Archives For January 2015

What’s a Polar Low?

January 29, 2015 — 1 Comment

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Update: this feature is under discussion on met-forums as to whether it was technically a polar low or not!  Some features (e.g. satpic) suggest it was but other features, such as upper temperatures, suggest it may have been a small scale feature with some similar characteristics (referred to below). References below contain more details. This post still valid as it describes an interesting weather feature with several polar low characteristics, albeit jury is out on final definition here!

Polar lows are small but intense maritime meso-scale cyclones, a few hundred km across, that form quickly in cold polar or arctic air advected (moved) over relatively warmer water.  They are much smaller than our usual mid-latitude frontal depressions.  They usually occur in winter in Arctic or Polar airmasses streaming Equatorwards and they form characteristically beautiful swirls of cumulus cloud and a comma cloud formation visible in satellite pictures (see above VIIRS sat pic 29/01/2015).  Sometimes these swirls form an eye reminiscent of tropical cyclones, to which they have surprising things in common.  Heaviest snow / rain occurs near the “eye”.  This similarity is why they are sometimes called “Arctic Hurricanes”, though they do not always produce hurricane force winds.  Despite their scary name they are relatively common over ice-free mid-latitude waters, they can produce rain or snow (due to “warm core” see below) and one was recorded moving down the North Sea in Dec 1995, so they are uncommon but not unheard of around the UK.

early evolution spotted on MetOffice fax

early evolution spotted on MetOffice fax

Polar lows tend to go through a rapid life cycle of a day or two which previously caught Arctic explorers unawares with hurricane force winds blowing up from nowhere and creating high seas in hostile Arctic waters.  Gale force winds wrap tightly into these features.  Polar Lows were invisible to meteorologists previous to satellite pictures and only with very recent upgrades in Numerical Weather Prediction models have Polar Lows been “visible” at all on charts.  They are still hard to predict and models sometimes struggle to track their intensity and path.

Polar Low circulations do not last long and, like their tropical cyclone cousin, they tend to decay rapidly once they move from warm sea over cooler land, because the convective energy and steep lapse rates driving the system are lost.  Cold upper air temperatures and a warm sea assist steep lapse rates that can cause thunderstorms with active convection.   Lightning strikes were recorded in this 29/01/2015 low as it came ashore over Northern Ireland, typical of polar lows.

Charts modelling the evolution of this low, albeit rapidly in the last few hours, have predicted tracks moving the system SE across the UK overnight.  Areas especially at risk from snow include Northern Ireland, N Wales and Midlands and possibly parts of the Southern England into early Friday.  However, the system is likely to fill rapidly overland as the sensible heat flux available for convection is lost over the colder land.  Additionally, the land is rougher than the ocean and this increased roughness increases surface convergence (air arriving faster than it lifts) and this causes the central pressure to rise and the system to decay.

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decaying polar low

Like hurricanes, Polar Lows form over oceans and gain much of their energy from them.  Polar Lows usually form in places where there is a rapid change in temperature and/or pressure horizontally, this is known as a baroclinic zone.  Edges of ice sheets or simply where warm and cold air meet are prime locations.

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baroclinic zone west of Scotland: breeding ground for this polar low

Warmer ocean water is another essential trigger for polar lows.  The warm surface water provides essential mositure and lift that creates convection, condensation in frigid air and the release of latent heat that develops cumulus and cumulonimbus clouds. The clouds then wrap into a tight circulation around a rapidly deepening low core not dissimilar to a hurricane.  Polar lows are much smaller and more transient than a regular mid-latitude depression.  Polar lows tend to form on the eastern side of a high pressure ridge and to the east of a decaying occluding mother low.  Both these features can be seen on charts.

An important feature of a polar low is the formation a warm core.  Charts below show this as milder surface temps at 950hPa and the theta e chart which appears to show warm air too.  Internal evolution of this system seem to suggest it has a warm core perhaps comprising Arctic flow chasing down a long-track of milder airmass originally sequestered by the extremely active cold front that swept through this location yesterday secluding a warm pool in the core of the mother low sat over Scandinavia.  This warm pool appears to have advected east to meet incoming Arctic air.  An area of positive vorticity (spin) contributed to the evolution by adding spin to the air that caused the low to form.

