Archives For March 2013

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

Polar air escapes in negative AO

Polar air escapes in negative AO

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


WEATHER ALERT FOR UK AND REIGATE:

Saturday am: heavy snow falling across our region: check twitter for updates.  A major wintry weather event is unfolding tomorrow and over the weekend across the UK.  This could bring potentially hazardous weather Friday through Sunday almost anywhere in the UK so if you are travelling please take care and check weather warnings for your destination. Reigate remains comparatively sheltered throughout but even here we have the threat of seriously miserable and cold weather with an evil mix of heavy rain, cold winds and sleet turning to snow over this weekend.  Snow amounts and depths for us still uncertain though.

Cold Polar Easterly winds will increasingly undercut and push back the advancing warm air brought in by a vigorous depression out west in the Atlantic. Where these air masses meet is essentially where the heaviest rain and snow will fall: the fronts move north on Friday and then move south on Saturday as the cold air wins out over the warm: it is the move south on Saturday which could bring some SNOW to Reigate. The SW is due to get torrential rain, whilst anywhere north of the M4, especially on high ground, could see heavy snow and drifting in high winds. Reigate remains comparatively sheltered throughout this episode BUT if you are travelling anywhere else do heed weather warnings.
Whilst Reigate will escape the worst of all this, we are due to have a pretty cold, windy, very wet and sleety and at times snowy weekend weather wise!
Friday sees fronts crossing the country and bringing increasing rain to Reigate during the day. Overnight into Saturday might see this rain turning increasingly icy and sleety. Wind chill 0°C to -4°C.
Saturday will be a truly horrible day with temperatures falling throughout. Cold winds (feeling -5°C) and heavy rain will turn increasingly sleety and turn to snow anytime but with greatest risk in the afternoon.  By the afternoon, however, the heaviest precipitation should be dying out over Reigate so any snowfall should be light by that stage.  Saturday could see totals over 20mm of precipitation (mostly in the morning as rain); snow lying could amount from 0cm to a few cm especially on the Downs by evening. Any snow conditions will be worse to the north of the region and over high ground.
Sunday sees even colder weather and a threat of further snowfall from nearby fronts to the south.  Still uncertainty around this so keep watching forecasts. Wind chill -8°C so feeling very cold indeed.  Drier later though.
The rest of the week looks cold, frosty and dry with gradual recovery of temperatures and the hint of more spring-like temperatures next weekend!
Take care this weekend if you are travelling anywhere in the UK: floods in the SW, blizzards and snow up North. If you don’t need to travel, you should probably wait until Sunday or Monday when things have calmed down.

What a difference a year makes: check out the daffodil pictures: taken one year apart!

update Friday pm: much as stated above with main snow zone staying north of London for longer on some models: filthy horrible day mostly sleet icy rain Sat; poss light snow later pm as cold air arrives; cold wind; colder on sunday and into next week. Cold and frosty next week. #notspring 

Update Thurs: things changing: snow being brought further south: cold air push back! snow risk for Reigate Sat and Sun! more later, mobile situation, keep watching but wintry weather certainly on the way!

The UK will see some heavy snow over Friday and Saturday as Atlantic fronts bring warmer air spiraling round a LOW pressure to the West bumping into the frigid cold Polar Easterlies blasting from the continent. It looks like the big snow action Friday and Saturday will miss Reigate and stay further north and we will just suffer cold rain, possibly a bit sleety and certainly not pleasant; wind chill temps down to freezing and plenty of rain, up to 10mm!  The North of the country, especially over the Pennines, could see record-breaking March snow.

There is a big North-South divide in air temperature caused by subtle but critical differences in source region: the wind in the North and the South is basically EASTERLY in direction but check the isobars over the country either side of the FRONT on the map showing Saturday’s weather. Over Southern England the isobars can be traced back in a long wide arc over Europe originating somewhere over the Atlantic off Spain and Portugal: much warmer than the source region for the cold Polar easterlies emerging out of the Poles and flowing across a still-cold Russia.  The Atlantic air has 850hPa temps of 0ºC to -5C, whilst the Polar Easterlies drop to -14°C or lower (850hPa is air temps at 1500m, commonly used to measure air mass temperature avoiding surface influences).  So Spring air from the west is trying to make a break through over the country but in a very roundabout way!  Sadly, the Easterlies win next week and flood the entire country with COLD air by Wednesday: expect chilly but dry conditions, frosty nights.

