Archives For Reigate

eclipse shadow

eclipse shadow

Sadly in Reigate and across the SE of England a blanket of thick cloud persisted throughout the eclipse period and we had no direct view of the amazing spectacle. Elsewhere in the UK views were mostly better, so we had about the worst possible situation: thick stratus and stratocumulus that stubbornly didn’t move until midday.  Nevertheless, effects of the eclipse were recorded and experienced. The afternoon was cloudless blue sky, so the weather played with us.  On the bright side, our student weather club eclipse forecast turned out spot on here! NewEx RESULTS: scroll down

cloudy in the SE

cloudy in the SE

Darkening skies: eyes adjusted to the fading daylight but the timelapse below shows the light fading as the camera exposure and shutter speed were taken off auto and set to manual. Spot the lights automatically coming on.

Cooling down: as solar radiation faded, temperatures locally fell a little but only by less than 0.5C or so.  As the eclipse was relatively early in the morning it seems that the usual climb in temperature was somewhat held back by the eclipse cooling.  Dew point, rather surprisingly, also dipped somewhat showing a somewhat drier atmosphere for a short period during the eclipse.

2015-03-20_21-20-06

Pressure change? Although we cannot directly experience this, pressure fell markedly towards the end of the eclipse period.  The overall forecast for the day was for pressure to fall… but was the pressure “held up” somewhat by cooling subsiding air from aloft?  Well, we won’t know for sure but the pronounced “pressure cliff” seems to nicely coincide with the maximum of the eclipse period.  The wind moderated somewhat through the eclipse which can been seen by the slight lowering of max wind gust speeds below.  Nationally, NewEx found little evidence of the “eclipse wind” (see below).

2015-03-20_21-28-25

Overall, the eclipse in the South East and for us in Reigate was spoilt by cloud and we didn’t get to see this rare event.  Nevertheless, there is some good meteorology that will come out of this, not least to investigate the influence of solar radiation on weather models.

another eclipse shadow

another eclipse shadow

National Eclipse Weather Experiment: summary of findings quoted from University of Reading Meteorology Department “StarGazing Team”.

“After the data had been uploaded, we collected together the observations from the different sites and averaged them. From combining the measurements from all the participants, these show a clear drop in temperature across the country.

Temperature

Temperature change

A reduction in cloud in central England during the eclipse is also apparent. This is a very interesting result for further analysis, and one which would be hard to obtain other than through the efforts of a disciplined group of distributed human observers such as yourselves.

2015-03-24_18-09-58

cloud cover change

This finding is therefore almost certainly unique to NEWEx. As you may have noticed, winds were mostly light across the country during the eclipse, which meant the circumstances were not well suited for detecting changes in the wind. The so-called “eclipse wind” unfortunately remains elusive, so more work will be needed on this.”

2015-03-24_18-11-02

wind change

more here from NewEx http://www.met.reading.ac.uk/outreach/newex_2015/index.html

Low cloud on a slack N/NE flow has sat over the SE overnight left by the weak Channel low.  This low cloud will clear during the morning, the question is will it clear by the eclipse maximum at 9.30am?  Low cloud can be seen on the satellite image below but the cloud mask image shows clearance occuring in Wales, parts of N England and E England.  This is behind a weakening frontal feature (no rain) which is slowing as it moves south. Our student forecast still holds true from yesterday.. so check our original BBC school report eclipse forecast here

Sat pics this morning suggest the NetWx charts are doing well with cloud forecasts. If this turns out to continue then we can expect a relatively prompt clearance of cloud between 9 – 10am. As cloud is likely to remain in some places locally it will be down to a lot of luck but regionally some in the SE, especially further NORTH should get a view of the eclipse between 9-10am. **as we know it stayed cloudy across Reigate and SE England during the eclipse and didn’t clear until midday. London had a better view so the clearer weather was never far away but took its time to arrive: better luck next time!** UPDATE ON IMPACTS OF ECLIPSE ON WEATHER FOLLOWING SOON

Watch out for subtle weather impacts such as an eclipse wind, changes in cloud formation and a slight dip in temperature during the event.  More on this from our weather students here 

Here is a reminder of the times and % cover of the sun during the eclipse.  Remember not to look directly at the sun at all.  Use a pin hole camera.