The polar low that developed today NW of Scotland has many of the hallmark characteristics of a Polar Low and seems to be generally accepted as such mainly because of the defining characteristic swirl of cumulus clouds round the low pressure core.

Note:  met community not all agreed whether this was a Polar Low or not : has many characteristics but some consider it too warm with rain in some areas rendering it a meso-scale slide low or similar. 

some links on Polar Lows

http://www.weatheronline.co.uk/reports/wxfacts/The-Polar-low—the-arctic-hurricane.htm

http://rammb.cira.colostate.edu/wmovl/VRL/Tutorials/SatManu-eumetsat/SatManu/CMs/PL/backgr.htm

https://polarlows.wordpress.com/tag/climate-change/

http://www.keesfloor.nl/wolken/sat/polarlow.htm

Reigate and the UK has a cold spell arriving imminently, but nothing as dramatic as the US NE snow storm Juno. Our cold spell has little to do with the US storm except that Juno is doing a good job of pumping SW warm air on its backside into building the mid-Atlantic high pressure ridge that will encourage Arctic air to plunge south over the UK this weekend, in conjunction with a LOW over Scandinavia.  So storm Juno is not arriving in the UK, and never will, but it has indirectly impacted our weather along with other storms in the US NE seaboard.  Theta E charts are good for looking at airmasses, the different air masses stand out clearly as different colours below. The metoffice fax charts are for comparison.

theta e chart: good for airmass spotting

theta e chart: good for airmass spotting

 

UK metoffice fax chart

UK metoffice fax chart

The active cold front sweeps across the UK tomorrow Wednesday bringing rain to the SE but more importantly ushering in a blustery NW polar maritime wind carrying air from a source region over the Greenland ice cap which will push temperatures from near double figures in the morning in Reigate to freezing overnight with wind chill making it feel more like it should at this time of year.  Reigate is unlikely to see snow during the cold front passage as the coldest air takes a while to arrive behind the frontal rain.

theta e

theta e

This cold front is interesting because it roars in so fast across the Atlantic, swinging around a low pressure between Iceland and Norway, that parts of the warm and more humid sub-tropical maritime air ahead of it is effectively chased up into the low core where it is secluded and trapped / sequestered by the advancing polar air.  This has implications for later when warm pools of air form occlusions that could enhance snow risk as they are dragged south by the advancing Arctic airmass through to the weekend.

arctic air with mixed up mT air

arctic air with mixed up mT air

UK metoffice fax chart

UK metoffice fax chart

The polar air from Greenland is a comparatively dry airmass but extremely cold aloft and becomes unstable as it passes over a comparatively warm Atlantic.  This combination is likely to bring considerable snow to high ground over the N/NW after the initial front has passed.  Usually snow showers in the NW would be it with polar maritime air but this wind will be strong enough to push showers across more of the country and even reach us in the SE during Thursday.

 

Expect many of these showers to be wintry in air that is below -5C at 850hPa (1500m).  Often snow showers form streamers / lines of showers that could accumulate reasonable snow cover for some places whilst others see nothing.

The secluded warm air trapped aloft will form occluded fronts that are due to move south later Thursday and into Friday as the isobars squeeze together between the building Atlantic ridge and the Scandinavian low pressure.  This will actually raise the temperature a tad for a while on Friday before the colder Arctic airmass arrives, direct from the north, into the weekend.   The warm secluded air will oddly make the advance of Arctic air initially warmer than the polar maritime, thus temperatures on Thursday are likely to be colder than those on Friday.

The coldest Arctic air is due to arrive through Sunday into Monday when some truly chilly air will make it down through the UK.

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A “potentially historic and crippling snow storm” is imminent for the USA NE seaboard especially New York State with cities from New York to Boston being warned by weather agencies of a potentially life threatening blizzard lasting some 48 hours.  Update Tues 27: whilst the worst of the blizzard missed New York heavy snow occurred further North.  The system tracked further east than modelled.

