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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!

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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After another big “50 year storm” battered the SW and south coast today, the weather next week unfortunately looks to continue very unsettled with the possibility of stormy conditions at times, especially for the South and west.  Reigate, though more sheltered than most, will not escape the action. Mid-week Tues/Weds sees a likely cold snap with a chance of snow mainly falling over high ground to the NW but also snow showers possibly falling anywhere, albeit briefly and probably not reaching the SE. Before all that, let’s do a quick review of the interesting weather today…

In Reigate there was some interesting weather to report today as an unstable brisk SW airflow picked up moisture from the Channel and built cumulonimbus cells that rolled up to Reigate in the morning… see below:

In Reigate gusts peaked at 39mph, more widely gusts reached a measured 43.5mph on N Downs and exceeded that elsewhere to reach 50mph in places. The SW and coastal areas had gusts well over 70mph. In the morning some well developed cumulonimbus thunderstorms rolled across the area and produced hail and some lightning around Reigate, Dorking and Guildford. During these storms downdrafts caused temperatures to drop sharply but also the pressure to rise locally.  Now, let’s look at some charts for next week… 

It’s worth noting that the Northern Hemisphere is exceptionally cold and snowy this year… except for UK where it is exceptionally stormy and wet!  There are signs that things will calm down after this coming week as pressure creeps up.  This week, however, looks a little rough, windy, wet and unsettled.

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Models are still battling with the precise intensity and track of storms next week. ECM wants to bring in a southerly tracking rather intense low with some big gales through the South and the Channel prior to opening the door to a cold plunge of NW winds fresh from Greenland on Tuesday / Weds. The NW is not the usual direction for country-wide snow, at least not in the sense of prolonged bitter temperatures like 2013 but this particularly unstable polar maritime airmass is being delivered direct from Greenland/Canada and has some chilly upper air wrapped up in a low pressure behind an active front that will cross the UK through Tuesday and drop plenty of rain / sleet /snow. It’s the air behind this front that may bring snow.  Whether it reaches SE is not certain, but the fridge door will not stay open for long as a bigger Atlantic storm sets to arrive at the end of the week.

Finally, at the end of the week there is a sting in the tale… another almost identical storm to this weekend appears on the scene on Thursday and runs through Fri/Sat.  Some models (GFS) show extraordinary wind speeds for inland at the moment but often they over-do things at this early stage and they usually calm down nearer the time. Lots of interest this week so watch this space for developments.  As weather is so fast moving our twitter feed is the place to get best up-to-date info. @RGSweather

Quick interim storm summary graph with data from our Reigate weather station. Analysis and links to follow.  Below is a medley of images… analysis to be continued after festive season!  Meanwhile… hope you survived unscathed and Happy Christmas!

70mm total rainfall (manual rain gauge reading) now confirmed for Reigate during the storm 24hrs.

which was bigger, StJude or Caspar? (our name, not Dirk!) Caspar of course! Caspar goes on to Scotland today… 24 Dec

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Dramatic VIIRS sat pic from 24 Dec midday: note boiling clouds along Atlantic front

http://www.getsurrey.co.uk/news/local-news/fallen-trees-floods-power-cuts-6444107

http://www.environment-agency.gov.uk/homeandleisure/floods/riverlevels/136484.aspx?stationId=7262

River Mole 3.45m above normal level: severe EA flood alert issued: the only one in the UK at the time.

Flanchford Bridge wall collapse

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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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

Friday update: looks like heavy rain will go thru N France, so S and SE can expect a damp day with rain am and more pm, possibly heavier pm.  But not the deluge forecast a day ago. This situation is changing rapidly so check back.

A fast moving situation and not certain of track yet but … heads up that TWIN STORMS are set to hit the UK this Friday and Saturday bringing peaks of further heavy rainfall and gales. The first storm has uncanny similarities to StJude but is a weaker affair… and a warmer and wet version of StJude which is set to bring a lot of rain and windy conditions to the south sometime on Friday, probably pm. Stress this is not as powerful as StJude. Wind speeds probably max 30mph in Reigate, so not damaging as modelled currently. This first storm will be shifting quickly SW-NE, arriving SW Friday am and leaving by midnight, clearing the country in less than 12 hours.

