Archives For jetstream

2016-03-21_19-02-57

The Atlantic is showing off some classic visible cloud features of cyclone birth and decay today.  Systems labelled 1-4 on the satellite photo above show different features including stages of cyclone / mid-latitude depression formation and decaying high pressure ridge all on one satellite picture.  The chart below shows the same view with fronts.

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Starting with LOW number #2 (why not?!): the spectacular classic cloud spiral of LOW #2 indicates a mature low occluding and filling.

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This maturing occluding LOW has a couple of interesting extra vortices near the low core.

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Despite their angry look, classic cloud spirals like this on satellite photos are actually decaying and filling lows, losing their strength as pressure rises in the low core.  This particular LOW has a spectacular cold front of over 1500 miles stretching from 60N to the sub-tropics. The red colours on the RGB false colour eumetsat image below shows the cold continental polar air surging in behind the cold front.

Low #1 is a rapidly intensifying LOW off the coast of Labrador.  It looks harmless as a smudge of cloud but this shape … a so called “baroclinic leaf” indicates the birth of an angry developing storm: rapid cyclogenesis.

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This will deepen and pressure will fall rapidly in the next 24 hours as frigid continental air collides with humid maritime air under the influence of an active 200mph jetstream.

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LOW #1 is expected to form a big storm in the Labrador Sea by Wednesday. The fronts on this storm are then forecast to stretch clean across the Atlantic and bring the SE our first frontal rain for over a week by later Thursday.

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System #3  on the top satellite photo shows the HIGH pressure lingering over the South of the UK but regressing into the Atlantic.  This ridge has dominated mid to late March weather in the UK but brought a lot of anticyclonic gloom to the SE.  The deflating ridge will allow a more unsettled Atlantic westerly regime to dominate late March and early April weather.

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Low pressure #4 is an interesting developing depression in the Mediterranean, courtesy of a southward limb of the jetstream. Currently a disturbance dumping snow over the N Atlas in Morocco, this LOW is set to deepen across the Mediterranean through the week.  It will track directly ENE through the Med and bring snow to the Atlas mountains, rain to N Algeria and foul wet, windy and wintry conditions to Italy and then more snow and wintry weather to the Balkans.

Finally, for the UK our weak ridge is deflating to the SW and this will open the door to zonal westerlies and frontal systems bringing rain and wind from later Thursday and into the weekend.  Ensembles below show the dry spell ending this week and some notable rainfall spikes in the days to come, especially over the weekend.

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2014-02-10_22-24-13

Four days of UKMO weather warnings and this hasn’t been unusual this winter!

Unsettled is an understatement, the weather is still on over-drive! Here’s a quick update on rain and storm events upcoming this week for Reigate, as the weekend summary didn’t quite do it justice!  The jetstream is still blasting across the Atlantic and giving birth to storm after storm.  Causes of this extraordinary jetstream are complex, but most “local” to the UK is the extreme contrast in temperature across North America with a buckling jetstream dragging frigid conditions across much of that continent.  This creates a steep temperature and pressure gradient that kick-starts a powerful jet across the Atlantic.  Storm force lows are being created amazingly quickly and most of them are being directed at the UK.

So, here’s a summary of storms on offer this week for Reigate and Surrey…

Tuesday: active front with cold air behind sweeps across the UK and arrives in Reigate am for heavy rain at lunchtime with gusty winds for a time. 10mm is possible in a short space of time which could lead to local FLOODING.  The front will whizz through quickly by the afternoon and leave a brighter but cooler feel pm even with possible wintry showers on a brisk cool W/NW wind.

Wednesday: looks grim… another tight storm approaches from the SW.  This one brings more gales to the SW, Wales but also pushes them further inland across the Midlands.  The south coast and the NE coast (on departure) will also see gales.  Wind for the SE will be pushing in off the Channel at possibly 50-60mph with 40mph gusts inland for inland places Reigate, peaking pm.  Rain is due to be heavy, especially pm and early evening and it will be cloudy all day.

Thursday… looks like a brighter showery day with possible heavy thundery showers at times. Friday / Saturday brings in the next storm with high rainfall totals adding to already saturated ground.  The likelihood of more flooding seems high at this stage which is terrible news for those places along the Thames that have been so badly hit today.

Don’t worry, no icy blasts for SE and Reigate this week. Some cooler weather certainly arriving Tues (but dry and bright initially) and a slight possibility of snow flurries for Weds night/ Thursday am across the Downs for a little while but sleet more likely and rain either side of this momentary event, which most will miss anyway by being in bed. It’s interesting weather nonetheless. A LOW is developing rapidly as we speak between Greenland and Iceland. Freezing air off the Greenland ice cap is meeting warmer air brought north by a brisk jetstream and the two are colliding at a development zone for storms on the pole-ward side of a very frisky jetstream blasting west to east across Greenland from  Canada. The jet is in a meridional / wildly loopy phase and is directing this LOW pressure with attendant cold polar air straight across Iceland, across the North of Scotland and then down the North Sea brushing very close to the East coast and SE of England on Weds- Thurs.  

In Reigate this will bring chilly / “icy” rain overnight and a brief low risk possibility of dusting snow over the Downs. The LOW drifts off down to the continent where it brings snow and very cold conditions to the Alps: -25c is predicted on some models for the high Alps. So, there will be icy blasts here and there associated with this LOW but you’ll have to pop to Greenland or Iceland, possibly the mountains of Scotland or the Alps this weekend to experience this!  