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New York escaped worst, but Boston hit as expected with record snow in New England

 

Watch unfolding live pictures on webcams here:

http://player.theplatform.com/p/2E2eJC/NBCNewsIE8?guid=nbcnewslive_nyc1

http://www.earthcam.com/usa/newyork/timessquare/?cam=tsstreet

Potentially 36 inches (3 feet) of snow could fall in places (that’s waist deep!) as a Nor’Easter (NorthEasterly) wind buckles round a rapidly deepening low tracking NW up the coast over the next 48 hours. All weather agencies are busy getting the news out to people in the regions likely to be impacted.

Official warnings from NOAA and all regional weather agencies are giving these urban populations warning after the system has caught computer models napping.  The storm has rather unexpectedly “blown up” recently to become a potential threat only in the last 24 hours or so.

The charts below show the synoptic set-up that develops this system, named Juno, currently a rather benign low tracking west out of continental US.   This LOW is forecast to deepen rapidly due to it’s location in relation to a fast meridional jetstream looping round a HIGH pressure ridge in the US west.  The LOW is dragging cold air from the continent interior and, as this cold air interacts with humid air and plenty of moisture over the Atlantic, it will turn into a snow-making machine that could last 48 hours and dump feet of snow widely across many places indicated on maps above.

Here are some synoptic charts showing the development.  Note the track sliding up the NE coast.  The Nor’Easter wind itself can be seen developing on the northern edge of the low which, as it is forecast to slow and stall while it deepens (a common feature of bomb depressions) the strong NorthEasterly gales, some up to 70mph, will continue to pump snow across parts of the NE building up depths to 2 – 3 feet in places.

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Note the warm sea surface temperatures off shore which will provide enormous quantities of moisture for snow production on the NE wind.

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Here is a chart showing minimum temperatures and possible snow cover by Wednesday.

The US is more used to heavy snow and more able to cope with it than the UK because the USA has a continental climate which experiences colder winters with more regular and significant snow events.  Earlier this winter Buffalo recorded feet of lake-effect snow.  Even so, this snow storm is serious and a state of emergency has been issued for New York.  The difference this time is that this storm will dump a lot of snow in a very short space of time and more people will be impacted in a densely populated and urbanised part of the USA.

The National Weather Service and regional weather agencies and the New York Mayor have all issued weather warnings for the blizzard urging people not to underestimate the possible impacts of 2-3 feet of snowfall over the next 48 hours.

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http://www.nbcnewyork.com/news/local/NYC-New-York-New-Jersey-Connecticut-Blizzard-2015-NorEaster-Snow-Wind-Whiteout-Record-289770081.html#

Here are some tweets and information emerging on the eve of this storm:

 

More info:

Excellent write up and explanation of the evolution of this Nor’Easter storm http://www.washingtonpost.com/blogs/capital-weather-gang/wp/2015/01/28/raging-snow-howling-wind-the-meteorological-evolution-of-the-blizzard-of-2015/

http://forecast.weather.gov/product.php?site=NWS&issuedby=OKX&product=WSW&format=CI&version=1&glossary=1&highlight=off

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2902

http://edition.cnn.com/2015/01/25/us/weather-storm/

http://www.bbc.co.uk/weather/features/30989760?ns_mchannel=social&ns_campaign=bbc_weather&ns_source=twitter&ns_linkname=news_central

http://www.climatecentral.org/news/what-warming-world-means-for-major-snowstorms-18594

http://meteorologicalmusings.blogspot.co.uk/2015/01/a-meteorologists-thoughts-on-blizzard.html

