Archives For snow

2016-02-29_19-54-33

8-10 day mean upper air chart

The most wintry set-up of the “winter” has decided to arrive at the start of Spring! High pressure over the Pole is still keen to push out polar air into mid-latitudes as the meteorological Spring starts tomorrow.  The Arctic Oscillation shows this tendency as it has been dipping negative, showing relatively high pressure over the Pole and lower pressure in mid-latitudes. The belated rise in Polar pressure has been attributed to the stratospheric warming which occurred around a month ago.

2016-02-29_20-01-32

Unfortunately, for most of late winter the jetstream has remained strong enough to push a predominantly Atlantic flow into the UK and breach any blocks attempting to drag in sustained cold air. This is shown by the slight but persistently positive North Atlantic Oscillation below: this set-up spoilt any chance of proper cold this half of the winter.

2016-02-29_20-00-40

However, this week the wind is expected to turn more northerly as pressure lowers first over the North Sea and then the Channel / N France. By the weekend the UK will be in a cold Arctic flow courtesy of the same LOW lingering over Europe and a blocking Atlantic ridge. Between them and the jetstream they will do a good job of pulling down a cold Arctic flow this weekend. Spot the cold anomalies in the chart below. Lots of lying snow is not likely but some wintry precipitation is possible at times here, especially a marginal chance on Friday am.  Things change though, so check weather professionals like the MetOffice to make any weather related decisions.

Here is the story of how we get to some belated cold by this weekend.  Tuesday sees an end to the cool clear HIGH that brought Spring-like sunshine to Reigate and Tmax 9C.  A warm front will sweep in tomorrow morning and bring rain for most of the day for the SE.  It will be breezy, though relatively mild in the warm sector shown below, Tmax 10C.

2016-02-29_21-09-40

warm sector Tuesday

Things cool off into Wednesday as an active blustery cold front ushers in colder polar maritime air through the morning.  Any snow is restricted to the NW of the UK.

2016-02-29_20-38-37

The flow swings increasingly to the north west during the day with the possibility of showers later here, some quite heavy, maybe even with the outside chance of hail and thunder thrown in.  Tmax 6-7C.  Update: The video below shows how that showery trough passed through Reigate during Wednesday:

2016-02-29_19-39-43

Thursday is the crux to building a set-up capable of producing any snowfall at all for the SE.  A LOW is forecast to develop from a system off Greenland and cross the Atlantic smartly from the NW.2016-02-29_19-50-10

This low, with rapidly occluding warm sector, will deepen slightly and meet the cool pool sitting over the UK from Wednesday’s Polar attempt. The LOW is expected to arrive late Thursday and track south east overnight into Friday bringing in a cool NE flow capable of wintry precipitation for a while on Friday morning.

2016-02-29_19-43-04

The latest charts suggest the track into N France will result in NE winds which could bring snow for a time in the SE on Friday morning.

2016-02-29_21-52-18

meteoearth Friday ppt

It is all rather marginal for us in the SE and Reigate though. A rain / sleet event with a possible wintry mix at times is more likely and nothing much is expected to settle.

By Saturday and into the weekend a cold northerly / NE flow sets as the “Greenland” LOW settles over Europe. Wintry showers could develop across the SE, especially in any distrubances in the Arctic flow.  However, pressure is likely to be on the rise as the Atlantic ridge creeps in from the west under an increasingly anticyclonic jetstream.

2016-02-29_19-57-56

How long any early Spring cold lasts is not certain, the Atlantic ridge looks like toppling over to bring in warmer conditions later next week.

2015-11-19_21-56-59

MetOffice Fax chart Sat 21 Nov: cold UK weather

A cold Arctic surge will spill south over Friday night bringing a shock to the record-breaking warm November so far.  The airmass trajectory shows the extraordinary journey the air has taken to get to London.

2015-11-20_21-39-59

airmass trajectory (GFS)

It’s a brief Arctic attack but rather potent and Saturday morning will feel especially cold in strong northerly winds.

Snow is also forecast for Scotland and parts of the North of England as the Arctic air is brought south with a North Sea low and attendant fronts.  This system will reach Reigate and the SE early Saturday morning and likely bring a wintry miserable start to the weekend with sleety rain, maybe even some wet snow for the Downs. (in the event: snowfall in Reigate for an hour or so as front passed through, settled momentarily. front cleared off quickly)

In the South East, whilst the airmass temperature is just cold enough for snow and sleet, falling to a cool -5C at 850hPa by early Saturday morning, the dew point is shown to rise quickly above freezing as precipitation arrives in the morning and this will reduce the chance of any proper snow fall. This is fairly typical of the SE!