For the UK a HIGH pressure builds through to NW later this week / weekend so things look to be staying cool but rather settled with a N/NE wind dominating.

Weds night into Thurs: looks to be that wintry ppt on the way 🙂

check out latest radar 21:00hrs weds: please follow our twitter feed for latest updates on local winter weather @RGSweather

20-11-2013 20-59-32

next update: Sunday 24 Nov…

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.

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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!**

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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

GALE WARNING: Reigate could see 40mph gusts in exposed places building through Sunday peaking in the afternoon and early evening.

Sunday will see the first Autumnal storm bringing rain and strong winds and a cool plunge of air from the Poles through Sunday and influencing our weather for much of next week.

Rain is due to arrive Sunday pm, and winds will build through the day to reach peak gusts possibly close to moderate gale force in exposed places 40mph into Sunday evening. Rainfall totals overnight into Monday could amount to 6-10mm, less than the rain yesterday and overnight, but heavy at times.
The cool air will be a feature as it follows the cold front with a Polar airmass fresh from Iceland. Expect temps to struggle into low double figures and feel especially cool in the breeze.
A gradual recovery through the week but the LOW will never be far away, lingering in the North Sea / Atlantic between Scotland and Norway: bringing northerly winds much of the week.
The end of the week might see ex-hurricane Humberto knock on the door. At least he will bring tropical air instead of Polar! Autumn is winding up!

Formation of our first Autumnal storm… 14 Sept

surface low formationThe chart above shows an unusually fast 200mph jetstream at 30,000 feet blasting across the North Atlantic from S Greenland to to S Iceland.  This jetstream then loops south and changes speed.  Any alterations in the speed and direction of an active jetstream are potential development areas for mid-latitude depressions.  The one South of Iceland, developing as we speak, is being created by the sudden decelaration of the polar jetstream and a steep temperature gradient between each side: warm to the south, cold to the north of the jet.  This steep temperature and pressure gradient across the jetstream axis encourages rising air (drags it off the ground), a fall in air surface pressure and convergence of airmasses at the surface: a cyclone.

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

 

Interesting subtle switch in the weather coming up… surface winds today and tomorrow will slowly shift from a warm light easterly, round to southerly and thence to westerly usherring away the mini-Euro heat wave and bringing cooler normal Atlantic conditions by the weekend.
You might be surprised to know that, despite slack humid lazy conditions down here at the surface, the jetstream is alive and well and blowing warm air over SE England from Spain at a healthy 60mph+ at 5500m. This warm plume aloft is SO warm that it has created a temperature inversion at about 2-3000 feet, where temps actuallty increase with height. This effectively means rising thermals which could create thunderstorms do not break through the inversion cap. This is why we’ve had no spectacular thunderstorms…yet. The daytime temps have not risen high enough for the cap to be broken. Today this might change but it’s a low risk again and nothing may happen at all. Enjoy the last moments of continental heat… it’s drifting off today, albeit slowly, and being replaced gradually by cooler Atlantic air.

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

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

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

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

This Atlantic chart for Wednesday shows a classic run of LOW pressure systems (depressions) charging in from west to east towards the UK associated with an active jetstream. This week LOW pressure will bring rain and wind to western parts but most of these LOWS will skirt to the NE and hit Scotland and Northern England hardest whilst any trailing fronts reaching Reigate will have weakened considerably.  In Reigate we can expect occasional light rain and some breezy weather (30 mph gusts) most days this week, particularly on Wednesday, but nothing dramatic and rain only amounting to 6 mm or so for the whole week.  HIGH pressure ridges separate each depression bringing a few days of calm, dry and brighter weather. The next significant anticyclone (HIGH) is expected to build across the UK just in time for the weekend and London Marathon, hopefully bringing dry and pleasant weather.  For Marathon day the weather currently looks like being 11ºC with light southerly winds and no rain.

Next week is a real chiller for the whole of Europe. Average temperatures are 6°C below normal for this time of year in the UK.  One reason for the continued cold spring weather is exceptionally high pressure over the North Pole compared to that in the mid-latitudes which is measured by an index called the Arctic Oscillation.  The HIGH pressure BLOCKS out milder air and causes Polar air to flood further south than usual, reaching Reigate on several occasions this winter (albeit modified and warmed up along its journey).

Polar air escapes in negative AO

Polar air escapes in negative AO

The Arctic Oscillation index is currently “strongly negative”.  This indicates much higher than normal pressure over the Arctic and a weak jetstream in low pressure further south than usual.  Air flows from HIGH to LOW pressure.  This means that cold Polar air can easily push out from the north unimpeded by weak SW winds: hence the cold weather brought by Polar easterlies reaching the UK.  The jetstream also “holds in” Polar air like a belt, but the belt has slipped down well to the south of the UK causing “pants” weather for the UK.  Usually the jet migrates north of the UK at this time of year bringing in milder SW air at the surface.
The question, of course, is not “if” but “when” exactly Spring weather will arrive. The maps and ensemble graph seem to suggest that next weekend could see initial improvements with milder SW air reaching the UK and a break down of the Polar block… let’s hope this proves to be accurate!