http://www.bbc.co.uk/news/world-us-canada-30996010

The ensemble charts above show that we can expect a week of change ahead.  At the start of the week fronts will bring modest episodes of rain and swings in temperature to the SE followed by a notable change mid-week as both temperature and pressure fall (see charts above) with the possibility of snow for the SE.   By next weekend there is a risk of some proper cold into the start of February.  As usual Reigate and the SE will be sheltered from most of the action but the weather will take interesting and notable swings in a predominantly downward direction nonetheless.  Models agree on how this transition will happen and it is largely based on the evolution of a LOW starting life off Canada near Newfoundland …. here are charts from the ECM that shows the story of our Canadian LOW and how it is likely to bring another taste of winter to the UK.  (Update: worth explaining that the Canada LOW mentioned here is not the same as the Nor’Easter LOW that caused blizzards in NE USA Monday-Tuesday this week: it is a LP preceding it.  If you look below at Chart 2 for 28 Jan you can spot the infamous Nor’Easter bashing the NE coast well modelled on this ECM chart from back at the weekend. The US Nor-Easter Blizzard2015 storm is responsible for building the Atlantic BLOCK helping to push Arctic winds our way but it is not travelling to the UK. The NYC storm looks to travel North up the Canadian coast, filling near S Greenland, unable to break through the Atlantic high pressure ridge extending north that it, in part, helped to build. Hope that helps!)  In any case, the weather set-up gives Reigate another flaky chance of some snow.

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1. The story starts now with the Azores HIGH pressure giving a dry and pleasant weekend for the SE, Saturday has seen brilliant blue skies as a result.  The Azores HIGH has been dominating the Atlantic recently and our Canada LOW, deepening rapidly off Newfoundland on Monday, will be forced round the HIGH to the north west to Greenland. This LOW will deepen rapidly because of the great temperature difference between frigid air pouring off Canada and humid sub-tropical air fed up from the south courtesy of the Azores high.

For us in Reigate this period sees a ridge of the Azores high pressure ebb away slightly during Sunday and this will nudge a mild SW flow to raise temperatures temporarily high into the UK and SE overnight into Monday morning.  This minor weather episode (marked by the ups and downs in temperature on the 850hPa ensemble chart top of page) will be heralded by increasing cloud on Sunday as fronts bring some patchy rain in on a warm front overnight into Monday.  Polar air will follow a cold front later on Monday and a ridge of high pressure will build quickly overnight turning the winds into the NW with a dry chilly night in store for us in Reigate and SE into Tuesday.

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2. Later on Tuesday the Canada LOW is forecast to move NW to near southern Greenland where it will feed on a brisk northerly wind of freezing cold air from the Greenland ice cap.  This freezing cold air will create a vigorous cold front that will reach the UK early on Wednesday.  This active cold front will usher in an unusually cold Polar Maritime NW’ly wind across the country reaching the SE late Wednesday with the possibility of heavier rain or even snow. One to watch carefully.

A point to note from the charts above is that the central Atlantic is anomalously COLD at the moment and so the brisk NW’ly wind will not warm up as much as usual on its journey over the Atlantic to the UK, increasing the likelihood that it will bring wintry stormy gales to the NW, with some models even showing wintry precipitation for us in the SE too, perhaps reaching us by Thursday, though these are likely to be sparse unless the cold front stalls in which case more significant falls are possible. (Update: GFS 18Z suggests this cold front could be active and bring snow across entire UK in its wake…)

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NW airflow 2m temperature anomaly

Check the temperature anomaly chart above which shows how unusually cold this NW’ly wind is going to be.  Usually NW winds do not impact SE England with snow unless fronts stall or there is an especially active undercut of unstable cold polar air.

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3. Our Canada LOW that started life just off shore from Newfoundland is forecast to cross Iceland mid-week and then slide SE into the North Sea, around the blocking high extending north through the Atlantic to meet rising pressure in Greenland, a good scenario for a COLD Europe!  As the LOW transits SE into the North Sea it will bring down Polar and then Arctic northerly winds on its back and snow for the North, NW and NE coasts.  Arctic air rarely brings snow to Surrey or Reigate … it usually runs out of moisture and lift on its transit across the cool land and instead the south and SE usually gets azure blue skies with frosty nights.  NE and E coasts can get snow showers.  Daytime temperatures everywhere may well stay near freezing if this comes off.

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It’s too far ahead for any detail but some model runs show Polar LOWS sliding round the edge of the trough on the left exit side of the jetstream (where lows develop rapidly) as the trough moves east .  These daughter lows can bring snow to the SE but this is too far off to be certain.

So how long might this cold snap last?