2015-03-12_21-10-33

February 2015 Reigate weather summary

 

Weather statistics summary for Reigate during February 2015

Average temperature 3.9C (UK 3.5)

Tmax 10.8C 25/02

Tmin -3C 22/02

Total precipitation 59mm

max wind gust 32mph 06/02

February was around the long term average at 4C in Reigate.  The CET stood February at just 0.3C above the long term average.

After an intially cold start to the month the temperature rose.

Overall February came out around average temperature but note the much warmer continent.

2015-03-12_21-58-44

A notable cirrostratus halo occured around moon on 02 Feb and this accompanied by some brief excitement over a snow band moving south down the eastern side of the country.

This band delivered an insignificant snow flurry on 03 Feb, overnight, with  a cm or so of briefly lying snow that melted rapidly during the morning.

Models threatened easterly winds on occasion but this didn’t arise.  In any case Europe and the continent experienced a warmer than average month so the deep continental cold was not available.  In fact after another brief flirt with snow at the beginning of the month, the temperature climbed through the middle of the month with westerly influences with temperatures exceeding 10C overnight on occasions.

Rainfall 58mm was just a little below average and sunshine hours, at 90 hours, was about average.

 

 

 

MetOffice February summary

http://www.metoffice.gov.uk/climate/uk/summaries/2015/february

 

How high can we expect temperatures to go and how Spring-like is this weekend set to be in Reigate? Find out below!

This weekend and possibly for much of March, the weather is set to be dominated by HIGH pressure nearby to the south and low pressure to the NW which will bring in mild southerly or SW winds for Reigate.  Cooler and cloudy easterly wind directions are also possible later next week if the high pressure slips NE to Scandinavia which models are suggesting.  Whilst this weekend is expected to be pleasantly mild and spring-like and initially sunny on Saturday, temperatures are not going to break any Spring records because cloud cover is gradually going to spill from the north on a weakening cold front.

2015-03-06_20-08-11

synoptic chart Fri 6 March

The synoptic chart above shows the HIGH over the continent to the south of the UK and a deep low to the NW between Iceland and Greenland.  This is dragging in, with the help of a lively SW jetstream, a SW moist airflow over Scotland.  In fact NW Scotland has an amber warning for huge rainfall totals above 140mm over the next few days assoicated with the stalled cold front that will sit near or over Scotland for much of the time.  Warm air flow ahead of this cold front is advecting large amounts of moisture in a plume over the mountains which is causing the high totals over the NW. The charts below show the unsettled NW compared to the calm, mild and dry SE of the UK under the influence of the HIGH pressure.

The charts below show the story for this weekend.  Initially a dry airmass sits over the SE on Saturday morning but this is gradually replaced as cloudy conditions slip SE.  Temperatures through the weekend are looking mild, with Tmax 15C possible on Saturday, but anything higher is less likely on Sunday as cloud further thickens with the arrival of a weak cold front.  Saturday is probably the most pleasant day with brighter sunnier conditions especially in the morning.  The cold front in Scotland slowly migrates SE during the weekend but weakens as it does so.  By the time it reaches the SE on Sunday evening it is probably only going to bring low cloud and some drizzle.  Throughout the weekend wind in the SE is set to be light, especially on Sunday.  Misty conditions might occur overnight into Sunday and later Sunday evening in light winds.

Next week is looking generally mild and with HIGH pressure not far away to the south mostly dry.  A couple of LOW pressure systems are forecast to pass across the NW of the country and their trailing fronts will be weak in the SE but could bring cloud and some light rain.

2015-03-06_18-50-29

LOW passes to the NW on Monday

Overall the high pressure looks set to dominate Europe during next week bringing dry and warmer than normal conditions.

The GFS and ECM both suggest that the HIGH could slip over Scandinavia by the end of the week, as the chart below suggests.  This would introduce cooler easterly winds to the UK but nothing too icy at this time of year, it would also remain mostly dry. Unfortunately, easterly winds are often cloudy as they pick up moisture from the North Sea that creates days of anticyclonic gloom under an inversion.

2015-03-06_20-30-58

Scandinavian HIGH… would bring dry, cool but gloomy March weather

The charts below summarise the weather outlook: high pressure domintating bringing mostly dry and mild conditions.  Nights next week could turn colder with possible frost returning.  The longer range models suggest March could turn out to be a very dry month especially in the south.