Finally, here’s a special TREAT: take a quick look at the latest GFS forecast which shows a distinct rise in temperatures at the end of next week: a sign of Spring at last? Let’s hope so!


Weather is always interesting, of course, but this week is extra-fascinating! 3 weather things to watch this week in Reigate:
1. Watch out for… highly unstable cool air moving off the Channel onto warming surfaces during the daytime leading to convection, cumulonimbus clouds and heavy showers, maybe risk of thunder and hail over Reigate: especially Monday and Tuesday.  April showers in a cold March?
2. Keep an eye on … the big show-down between the powerful Polar North Easterlies spilling out from huge blocking HIGH pressure over the Arctic and the, so far, rather feeble South Westerly maritime air that has made little impression recently.  They are due to have a face-off on Thursday. The leading edge of fronts might be snow…but will the warm air reach Reigate or will the LOW retreat out into the Atlantic?
3. You can’t see this in the sky but… note the off-the-scale negative Arctic Oscillation graph that indicates a weak jetstream and high pressure over the Pole which allows Polar air to flood out over the UK easily. Will this mean another fake start to Spring this weekend (check the temperature graph leaping up at the weekend!) and then a return to cold next?
Reigate will mostly stay on the “warm” side of this LOW and avoid the very coldest easterlies this week which will stay up north (except mid-week which will be cooler down here) but the cold air will never be far away so frosty nights are certain, cool temps in slacker winds are definite, heavy showers hit and miss and a minor risk of snow anytime are still worth watching for!
Hope you enjoy the week weather watchers!

quick update Monday: cold weather continues to next week… polar easterly winds return Monday-Tues. after warmer spell this weekend but poss SNOW preceding warm front Saturday- update later.  Heavy rain with snowfall/sleet poss in between for SE: on weds, thurs & Fri. Very active, very odd, potentially extreme weather…watch this space for updates. So, for the moment the Easterlies win! some are calling this the MIA: Mini-Ice Age!

quick update Weds: snow threat Fri/Sat as Atlantic fronts on a vigorous depression move in from the west and move up against the cold air.  This may dump snow on Reigate Saturday.  Next week looks COLD … back in the freezer high threat: snowy Easter possible.  NOT the start of Spring by any stretch…someone tell those birds.

She’s old and her weather fronts have broken-up into a wild chaotic spiral but she’s due to deliver a powerful kick in the teeth this week!  Yes, the cool “Polar” LOW bringing heavy rain and miserable conditions to Reigate this weekend is due to edge slowly south and east Monday-Wednesday next week. This will, once again, open the door for North Easterly winds to swing across the country and pull temperatures well below Spring averages, just 3-4°C max by mid-week.  Snow is most likely up North where temps are lowest but in Reigate temperatures will still drop low enough for an increasing risk of rain turning to sleet and snow any time from Monday on.  The nature of this complex old LOW pressure means forecasting precise rain/sleet/snow fall is tricky at the moment as numerous troughs and lines of showers circulate around the edge but expect a miserably cold week with rain, sleet and snow at any time and especially Tuesday and Wednesday when upper air temperatures fall lower as North Easterlies kick-in properly.  On the bright side, the winds will be lighter than last week so wind chill is less extreme.   Milder air may make a push towards the south east mid-week which could bring a more significant snow risk Wednesday if it runs up against the cold air but this is uncertain at present. 

Sunday forecast: rain most of daylight hours; moderate & heavy showers at times esp pm; temps 5ºC. and falling in the afternoon with slight risk of sleet later as temperatures fall away behind the showers. HEADS UP: OUTLOOK FOR THIS WEEK … MARCH = the new JANUARY! see below: Extremely negative Arctic Oscillation threatens continuation of cold March weather this week.

extremely negative arcitc oscillation

21 March UK north south divideThe low pressure bringing rain and cool conditions this weekend looks like dragging in a return of north easterly winds as it moves south over the UK.  This means continued cool/cold weather for Reigate Mon – Wednesday next week.  Temperatures, whilst not as cold as the last spell of Easterlies, will bring some rain and maybe even sleet or snow at times through mid-week.  There will be a big battle between milder air to the south and cold air to the north of the UK next week and significant snowfall is a risk where these air masses meet.  Currently, it looks like the front will be further north than Reigate and in the slightly milder air so we will escape the worst of any snow threat poss Tues/Weds and may miss it completely but cool damp unsettled weather is certainly sticking around next week so keep watching for updates!

