy” / sleety mid week, possibly with dusting of snow too for higher ground locally.  The complicating factor in this Arctic plunge scenario is a vigorous area of LOW pressure regressing down the North Sea, driven by the northerly jetstream, which has some cold polar air from Greenland wrapped into the circulation.  The North Sea is about 9c so this will have warmed up considerably since its departure from the Ice Sheet but the warming from below could invigorate wintry showers for the East coast. The GFS model has the additional possibility of the cold front dusting the SE with snow overnight Weds into Thursday am as this cold air sweeps over and freezing levels touch down on the Downs. As usual, check back for updates as this interesting wx takes shape this week as models are hinting at further COLD for next weekend. Follow @RGSweather on twitter too! 

So, prepare for the arrival of the Queen of Freezer!