There are indications both for and against a more prolonged cold snap but, on balance, the peak of any Arctic cold looks likely to be shortlived as the high topples east, ebbs south and allows gradually more westerly winds back across the UK.  For enduring cold we really need the LOW to move south into Europe and pressure to build to the north bringing in a blocked situation allowing cold easterlies into the UK (beast from the east).   However, this scenario looks unlikely because the Azores HIGH is likely to remain relatively dominant.  This is shown by the generally positive North Atlantic Oscillation chart above.

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North Atlantic Oscillation

The NAO is a forecast measure of the sea level pressure difference between Iceland and the Azores.  When the NAO is positive it usually indicates a strong pressure difference with a big HIGH pressure over the Azores and a LOW over Iceland.  A positive NAO correlates with a fast zonal westerly jetstream and mild wet winters for the UK.  A negative NAO indicates colder winters in which pressure rises to the north (Iceland) and allows easterly or NE winds to bring cold airmasses from the Arctic or more commonly Siberia into the UK.

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Arctic Oscillation

The Arctic Oscillation (AO) is a related cousin of the NAO.   The Arctic Oscillation is a bigger scale comparison of pressure at the Pole compared with that in the mid-latitudes.  A positive AO usually correlates with lower than normal pressure over the Poles compared to the higher pressure further south in the Atlantic.   This brings mild wet conditions to Europe (zonal westerly winds).  A negative AO sees pressure rise higher over the North Pole and this acts like a balloon to push polar air out into mid-latitudes… a cold winter scenario.  The AO forecast shows it going negative by early February and this correlates with the cold snap.  Unfortunately it looks like the AO will go positive again thereafter but this is too far off to be certain.  On the other hand…. !!

The charts above show a Polar view of the Northern Hemisphere.  They show that pressure is forecast to rise over the Pole disturbing the zonal westerly flow of the polar vortex by a displacement of the polar vortex away from the Pole (see below).  This might increase the likelihood of a prolonged cold plunge of Arctic air reaching the mid latitudes including the UK  This is good for cold weather enthusiasts in Europe!

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The other longer-term chart shown above in favour of a cold late winter is a sudden stratospheric warming forecast in early February.  A SSW can lead to pressure rising over the Polar regions a few weeks later, disrupting the upper westerly winds, potentially upsetting the jetstream and bringing cold to mid-latitudes.  This is well correlated and was significant in bringing a late winter in 2013.  So… much of interest at the moment as we enter the final third of winter 2014-2015.

note: for all decision making purposes and forecasts please consult professional agencies: e.g. MetOffice at http://www.metoffice.gov.uk/public/weather/forecast/gcpg7rs0t#?tab=fiveDay&fcTime=1422057600

http://www.nc-climate.ncsu.edu/climate/patterns/NAO.html

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Water vapour EUMETSAT rgb airmasses: Arctic red, tropical blue

Update Tues pm: system not bringing any significant snow to Reigate, Midlands north might see accumulations but even this is not likely to be disruptive..  The satellite image above shows water vapour and airmasses.  Notice how tropical (blue!) and Arctic (red/mauvey) airmasses are intruding across latitudes – plunging respectively north and south of their source regions.  Cold Arctic air is reaching right the way down to Spain.  Here’s a look at what is going on and how things might develop for Reigate and SE UK especially this week.  Overall things look cold and wintry and there is a slim possibility of snow for us but it’s worth watching the forecasts and twitter updates because it’s very marginal and things can change locally quite quickly.

The Arctic air arriving in London Monday left the Kara Sea a week ago. This Arctic air is arriving over the UK courtesy of a high pressure ridge over the Atlantic and Greenland / Iceland which blocks mild maritime air in the Atlantic from reaching the UK. A low (the remnants of storm #Rachel) over Scandinavia is dragging down cold northerlies assisted by a northerly jetstream aloft.  This set-up makes this week the coldest since March 2013 and, as some light snow fell in Reigate this morning (Sat 17) then that was the first snow fall here for 2 years.