The main feature of Saturday morning in Reigate will be a truly miserable wet, cold windy start with an impressive wind chill well below freezing lasting all day, something we haven’t experienced in Reigate and Surrey for a long time!

2015-11-19_22-17-59

wind chill November 21 2015

Overall, this cold snap is not due to last long, warming slowly into next week but turning unsettled as the jetstream blasts back from the Atlantic. It will not, however, become as warm as it has been.

2015-11-19_21-26-37

unsettled due to return as jet bites back

Winter forecasters look high into the atmosphere to get an idea of what the winter will bring.  One feature they look at is the Polar Vortex.  The PV is currently getting stronger, which it usually does this time of year.  The vortex is the winter circulation of high altitude westerly winds in the stratosphere that keep the cold polar air locked up in the Arctic.  To get sustained cold outbreaks the vortex needs to be broken down: something that can occur when the polar stratosphere warms, sometimes suddenly.

However, with North Pole stratospheric temperatures taking a steep dive, lower than average, the temperature gradient between the Pole and mid-latitudes is increasing and so the subsequent pressure gradient is also steep: with generally low pressure over the Pole and high pressure further south.  This all really means cold air is less likely to leak south.

2015-11-19_22-27-24

The Arctic Oscillation is an index showing how atmospheric pressure varies between the North Pole and mid-latitudes.  In a positive phase the westerly winds are stronger and the jetstream more powerfully moves depressions across the Atlantic to the UK.  A negative AO is required for wintry outbreaks… El Nino can induce stratospheric warming episodes and many winter forecasters are expecting the Polar Vortex to come under attack later this winter, after Christmas, and possibly produce a colder second half.  meanwhile, expect more unsettled, relatively mild weather to return after this cold snap.

2015-11-19_22-34-50

The brief cold snap is shown in the ensemble forecast from the GFS and ECM models below: cold snap followed by a return to mild.

 

Is this it for winter? Read on…

Reigate stays cool for the first week of February after a couple of modest wet snow non-events overnight this weekend.  Snow thawed in the morning each day as temperatures climbed above freezing.  Whilst the air temperature was comparatively high at 5.8C Tmax at midday, the stiff northerly wind in Reigate was gusting to 30mph in town which brought wind chill as low as -5C at times.

The cause of the current cold weather is a northerly wind set up due to HIGH pressure ridging north in the Atlantic and a LOW over Scandinavia. This sets up a northerly flow, called an Arctic airmass, albeit modified by its significant journey over relatively mild seas.  Also, this particular Arctic airmass is not direct from the Pole, if you follow the isobars back from the UK you can see the air originating from southerly areas in SE Europe, so not truly polar.  In any case, it’s usual for Arctic airmasses to bring dry weather to Reigate and the SE: the long transit over land means it lacks moisture, usually dumping any significant snow over NE facing coasts well before it gets here.  More locally, our sheltered location beneath the North Downs, facing south, also affords good protection from Arctic airmasses and N or NE winds.  So, either way, Reigate rarely gets lots of snow from this airmass.

2015-02-01_17-28-01

arctic airmass

This week is likely to see further cold weather continuing as the HIGH slowly nudges in from the west by mid-week.  With HIGH pressure not far away and a relatively dry northerly airmass, a major snow event or indeed much precipitation at all is unlikely.

2015-02-01_17-27-26

So as pressure rises we can expect a cool mostly dry week with frosty nights and possible fog on occasions in lighter winds (fog not really ’til end of week though as wind remains significant running round high). In this set up a big snow event for Reigate seems most unlikely.  Nevertheless, an easterly / NE’rly wind for a time is a strong possibility, initially Tues-Weds as a front moves south, and so modestly disruptive snow showers reaching us cannot be ruled out.  Also, don’t forget icy roads and fog are arguably the most risky of all winter hazards so this kind of high pressure wintry weather should be handled with care if travelling.

At the moment  GFS, Ensemble and ECM models are agreeing that the HIGH pressure is likely to land up sitting somewhere to the north / NW of the UK by the end of the week.  With unusually LOW pressure in the south of Europe, this could set up a cold easterly wind especially across the south east, albeit this might yet not come off as other runs show the HIGH right over the top of the UK cutting off any easterlies very quickly.