The second storm is bigger and currently set to cross the UK further North and bring stronger gales across the South and SE, possibly gusting in excess of 40mph in Reigate and 50mph in exposed places nr coast; it will be wet but the wind is more likely to be the defining feature especially in the SE.  The Channel will likely see strong winds throughout this period.  Details are still emerging but twin storms, yet to be named, both deepen on their track across the country due to a lively jetstream. Once again, batten down those hatches, check the fence posts and tie down the trampolines!
More on this rapidly developing situation later as things emerge but prepare for a potential windy and wet fireworks on Friday and Saturday.  Watch forecasts.

She just won’t let up… enjoy the next few days of relative calm because the Atlantic is going to throw more storms at the UK Friday and through the weekend, so bonfire goers and organizers please check forecasts for details as they emerge!
For the SE and Reigate it is looking wet and windy with Friday being possibly very wet indeed in the SE with a weak Jude-like copy-cat appearing from the SW. By no means as strong as Jude, but it has uncanny similarities.  Saturday sees a monster appear to the NW. It has a deeper central pressure than Jude had when she left but, crucially, this one FILLS and weakens as she crosses the UK. Very tight isobars off shore will give NW UK a bashing on Saturday and the SE and E windy conditions on Sunday. Wind speeds will be less than Jude for us but still pretty blustery. These two storms may well pour cold water on bonfire night for many around the country.
Things still uncertain about this family of LOWS so check forecasts and details as they emerge.

30-10-2013 07-08-45

The popularly named St Jude storm (officially named Christian) of 27-28 October 2013 was the most severe to hit southern Britain for over a decade. Whilst it was less powerful than the 1987 Autumn storm, St Jude lived up to its forecasted strength and caused an estimated £1 billion worth of damage and losses across the southern half of the UK.  Here is a round up of  causes and some impacts of this severe mid-latitude storm.  Locally, @RGSweather covered the storm continuously overnight, providing updates and advisories on twitter as things developed minute by minute.  This is a summary of causes, what happened and what we have learned from this storm…

Causes

The “ingredients” for the birth of St Jude include…

1. A big warm soup:

A warm Atlantic Ocean, some 3-5c warmer than 30-year average, acted as a perfect birthing pool and nursery for StJude. The warm sea surface temperatures provided plenty of extra water-vapour, heat energy and lift ready for stirring up a potentially big storm.

Warmer than usual Atlantic

Warmer than usual Atlantic

2. Add some extreme pressure!

The North Atlantic Oscillation is a measure of the difference in pressure between Iceland and the Azores and it was in increasingly positive phase during the storm build-up.  Whilst this is more of a measure than a cause, a positive phase nevertheless indicates LOWER than average pressure over Iceland and higher pressure over the Azores, which usually indicates a strong zonal west to east flowing jetstream across the Atlantic and fast moving weather with the potential for plenty of low pressure systems from the west.  This rapid west-east flowing weather was a necessary ingredient in the set-up for St Jude.

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bubble bubble toil and trouble

bubble bubble toil and trouble

3. Throw in Mr Muscle

A very strong jetstream: blowing at 240mph across the Atlantic towards the UK acted as the main ingredient in the birth of storm St Jude. The jetstream directs weather on the ground.  The jet over the Atlantic in the days leading up to St Jude was extremely strong and blowing directly across UK latitudes. The jetstream is a product of the temperature and pressure contrast between cool Polar air to the north and warm Tropical air to the south.  The temperature difference between polar air and tropical air is particularly marked at this time of year: with the tropics still very warm, while the Polar ice sheets seeing a marked fall-off in temperatures with their attendant air masses.  This builds steep pressure gradients and a strong jet. The jet is also a key factor in creating and guiding LOW and HIGH pressure systems on the surface.  Like a dog on a lead, St Jude was dragged across the Atlantic by it’s angry owner, the jetstream. At times during the passage of the storm wind speeds above the Channel exceeded 180mph.