The result of the pressure pattern is airmass temperatures lowering at 850hPa to -7C or lower over the course of the next 48 hours due to the steady invasion of Arctic air. Overnight tonight into Sunday morning a front in the Channel could bring light snow showers to southern England early Sunday morning.  The situation is “marginal” as the truly cold air has yet to arrive in the south and dew points are hovering around or above freezing tonight which can make a difference between whether it snows or rains.

Over Sunday the Arctic air will arrive and the upper air mass temperature at 850hPa (1500m up) will fall from -4C to -7C by Monday.  In such a cold airmass the surface air temperatures on Monday will struggle above freezing during the day to about 3C and wind chill will make it feel colder.

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An important threshold for snow is an upper air temp at 850hPa of -5C or lower.  So from Monday any precipitation might fall as snow, so long as other factors are in place.  The chart below shows the movement of the Shetland low into the N Sea and eventually further south which ushers in northerly winds.

LOW sinks south bringing Arctic air

LOW sinks south bringing Arctic air

There is reasonable model agreement as to what will happen into mid-week but uncertainty thereafter as to how long any cold will last.  High pressure is set to build over Scandinavia blocking the NE track of a low S Greenland.  A trough disruption is set to occur when the LOW near Iceland splits from the main trough and slides down the edge of the Atlantic high and sinks SE over the UK (called a slider low).  Trough disruptions are notorious for causing models problems with accurate forecasts!

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trough disruption

The slider low will bring attendant fronts with milder air mixed in, thus complicating chances of widespread snow and making forecasts tricky.  Tuesday is the first chance of any snow this week for the SE as a front moves in from the west to reach the SE around the afternoon, though details this far out cannot be certain.  As the front moves into colder air the rain could turn to snow, especially on the back edge as overnight temperatures fall.  It is very likely to be snow across the middle of the country and certainly over high ground but snow for the SE is less certain, it could be just sleety or rain depending on the mix and location of mild air from the south in the occluded front. In any case Tuesday looks light precipitation as the front weakens to the east.  This doesn’t help snow formation in marginal situations because less cold air is dragged down from aloft in light rain and there is less evaporative cooling in light rain.

Charts currently show that Wednesday has a better chance of snow action for the SE as another front, this time with heavier rain, clears east later on Wednesday or overnight into Thursday.  Exact timing is uncertain and indeed the development of this might all change despite there being sound model agreement as to the overall synoptic situation into the mid week period.

The latest UKMET chart for Thursday shows the SE in a COL between HIGH pressure SW and NE and LOWS NW and SE. This looks like a cold wet day with sleet for Reigate, but snow is again possible, especially as the LOW drifts south and introduces a cold NE continental flow for a time on the northerly edge. This is most likely to restrict to inland areas or those higher up locally but it needs watching carefully as heavy rain might tip over into sleet then snow due to evaporative cooling.

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After mid week things look like turning somewhat milder for a while as westerly winds eventually break through properly, possibly by late week or the weekend.  This means snow chance reduces to nothing as milder Atlantic winds return.  Nevertheless, the long range models still show some propensity towards building further chilly Arctic incursions later too.

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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Reigate 2014 weather report

2014 weather in Reigate, like the rest of England and Wales, broke some records for rainfall and temperature but in quite different ways.  To compare with 2013 visit here.

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South East England JANUARY rainfall 1873-2014

The annual summary chart (top) shows that Reigate had an exceptionally wet start to the year with January being the wettest on record (MetOffice) with over 180mm of rain (see chart above). The annual total rainfall for Reigate was 931.8mm which is above average for the area and well above the 2013 annual rainfall total of 654mm.  The rainfall record that fell in 2014 was for the highest January monthly precipitation total since accurate UK rainfall records began in 1873, exceeding 180mm in Reigate and the SE.  The January rainfall record fell in a winter that will be remembered for being exceptionally stormy in the SE, beginning with St Jude storm in October 2013 and persisting with numerous Atlantic storms battering the west coast and delivering high rainfall totals and significant flooding events across the SE, including our own River Mole and impacting Gatwick airport.  This blog covered numerous accounts of flooding and storm damage throughout that exceptionally stormy winter.