The exact position of the HIGH will therefore make a big difference to whether we get much or any precipitation.  NE’rly or easterly winds, depending on their strength and track, can bring snow off the North Sea and inland into Kent and Surrey as well.  There is already a MetOffice yellow weather warning for the possibility of such an event mid-week, although these often don’t come to much they should not be ignored as the potential for perky snow showers causing local traffic problems has already been experienced with one minor brief thundery shower wintry episode last week.

The time-averaged charts show the overall story for the next week as being dry and colder than normal.  Throughout this episode the jetstream is wrapping round to the north of the HIGH, actually building it with milder upper air from the SW, and, eventually, it could help to deliver our easterlies at the end of the week.  As usual, for exact details check the twitter feed and consult official sites like metoffice for decision making.

refs

http://www.metlink.org/wp-content/uploads/2015/01/arctic_maritime_jan15.pdf

What’s a Polar Low?

January 29, 2015 — 1 Comment

2015-01-29_18-19-06

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.

2015-01-29_19-34-48

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.

2015-01-29_20-29-45

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

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.

2015-01-24_15-47-35

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.

2015-01-24_15-47-56

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…)

2015-01-24_21-18-11

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.

2015-01-24_15-48-26

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.

2015-01-24_17-03-53

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.

2015-01-24_19-28-39

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.

2015-01-24_19-27-51

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!

2015-01-24_19-39-11

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

2015-01-18_08-08-35

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.

2015-01-17_08-20-43

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!

2015-01-18_07-57-19

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.

2015-01-18_09-17-49

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.

Whilst this storm was uneventful for most in the UK the LOW is forecast to bring extreme cold to parts of Europe.  Here’s the story… see below for “in the event” 

Update Fri 8am! MetOffice chart brings LOW further South… this would bring in colder air more quickly for SE on Sat am possibly interacting with the frontal rain and turning it to snow for a short period, check weather warnings before travelling, especially north through the Midlands and N England and Wales tonight.

Quick update on weather prospects for Reigate and SE Boxing Day night and into Saturday 27 Dec.

The low pressure storm arriving Boxing Day afternoon is set to bring chilly conditions, lots of rain and some snow, but for who and when?  The LOW pressure is set to track overnight ESE through Wales, across the Midlands and exit through the Thames Estuary sometime in the early hours on Saturday. Strong winds are likely during this time, initially W/SW winds for the SE and Reigate building Friday pm/ evening and then veering to brisk cold winds NW/N winds by Saturday morning. Winds could reach 30-40 mph in places and more on coasts possible. Wind chill temperatures overnight will be cold, down to -6C in places. There will also be a lot of rain overnight: possibly 10-20mm in places. It is of course the snow that people are interested in.  Whilst snow is likely on the northern side of this low across parts of Wales, Midlands and N England, parts of East Anglia, for us in the SE on the milder south side of the low the snow is initially unlikely and much more marginal and more difficult to forecast.

A bit cheeky but couldn’t resist this apt tweet from WindyWilson in Scotland!

Overnight snow is unlikely for much of the SE because for much of the night we will sit in the warm flow of air to the south of the low centre.  Rain is forecast to arrive sometime mid-late afternoon. Friday will then actually warm up to possibly 6C in the evening in an occluding low as the warm front arrives.  This is called warm air advection and is what drives the lower pressure down as air rises.  It is also likely to keep any precipitation as rain for most of the night.  Overnight this warm air will be forced aloft by the chasing cold polar air sweeping down from the north, this can be seen on upper air charts below.

The charts below show snow fall is only likely on the very back edge of the system as the low pressure slips away across the Channel and drags in the coldest air from the N / NE in its wake. It is only at that point , from early on Saturday am, that the Downs and Reigate and the SE might get some snow but, even then, it is only 50% chance (see skew-t below).

On the atmospheric cross section (skew t) below for Heathrow the warm air can be seen on the 21:00hrs GMT chart as a slight bulge with increased height.  This is an isothermal layer which shows warm air is in the system at this point and likely to melt precipitation starting as snow higher up.  The skew-t diagram on the right is for some hours later at 03:00hrs GMT when temperatures can be seen to have fallen at the surface, winds veered to a cool Northerly direction after the cold front has passed through.  At this time a rough calculation of Dew Point + Temperature yields 3.3, which would give a 50% chance of snow at this location.