BBC summary

http://www.bbc.co.uk/weather/feeds/31774369?ns_mchannel=social&ns_campaign=bbc_weather&ns_source=twitter&ns_linkname=news_central

 

A system stirred up by a low pressure tracking out of Mexico into the Gulf of Mexico this week is on track to bring unsettled conditions for part of the weekend to the UK.  It’s nothing too severe for most but is an interesting feature that will bring some wind and rain everywhere.  Below is a satpic showing the development of this system as it interacted with a lively jetstreak on Saturday 28/02/2015.

development of LOW on jetstreak

development of LOW on jetstreak

 

This LOW illustrates nicely how extra-tropical systems can rattle clean across the Atlantic in a few days if they are picked up by an active jetstream.   This one does precisely that.  Spot the system leaving Florida on the chart below for today and the sat pic.  This system started as a low pressure crossing from Mexico into the Gulf of Mexico mid-week, so for weather systems it will be an aged fellow on arrival here in the UK.  Its’ longevity is partly due to the exceptional COLD over NE US which interacts with the warm tropical air and causes further deepening.

The Gulf low pressure is tracking quickly NE skirting the US east coast before being picked up and deepened further by an active jetstream.  The jetstream itself is particularly powerful at the moment due to intense cold spilling out of an exceptionally wintry NE USA meeting warm tropical air issuing from a strong subtropical Azores HIGH pressure converging with the moist Gulf airmass.  A result of the powerful jetstream is a positive North Atlantic Oscillation: the NAO is a measure that indicates the difference in pressure between Iceland and the Azores.  In positive NAO conditions the jetstream is often active, producing a strong westerly zonal flow keeping Europe mild and unsettled especially in winter, or early spring!

Our Gulf LOW is due to pass over Reigate fairly rapidly through Saturday pm and overnight into Sunday am and bring some moderately wet and windy weather, likely to go unnoticed because of the nocturnal transit.  Winds gusting in excess of 40mph are possible for Reigate into Sunday am in exposed places.  Notably, due to the TROPICAL origins of this airmass the temperature overnight Sat-Sun could climb to double figures in Reigate.  Tropical air crossing the Atlantic also picks up a tremendous amount of moisture so attendant fronts are likely to bring a lot of rain too, possibly exceeding 10mm overnight, which is a moderately wet night.

Here are the synoptic charts showing the Gulf low progress across the Atlantic, deepening and occluding into the North Sea.  Note the secondary wave which could bring additional rain later on Sunday to the South.

This slideshow requires JavaScript.

The additional rain later Sunday afternoon / evening looks potentially heavy for the South and SE.  It’s a rapidly developing wave feature that needs attention as, on the northern edge, it looks to raise the possibility of snow across the Midlands.  Heavy rain is possible for the SE and #Reigate with a period of gales on the south coast.

2015-02-26_22-44-28

The outlook for next week is for the Azores HIGH pressure to extend a ridge to the north and cause a NW then northerly airflow for the UK.  This will bring cooler temperatures to the UK.  Whilst it is likely to be mostly dry for Reigate and the SE with pressure rising, wintry showers especially on east facing coasts of the North Sea could be possible depending on how the HIGH develops.  Frost is likely with temperatures dipping below freezing at night from mid-week.

2015-02-26_19-17-13

Atlantic ridge builds to the west and brings in northerlies

How long this Atlantic block persists, and the cooler weather, is uncertain.  The coldest scenario would depend on the HIGH moving north and east and building over Scandinavia to pull in easterlies from a cold continent.  This scenario is preferred by the ECM by later next week whilst the GEFS topples the high to the SE and brings back a zonal mild westerly flow from the persistent Icelandic LOW pressure that erodes the edges of the HIGH from the NW.  The charts below show the uncertainty as a wide spread of possible pressure and temperature towards the end of the first week in March.

2015-02-26_19-33-18

Gulf low for the weekend, then pressure builds

The cool start to March is shown below.  The overall outlook is for a persistent positive NAO and Arctic Oscillation to persist and this would suggest a brief cool epsiode without the formation of a persistent Scandinavian High.  Models have flirted with possible easterly winds by the end of next week but the outlook is for the positive NAO to persist and this rather suggests a quick return to milder zonal westerlies.  As the high builds in early in the week various troughs and fronts could even push some wintry precipitation as far as the SE on Tuesday (spot the pink on the rainfall chart below for Tues)

First week of March starts cool: MetOffice synoptic chart for mid-week shows Azores high briefly ridging north to block mild zonal westerlies and usher in a cold polar airmass, albeit briefly as this ridge looks to topple SE and by next weekend we could be in a pretty mild SW flow hitting mid-teens possibly.  So a cool, mostly dry middle part of the week for Reigate and much of S England but precipitation, some even wintry, pulling in on NW / N winds is not ruled out with a North Sea low possible.  As the Atlantic is likely to push westerlies back in later in the week we can expect more purposeful frontal rain pushing east across the whole country.