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UK rainfall anomaly 2014

In SE England and Reigate January and February 2014 were 260% and 274% wetter than average respectively (MetOffice).  Of the remaining months March, April, June, July, September and December were at or below average.  September in Reigate was particularly dry with only 18mm of rain, compared to the average annual SE total nearer 80mm. Nevertheless, despite these dry months for SE England 2014 was the 3rd wettest year on record, mainly due to the exceptional rain totals in the first 2 months.

2014 rainfall and pressure for Reigate

2014 rainfall and pressure for Reigate

From the rain chart below it is notable that for the second year running Friday is the wettest day of the week in Reigate, Fridays delivering 20% of our rain in 2014.  There have been studies showing that this effect of wet ends to the week is due to the build up of aerosol pollution during the commuting week.  This encourages the formation of condensation nucleii and higher rainfall totals on friday…it’s a theory worth pursuing because this is the second year running showing this effect in Reigate.  The other chart shows that southerly winds (SE-SW) brought most rain to Reigate in 2014.  This continues the trend from 2013 where southerly winds from SW to SE similarly accounted for the vast majority of rain arriving in Reigate.

2014 was the warmest year on record according to the Central England Temperature record stretching back to 1772.  The UK mean temperature for 2014 was 9.9C, 1.1C above the 1981-201 average and 1.47C above the longer term CET average.  The Reigate mean temperature for 2014 was 11.5C, with a max 30.2C (23/07) and min -2.9C (31/12).  The temperature record was broken by stealth and not by extreme values.  11 months of 2014 remained above the long term average.   4 months exceeded 2C above average, a further 7 months were above or very near 1C above average and only August dipped below courtesy of extra-tropical storm Bertha.  Extreme heat was mostly absent, as was extreme cold.  There was no snowfall in Reigate during 2014 and a notable absence of frost until some crisp days during December.

2014 warmest year on record

2014 warmest year on record

The annual ring chart below for Reigate also shows how August dipped significantly from the curve.  The rest of 2014 however was much warmer, resuming the upward temperature trend globally after the hiatus or temperature pause over the last 18 years.  Nevertheless, it wasn’t the warmest year for every part of the UK and a useful chart by Ed Hawkins (climate scientist Reading Uni @Ed_Hawkins http://www.met.reading.ac.uk/~ed/home/index.html ) reveals that parts of the UK had a cooler year than others and their warmest annual temperature record, technically, has yet to fall.

The animation below shows Europe bathed in above-average anomalous temperatures for the entire year, except momentarily dipping around August.

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2014 started stormy and windy.  The 3 month mean pressure charts below shows the mean pressure pattern during early 2014 and illustrates the air flow yielding our stormy and wet Jan and Feb: note the strong SW winds that result from the location of the LOW pressure in the Atlantic near Iceland up against the strong Azores high to the south creating a steep pressure gradient and resultant high winds.  This situation yielded a positive North Atlantic Oscillation which correlates with a warm, stormy wet winter. Contrast the winter chart with the rather slack flow of 2014 summer.  These are mean sea level pressure charts for those 3 months seasons.

The charts below show wind regime and gusts in Reigate during the year.  The highest gust of 52mph occured on 25 January.

In August the notable Ex-Hurricane Bertha, despite limited realtime impacts on the UK, delivered a sting in her tail by lowering temperatures across the UK in her wake by introducing a cooler-than-average NW flow for August.

Finally, Reigate had 1560 hours of sunshine in 2014.  2014 was a sunny year though not outstandingly so.  December was notably sunny for the time of year with over 100 hours of sunshine, over 160% more sunny than usual!

refs

http://www.metoffice.gov.uk/hadobs/hadukp/charts/hadukp_daily_plots.html

http://www.metoffice.gov.uk/climate/uk/summaries/2014/annual

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Reigate December 2014 (2013)

  • Tmax 12.7C (11.4)
  • Tmin -2.9C (-1.9)
  • Average temp 5.3C (5.8)
  • Total rainfall 49.4mm (CoCoRaHs) (110mm)
  • Max gust 38 mph (47)
  • Total sunshine 100 hours  (92.8 hours)
  • No snow

For Reigate and the whole of SE England, December 2014 was drier, warmer and sunnier than the long-term average (1961-90), but was it less windy than December 2013?