 

2014-12-26_20-14-52

Snow might also fall as heavy rain drags colder freezing air from above to the surface. Evaporative cooling, however, is unlikely as a snow making process because it requires less windy conditions.  By Saturday morning the polar Northerly air has arrived and this has dew points low enough for any showers possibly pushing in on the NE breeze to fall as snow at any time during the next 48 hours or so.  So… snow is unlikely for Reigate first thing overnight Boxing Day, there is a 50% chance of snow for the second half of the night, especially over high ground like the North Downs and especially if the rain is heavy enough.  Finally, as dawn breaks on Saturday any rogue showers penetrating our area could fall as snow in the frigid air.

This system is set to bring in a cold weekend and early next week a cool high pressure will keep things dry and frosty.  After New Year things look to be warming up and turning more Atlantic driven on the latest charts as winds bring rain back from west.

 

Why is forecasting snow so tricky?  http://blogs.channel4.com/liam-dutton-on-weather/snow-challenging-forecast-uk/2568

In the event:

Snow fell as forecast by MetOffice across Northern England / Midlands and caused some traffic problems and flight cancellations out of regional airports Manchester and Liverpool.  Snow accumulation up to 10cm was reported and some low wind chill.  The snow zone was a discrete area and to the south the warm sector kept everywhere south of the Midlands free from snow and, as expected, mild throughout (8C) until the polar air arrived behind the LOW. Some sleety showers and possible snow flurries came and went uneventfully across the SE and especially Kent but no accumulations were reported.

The LOW had much more wintry impact into Europe: with Netherlands through to the Alps receiving significant snowfall.  The LOW is forecast to continue SE into SE Europe and drag in some freezing polar continental air into the Balkans, Greece and even reaching as far as the North African coast.  Temperatures as low as -20C are expected across Serbia.  SNOW fell in Algeria.

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.

2014-02-08_23-09-31

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

(Got pics of “mini-tornado” damage?! Please post them on twitter or send them to me here for a post on the subject to be posted shortly, thanks.)

QUICK UPDATE Sunday: Reigate mixed cool wintry weather this week with always marginal conditions between sleet/snow but mostly rain midweek as the active LOW sinks south and fills and produces some interesting but uncertain possibilities for unstable bands of heavy rain circulating round the LOW in troughs as the week progresses. Convergence at times on the S coast could cause thunderstorms at times.  The coldest air overlying SE is due Sunday-Monday but, given the subtle nature of wind directions from the continent and the uncertainty of models as to how this scenario ends, anything could happen regarding next weekend… heavy Atlantic rain and then cooler could be possible. Watch UKMO warnings carefully for updates regarding Tuesday and Weds rain. 

Temperatures are set to dip next week below the balmy winter conditions we have been used to recently.  Below average temps will arrive for the first time this winter to Reigate.  It has been mild so far this January with CET temps over 6c average, courtesy of an extremely lively east-bound jetstream that has flooded (literally) the UK with mild and moist westerly winds from the Atlantic.  A cold plunge has been on the model-cards for a while but has come to nothing so far. However, a LOW pressure spinning up in the Atlantic on Friday is set to bring stormy conditions especially to the W and NW of the UK this weekend with fronts sweeping some significant rain right across the country, including SE and Reigate on Sunday.  It is the track of this LOW pressure that is likely to cause the dip in temps next week. Whilst significant snow is not likely for the SE it is possible that we might get some wintry mix of sleety/ snowy /rain precipitation on occasions through next week and, as Atlantic fronts possibly push in across the region at the end of the week / next weekend these might bring more significant snow on the leading edge of fronts, possibly for the end of Jan or start of Feb.  This is a way off so not certain at all but a possible interesting outcome to ponder that might bring at least fleetingly proper wintry weather to our winter-starved region.

The cold air is set to be dragged down across the UK as the LOW pressure over NW Scotland moves south and east across the country eventually sitting, cut-off, over the continent through next week.  A cut-off LOW is a cold low with plenty of cold air wrapped up in the circulation (called “cold uppers”: cold air throughout the depth of the atmosphere) that becomes cut-off from direct polar or tropical feeds of air.  It becomes a closed circulation with the cool air gradually sinking down to the surface, chilling things further.  Convective showers are also possible as these lows “warm through” from any surface heating.

When this “cut-off low” or cold pool settles over the near continent by early next week, the winds on the northern back edge of the LOW will pull in cool easterly and NE winds that could be cold enough to produce wintry precipitation on occasions, certainly to the North of the UK and maybe to the SE as well, but this is by no means certain for us and probably remains unlikely at the present time until maybe later in the week.  Whilst the source region for the wind is not as cold as 2013, the continent is nevertheless pretty chilly at this time of year so expect Tmax temps to crawl to just 5 or 6c in Reigate but not much higher.