24-10-2013 08-21-55

4. Whisk up some bad parents!

So all the ingredients are ready, but no storm yet!? The mother of St Jude was a deep robust LOW south of Greenland.  This formidable storm produced hurricane force polar winds directed from the NW in the days before St Jude was even a twinkle in her eye.  The father was a weakening and slow moving tropical storm called Lorenzo.  He had spent the week meandering slowly in the Mid-Atlantic but Lorenzo, despite his old age. still arrived with plenty of hot air from the Tropics.  Their respective air masses collided in the mid-Atlantic some 1000 miles off the SW coast of the UK and, encouraged upwards by the jetstream, they produced their only child, St Jude!

baby bomb is born

baby bomb is born

5. Give it a stir!

Cyclogensis is the process of rapid growth of a baby storm in the mid-latitudes: due to converging warm and tropical air and, encouraged by the jetstream, air rapidly lifted off the surface and Jude’s central pressure, as predicted by the UKMO fell steeply.  This so-called meteorological BOMB exploded (or perhaps imploded, as air was dragged into the low pressure causing all that wind) formed a deep wave depression LOW that charged across S Wales and England in less than 12 hours.  It made a rapid exit from the UK via the Wash and then continued to deepen across the North Sea before smashing into Scandinavia. Arguably the storm did not deepen over the UK quite as spectacularly as some models forecast, but nevertheless, the track and winds were much as predicted and the storm went on to cause significant damage. The chart below shows the pressure falling at a NOAA weather buoy in the development zone of St Jude some 300 miles SW of Cornwall. Note the INCREASE in pressure before the sudden drop-off.  This is entirely in line with cyclogenesis: pressure builds ahead of rapidly developing warm fronts as isobars are buckled up ahead of the storm.  This is popularly known as the “calm before the storm” where winds die down before the maelstrom hits.  This was marked across the country on Sunday evening. At that stage people wondered “what storm?”.

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calm before storm

The satellite picture below of St Jude still in development phase shows the characteristic wave form kink of a rapidly developing storm.

6. Watch out for that sting in the tale!

More immediate “causes” of storm damage from StJude, making it extra-powerful, include the relatively newly discovered weather phenomenon called a stingjet wind.  These are isolated fierce gusts of wind experienced behind a departing deep area of low pressure, often behind a cold front. Oddly, they tend to occur as conditions more widely are improving. In very tight depressions descending air from the upper troposphere pushes gusts to the surface and, like a giant invisible hand, these can, in a careless whim, push down whole swathes of mature forest, take rooves off houses, rip down scaffolding, push over cranes, roll over double decker buses and blow trampolines clear out of your garden!  The sting in the tale is an appropriate analogy, as the curl of winds descending round from the NW of the departing LOW are frequently the last hurrah for these storms. **NOTE: Stingjet NOW confirmed!**

28-10-2013 06-27-15

The storm was perfectly forecast by the UKMO up to a week before the event.  It was always going to be hit-and-miss up to the last minute, not least with inevitable media-hype; but the consequences of playing this down would have been potentially disastrous.  Overall, it was well predicted and people were warned effectively days beforehand.  Whether they prepared effectively or took warnings seriously is another matter.

Below is a slide-show of synoptic charts showing the progress of the storm.  Note that the central region of LOW pressure experienced light winds like a hurricane “eye” (but not as extreme in contrast!). Below this is a brief list of impacts.

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Impacts

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The highest winds were largely restricted to places south of the M4, excluding Wales which had 80mph+ winds in the run-up to the storm on Sunday but calmed down for a time overnight Monday as the centre of the LOW passed over the Severn Estuary and S Wales. Across S and SE England wind speeds were widely 40-60mph and 70-80mph+ on the south coast.  Remember that average wind speed across an area can seem surprisingly low during a storm.  The average wind speed across Reigate from 5-8am during the height of the storm was only 16mph! It is, of course, the random gusts that cause the most damage. The highest gust in Reigate was 48mph at 6:20am on Monday morning.  The highest official max gust was 99mph on the Needles, Isle of Wight, other notable wind speeds were Heathrow 70mph and 62mph at Redhill aerodrome.  Reigate, as predicted by @RGSweather, was spared the worst as our max wind gust was 48mph. Our location in Surrey is away from the coast and locally the town is low down in a vale with low wooded hills to the south, Priory Park, that shelters the town from S or SW gales such as the ones StJude produced during the worst of the storm.  More exposed parts of our local area certainly experienced higher wind speeds.  Rainfall was intense for a period of time and caused localised flooding. In Reigate 25mm of rain fell overnight, which is more than for the whole month of July or, August, in less than 10 hours!  St Jude crossed at night and only keen meteorologists were awake to see it go through.  If this had been a daytime storm, impacts listed below are likely to have been worse with more people getting out and about, or attempting to.