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In Reigate and for much of Southern England a HIGH pressure moved over to create a cool calm frosty end to the year with more sunshine than usual with a total exceeding 100 hours (92 hours 2013).  Rainfall was also less than usual for December at only 49.4mm (SE Dec average 96mm).  This was despite two low pressure systems this month labelled as “bombs” in the media.  Notably there was no snowfall recorded for Reigate in December and it maintained the warmer-than average theme making 2014 the warmest year on record for England, in the longest running continuous temperature record stretching all the way back to 1772 (more on this later).

2013 was remembered for being extremely stormy for good reason as gales and rain swept the UK through the winter period causing floods and travel chaos.  However, was December 2014 really so much less windy? One storm worthy of note struck this month but the weather “bomb” on the 12 December hardly caused a stir in the SE, with most of the impact being restricted to the NW of the country. The diminutive December max wind gust in Reigate of 38 mph was caused by the non-bomb LOW that sank SE across the UK that introduced the cold final days to 2014, the so-called “arrival of the queen of freezer”!

2015-01-01_15-07-49 december 2013-14 highest av wind speed

The chart above shows the daily highest average (10 minute) wind speed for each day through December 2013 (red line) and 2014 (blue line) as measured in Reigate. The chart shows that December 2013 seems to have started less windy than 2014 but caught up and finished on a more consistently breezy note.  Nevertheless, the difference is perhaps less than might be imagined considering the stormy label given to December 2013, though of course Reigate never experienced the highest gusts nationally which were reserved for coastal areas.  Nevertheless, equally surprising is that there were 14 days when average winds exceeded 10 mph in 2014 while only 11 days exceeded 10mph in the supposedly windy 2013.  The data for 20 mph starts to show the difference between 2013 and 2014: only 1 day exceeded 20 mph in Dec 2014 while 2 days exceeded 20 mph in 2013.

The data with respect to max wind gusts also confounds the idea that Dec 2013 was windier than Dec 2014.  The average maximum wind gust in Dec 2014 was 23 mph whereas the average maximum wind gust in Dec 2013 was 22 mph, no significant difference.  Of course, this all hides the crucial MAX GUST data (peak wind gust in 24 hours): but this doesn’t help either much because there were 18 days with max gusts exceeding 20 mph in 2014 while only 15 days exceeded 20 mph in 2013.  It is left to the absolute value for maximum wind gust to distinguish the two years because it is only in this category that December 2013 markedly exceed values in 2014 with 2013 max gust being 47 mph and 2014 just 38 mph. So it is these maximum gusts that people remember and associate with “storminess” because they do the damage, even if they are only brief moments in more average wind events.  On all the other data December 2014 was windier than 2013!  This also shows, of course, how different weather data can be used to illustrate different angles on a story.

  • Max Gust 2013 47 mph
  • Max Gust 2014 38 mph

It could be concluded that the collective memory of “severe weather” is often down to a few key events that raise public awareness, more than the weather itself.  There might also be thresholds which hit the news and chime with our collective severe weather memory: flooding and snow being the obvious triggers.  December 2013 was as much to do with heavy rain as it was to do with gales, especially for the sheltered inland areas of SE England.  Rainfall this December has certainly been lower than the long term average. The chart below shows the December precipitation average as 75mm for England and Wales and just 56mm for South East England, December 2014 in Reigate is lower still at 50mm, half as much as 2013.

 

So December 2014  has been an unremarkable month for Reigate and SE England although it was nearly as windy as 2013,  but it didn’t quite hit the extremes that make headlines!

More widely December caught the imagination as freezing weather sank south across Europe and the USA where huge snowfalls were recorded in Buffalo NY and in parts of the Alps.  It snowed in Algeria while Iceland experienced balmy December days as warm as 15C.  In the SE we were stuck under a very HIGH pressure (record breaking 1044mb) that gave us the sparkling final days in 2014 with pleasant winter sunshine and some cold temperatures as low as -7C in Redhill airport.  In Reigate our lowest temperature for December of -3C shows how a town location can ameliorate extreme temperatures.  Happy New Year!