So, we are set for a “cold rinse cycle” next week as this cut-off low circulates cold air over the SE with resulting change in wind direction from mild westerlies, through cool showery NW and, eventually easterlies with the potential for incursions of even colder air from the continent at times.  This is not a severe winter episode, merely a cool/cold snap with some wintry precipitation possible.

http://www.bbc.co.uk/news/uk-england-25902371

http://www.dailymail.co.uk/news/article-2546150/UK-weather-More-heavy-rain-way-flood-hit-parts-Britain-mini-tornado-hits-southern-counties.html

http://www.epsomguardian.co.uk/news/10965414._Mini_tornado__brings_down_trees_in_Epsom/?ref=rss&utm_source=twitterfeed&utm_medium=twitter

STORM SURGE WARNING FOR NORTH SEA COASTS WIND and North Sea storm surge update later today: check twitter @RGSweather for updates!

Check environment agency website for flood warnings http://www.environment-agency.gov.uk/homeandleisure/floods/142151.aspx

Point to note: the original weather forecast (below) for Thurs-Sat dec5-7 emphasised a plunge of cold Arctic air on the back of the low passing across N Sea.  However, in the event, as things quickly transpired nearer the time the strong northerly winds became the focus and the storm surge down the east coast on the back of it. 528 dam (coldest air) stayed well to the east of the SE and so we had squally showers on the cold front with no wintry ppt.  

Express meaning rapid / short-lived icy blast for later this week is firming up in most model runs now, though with differences in arrival, departure and intensity (especially the GFS which is least chilly, but I understand this is not deemed as performing well on this event).  Anyhow, it would be sensible to assume that it will be chilly or cold to very cold for early December across much of the UK from Thursday onward and through much of next weekend with the North and coastal areas, especially North Sea coasts, getting some really icy gales at times, particularly Thurs-Fri/Sat.  

30-11-2013 22-29-39Some snow is likely for north facing coasts both East and West and Scotland and parts of Northern England.  Reigate and the more sheltered SE will, as usual, be protected from the worst but we can still expect cold blustery conditions at times with such a cold upper Arctic airmass incursion. Air mass temps are usually measured at 5000ft (850hPa) (to avoid surface interference and changes day/night, urban/rural etc above the so-called boundary layer). Currently our 850hPa temp is around 0c, by the end of this week they will drop to -8c or even -10c.  

30-11-2013 22-16-20

Include the wind expected, especially Thursday – Friday and this will feel cold if you are out and about, watching football matches on the touchline for example: much colder than this weekend!  Air temps may struggle to reach 2-3c in Reigate on Friday and the 15-20mph wind will make it feel well below freezing.  An interesting additional feature is a possible LOW forming in the southern part of the North Sea on those icy northerly gales.  If this occurs it might bring snow to the far SE of the UK and certainly into Netherlands and, later, the rest of N Europe. 

However, it is likely that this episode will be a brief SHOT of cold as a high pressure stirs in some warmer air later next weekend and the winds also die down making it feel more tolerable, though an anticyclonic gloom is more likely than sparkling sunshine. Latest Aus model shows an easterly picking up sustaining some cool conditions in SE in the week after this cold shot…check below…

01-12-2013 08-56-03

but a tad warmer later

but a tad warmer later

The culprit is the jetstream which is looping wildly around the Northern hemisphere at present.  The jetstream acts as a kind of “belt” around the Poles holding the cold Arctic air in.  If the jet starts weakening and wiggling from North-South it allows cold air to leak out from the Poles and plunge south in huge swoops of icy air.  The Arctic swoop expected later next week is going to inject icy weather to much of Europe from Greenland and the North Pole  The UK, being on the western edge of this swoop, will miss the most intense cold (that’ll hit Netherlands and Germany and, eventually, get as far as E Europe and the Alps).   The lead-up to this event is pretty benign with a high pressure giving calm cool cloudy conditions early in the week, turning into a more westerly zonal flow for a time before the northerly plunge hits from Thursday. More on this later in the week as more details emerge.

30-11-2013 23-41-45

A current picture of Greenland shows conditions this weekend.  Think of air leaving here this week and reaching the UK by Thursday: thankfully, it’ll warm up from around -40c on the ice sheet to about +4c by the time it reaches Reigate high street.

30-11-2013 12-09-50