Archives For arctic air

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

theta e chart: good for airmass spotting

theta e chart: good for airmass spotting

 

UK metoffice fax chart

UK metoffice fax chart

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

theta e

theta e

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

arctic air with mixed up mT air

arctic air with mixed up mT air

UK metoffice fax chart

UK metoffice fax chart

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

 

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

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

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

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Water vapour EUMETSAT rgb airmasses: Arctic red, tropical blue

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

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

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

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

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

LOW sinks south bringing Arctic air

LOW sinks south bringing Arctic air

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

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trough disruption

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

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

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

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

Weather Watch: update: heavy rain overnight Friday, moving away am; even HEAVIER rain overnight Fri – Sat, moving away am. Some possibly torrential and totals poss exceeding 30mm in next 48 hrs. 

The second significant Autumnal gear change is looking more certain for early next week as a big LOW sweeps between Iceland and Norway across the North of Scotland and a brisk active northerly jet combines to drag in a breezy and cool Arctic airmass from the Poles all the way across the UK.  The isobars on UKMO charts can be traced all the way back to the Poles, so expect chilly weather… possibly creeping into the low teens and feeling like 10c in the wind.

fri fronts

The process starts on Friday as fronts bring potentially heavy rain across the South, including Reigate, especially Friday pm and overnight into Saturday morning. The situation is complicated with fronts lingering over or near the south throughout the weekend so expect rain, some heavy, almost anytime during Friday through to Saturday. Saturday will be cooler than Friday.

The real news, though is the temperature plunge next week. Keep posted for more on this Autumnal development.

There might be a glimmer of some warmer temperatures returning by 21 Sept as a high builds from the SW … temps of low 20c’s might be expected to make a return but certainly not a heat wave!

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!