Here are some of the impacts from St Jude in the UK:

  • 3 people were killed by falling trees, 1 boy was very sadly swept out in rough seas on s coast in the lead-up to the storm
  • 147 flood alerts, 17 flood warnings issued by Environment Agency, including our own River Mole
  • In the English Channel and approaches there were 20-30 foot waves and storm force winds.
  • power cuts in SE across 270,000 homes, some for 2 days
  • 5 train companies cancelled all their trains in SE
  • 130 flights from Heathrow cancelled, delays at Gatwick
  • Port of Dover closed, horrifying stories of Channel ferry crossings
  • crane collapsed onto Cabinet office
  • Major bridges were shut in high winds including Severn Bridge and QE2.
  • Dungeness B nuclear powerstation had a power cut in 90mph winds and had to shut down both reactors
  • In Suffolk a double decker bus was rolled over by a gust of wind
  • Clacton pier helter skelter was blown down
  • 1000’s of trees blocked roads and caused travel delays and closures
  • other impacts across the SE here http://www.bbc.co.uk/news/uk-24699748
  • here http://www.bbc.co.uk/news/uk-24700611
  • and costs here (sorry, only one I could find 🙂 http://www.mirror.co.uk/news/uk-news/uk-weather-storm-st-jude-2651458

Emergency service response was predictably very effective in dealing with thousands of calls.  Public were advised not to call 999 for tree falls, and only call in real emergencies.

The storm went on to cause significant damage and some 15 people in total died across the UK, N France, Netherlands, Belgium, Germany and Scandinavia.  It deepened across the North Sea and became more intense with stronger winds, with 120mph reputedly being recorded in Denmark (http://en.wikipedia.org/wiki/St_Jude_storm)

So… hundreds of trees down, some scaffolding torn apart, helter-skelters blown away and some very unfortunate people killed out in the storm… plus £1billion lost through days off and travel chaos.  Inevitably, forecasters are stuck between over-blowing storms and under-playing them so as not to cause panic.  Personally, I think they got this spot on from the start, so congratulations UKMO! The fine balancing act between under-playing and exaggerating potentially serious events is not an enviable task for forecasters.  Despite being very powerful, computer forecast models were still flip-flopping 24 hours ahead with the exact track and severity of this storm.  It was an on-then-off affair right down to the line!  For the future it is worth raising awareness in the public that, despite computers producing forecasts (and who trusts them!?), predicting the weather is still based on the judgement of experts at the UKMO and elsewhere. (photo of clouds over Channel below left was taken by an airbus pilot on his way over Channel during storm). other resources for this storm:

http://www.theguardian.com/teacher-network/teacher-blog/2013/nov/03/st-jude-storm-extreme-weather-teaching-resources?CMP=twt_fd

http://www.independent.co.uk/news/uk/home-news/the-30m-supercomputer-that-helped-the-met-office-predict-st-judes-storm-8911510.html

Which was bigger, 87 or 2013? http://www.bbc.co.uk/news/uk-24708614

Finally, the old chestnut “was it a hurricane?”… Despite getting winds exceeding hurricane force ((74mph+) UK storms cannot be classified as hurricanes. Hurricanes are tropical weather phenomenon and do not form in the Mid-Atlantic at our latitude, neither do they ever get to the UK.  We may experience hurricane force winds in extreme low pressure systems which are confusingly also called cyclones, although they are NOT tropical cyclones!  At our location, on this side of the Atlantic and this far north, we have never experienced a true-hurricane.  Even the ’87 storm was not technically a hurricane despite having even stronger winds. We sometimes get “old” hurricanes impacting the UK but this is not the same, and neither St Jude nor 1987 were one of these characters. Handy pic below illustrates this nicely. Below this are a selection of photos posted on twitter mainly from our local area in E Surrey.

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Lead-up to storm in Reigate on Sunday pm