A cool LOW pressure tracking across from Greenland and Iceland will move over the UK on Friday and sit over Reigate for the weekend and into much of next week. Atlantic depressions usually have some warm tropical air circulating with cold polar air but not this one!  With high pressure to the west any warm SW air flow is being blocked out, leaving Reigate on a “cold-washing-cycle” with mostly swirling cold polar air circulating around the low. Cool air coupled with low pressure even in this cool Spring weather causes unstable airmasses.  Instability means that big showers can develop as thermals rise from warming surfaces even in the weak Spring sunshine we are experiencing.  Convective thunderstorms are a remote but interesting potential risk for Reigate over the weekend.
Various fronts and troughs will circulate around the LOW pressure which is blocked by a HIGH over Scandinavia and E.Europe and won’t move much. The LOW will bring showery rain, some of it heavy with the possibility of hail and even thunderstorms, especially over the weekend (50% chance of convective thunderstorms on Sunday over Reigate).  Temperatures will never climb much above 7°C and, with gusty winds of over 30mph, it will feel cool, but nothing like what we had earlier this week. Some models predict a return of cold Easterly winds as the LOW slips south over the UK and drags in cold continental air again. There is still uncertainty about this but it will certainly be an unsettled week. Other models see glimmers of spring after around the 23 March as winds turn more SW and a ridge builds over the country bringing drier conditions.  More on this later, keep your fingers crossed!


Today, a classic Arctic airmass brought almost every type of cumulus cloud over Reigate. Arctic air starts off very cold somewhere near the North Pole but quickly moves south over progressively warmer oceans and land surfaces. This makes it “unstable”, which means any heating of the ground by the sun will allow relatively warm bubbles of air to rise rapidly from the surface as thermals. Thermals will continue to rise so long as the air around them is COLDER than they are, which is pretty much forever in an Arctic air mass! When moisture in the thermals condenses, towering cumulus clouds form and showers become likely. Today these showers fell as snow despite the air temperature being +4.5°C! The dry Arctic airmass with a low relative humidity (56%) coupled with the rapid lapse rate (drop in temp with height) allowed snow forming at high altitude not to melt or sublime before it hit the surface. Cumulus don’t usually live long after sunset … they decay and die quickly once their supply of thermals is cut off.  Like many things of beauty, cumulus clouds are short lived so make the most of them when you see them next time!  The photos above show the wonderful cumulus clouds which formed over Reigate today, numbered in order of appearance: how many did you spot?  Tomorrow might be similar, so keep an eye out again…the best view is often up!


Some charts for next week show the Polar LOW which is heading our way for this weekend sitting on top of the UK and a HIGH building to the NORTH. This could drag in cool easterlies for some parts of the country again and keep Spring locked out. Look carefully at the chart which shows upper air pressure and temperatures at 500hpa: around 5000m (half way up through the atmosphere: a good place to predict air flows without the disruption of annoying surface features which complicate things).  It shows Russia warming up and Greenland distinctly spring-like but the UK stuck in a pool of cold air from N Scandinavia.  Details are not certain but don’t break open the beach-wear and bbqs just yet, looks like the Kangaroo Spring is set to continue with a hop back to winter yet again next week.

Tuesday 11 March in Reigate started with very cold wind chill temperatures down to -12°C at 5 am brought by strong NE gusts of +30mph blowing snow wildly into mini-drifts, especially on the North Downs.  Arctic air arrived from the North by mid-morning and, with slackening winds and breaking cloud, brought a much warmer and brighter end to the day with max temps of 4.4°C. The satellite photo shows the advancing Arctic air mass changing the cloud pattern as it nudges the cold NE winds to the south.
Calmer, brighter, drier HIGH pressure will remain in Reigate for a few days until a POLAR LOW from the NW brings rain and wind for the weekend. The arrival of this low from Greenland, wrapped with cold air, will not bring respite from the cool weather but temperatures will rise to 5-7C max so it will seem mild compared with our recent icy Polar experience!  More later on a potential re-arrival of Spring on 20 March!