2015-02-28_09-19-25

 

link to accuweather take on this system

http://www.accuweather.com/en/weather-news/winds-to-whip-uk-north-sea-coa/43063930

 

2015-02-04_18-44-20

January 2015 summary for Reigate, Surrey

Average temp 4.5C (UK av 3.7C, CET 1C above long term average)

Tmax 14.7C (12.2C 2014)

Tmin -4.9C (-1.5C 2014)

Total precipitation 69mm  (183mm 2014)

Max wind gust 42mph (52mph Jan 2014)

snowfall was recorded on 3 days, snow lying on 2 mornings

Total sunshine 96 hours (98hrs 2015)

2015-02-04_18-50-532015-02-04_18-50-11

January was mostly under the influence of an occasionally strong westerly and NW airstream with some stormy weather for the NW of the UK but, predictably, Reigate and the SE was sheltered from most of the weather action and our rainfall total was about on the long term average for the South East at 69mm. This was, of course, considerably less than the 183mm rainfall last January 2014!

2015-02-08_20-12-44

Overall, January was just about average temperature in the UK. This average hides the variation though… the start of January was extremely mild with some very warm Tmax temperatures early in the month reaching nearly 15C in Reigate.  The end of January was considerably colder than average although nothing extreme.

The end of January cold snap ran into the first week of February.  During this period Reigate saw temperatures drop modestly below average and we experienced 3 spells of modest wet snowfall, albeit lying snow from night time falls rapidly melted by early morning and the lowest temperatures we a mere -4C.  Reigate experienced one notable but brief “thundersnow” event on the afternoon of 29 Jan. This caused some local traffic disruption.

The cause of our swings in weather a neatly summarised in the mean monthly sea level pressure and 500hPa anomaly chart from the JMA.  Here you can see the building high pressure over the Azores towards Iceland and the low pressure to the North and NE nudging towards Scandinavia.  It is this configuration of building ridge in the Atlantic and LOW over Scandinavia that eventually brought our modified “Arctic blast” through the last week of January.  As is normal for an Arctic airmass the SE of England away from the North Sea coast of Kent rarely sees any prolonged snowfall and this was the case.  An interesting feature developed on the 29/01/2015 within the Arctic airmass: a polar low may have spun up and moved South through the Irish Sea.  This was controversial and not accepted as a true polar low by everyone but it seemed to have many of the characteristics.  A post on this polar low can be found here.

 

The evolution of the January 2015 cold snap was interesting because it was initiated by an unusually cold and unstable NW airstream on the back of a deep low that crossed Scotland.  The NW airstream was unstable enough to bring the thundersnow event to Reigate. Oddly the original NW “blast” from Greenland pushed through so quickly that a lot of polar maritime returning and tropical maritime air was secluded in the low core over Scandinavia.  It was this secluded / occluded warmer airmass that a) probably contributed to the polar low feature and b) modified the Arctic blast and , at least initially, made it much less cold than is usual for such an airmass direction.  It took several days for any truly cold air to reach the SE of England and, even then, 850hPa temps never fell below -6 or -7C.

The CET for January was nearly 1C above average for the long term average, quite a lot more than the MetOffice.  As can be seen from the chart below this January was not exceptionally warm, being moderated by the cold snap at the end of the month.  Only 6 of the last 20 years have come out colder than the long term CET average.

2015-02-08_21-09-03

Globally January was +0.35C above the long term (30 year) average temperature.  The northern hemisphere was +0.55C above the long term average.

2015-02-08_21-10-04

 

http://www.metoffice.gov.uk/climate/uk/summaries/2015/january/regional-values

http://www.metoffice.gov.uk/climate/uk/summaries/2015/january

 

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

Click to access arctic_maritime_jan15.pdf

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.

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.

2015-01-13_22-32-26

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!

2015-01-13_21-34-10