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Update #2 25/12/14: update: cold weather arriving after this LOW, heavy rain overnight Fri-Sat; snow marginal for SE early Sat am, more likely for Midlands and EA, cold weather arrives in lee of this system.  MetOffice warnings updated:

Update #1 25/12/14 latest MetOffice chart lifts pressure and pushes track further south, with low moving SE across our area.  This reduces wind speed, still brings in colder air flow though with risk of snow increased for back northern edge of the system with NE winds. For SE possible sleet/snow on Downs early Sat am. Evaporative cooling could yield more snow for SE if rain sufficiently heavy (drags down cold uppers). Gale risk gone but replaced by some heavy rain, marginal snow risk and retaining the cold easterlies in the aftermath on Saturday with pressure building to dry bright frosty conditions.

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After a pleasantly cool bright and dry Christmas Day, an interesting depression due on Friday and through Saturday is likely to usher in a period of colder weather for the UK and SE in particular. The situation is a little uncertain still but the run of warm mild gloomy temperatures lately this December, already pushed aside gently by a weak cold front passing south through the country today, are likely to be pushed further down into some “proper”cold after the storm passes through by Sunday. This storm, forms in the Atlantic along the polar front and quickly races east towards the UK on Boxing Day Friday.  Storms tend not to deepen much if they move fast, which this one does at first: crossing half the Atlantic in a matter of 24 hours. The storm is mixing some airmasses with contrasting temperatures: cold polar air in the north is about to get up close and personal to mild warm Tropical air from the south west.  They are due to meet in the LOW pressure over the UK soon, so expect some interesting weather!  You can spot the impact of the storm on the upper air temperature chart below but also see the steeper drop to colder conditions thereafter.

GEFS shows cooler days ahead

GEFS shows cooler days ahead

The ECM charts below show upper air temperatures at around 1500m. These “850hPa” charts are commonly used as guides to airmasses because air at 1500m (850hPa pressure level) is not affected by changes day and night or surface characteristics, it is therefore a good guide to true airmass characteristics.  Note the really cold airmass to the north meeting comparatively warm air to the south and SW in this LOW.

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For the South of England the LOW will initially push warmer tropical air ahead with rain arriving for us in the SE on a warm front sometime Friday pm (top diag above Sat 00hrs).  The warm sector is likely to be windy with gusty SW winds and a considerable accumulation of rain, 10-20mm overnight into Saturday.  The warm sector tropical air mass (upper air +5C) could have temperatures near double figures whilst the polar air bearing down from the north is a much more frigid airmass (upper air -6C).  The contrast between these two airmasses could make the frontal rain particularly heavy while the cold front contrast could even have an odd rumble of thunder as cold air undercuts the warm and forces it aloft.  The skew-t diagrams below show the contrast in these two airmasses.

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The LOW centre crosses the North of England and into the North Sea overnight into Saturday when, due to it’s location under the left exit of the jetstream, it is forecast to deepen to possibly around 980mb. quite low especially for a depression located so near the shore.  Deepening occurs as the jetstream aloft encourages air to rise off the surface because air is diverging aloft.  So air is rising off the surface quicker than it can be replaced by air arriving: hence falling surface pressure. This commonly occurs when lows interact with jetstreams on their left hand side, near the exit of a jetstreak.

The classic frontal depression with cold and warm fronts separated by a warm sector only lasts for a matter of hours before the cold front, pushing forward more dynamically than the warm, catches up the warm front and pushes the remaining warm air into the upper atmosphere.  This is an occlusion and signals the end of the development stage of a depression.  The central pressure usually starts to rise after occlusion has occurred.

Whilst the situation is still uncertain, it is likely that Friday afternoon and Saturday will be windy and increasingly cold as the winds veer clockwise from the SW through to North and finally NE and E.  It is the latter NE and E winds that will bring the colder air to the UK and the SE especially.  Continental Europe is currently very cold so any air flowing from this direction will be chilly.  Cold crisp continental air will stay with us for a while as high pressure builds to the west and pushes north over Scotland while the LOW moves over Europe.  This setup allows easterly winds to flow over the UK.  Dry cold is expected as the pressure is likely to rise and stay high.  Expect some frosty nights. The duration of the HIGH varies between models but certainly should keep things cold and crisp through to the New Year.

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cold new year

Further ahead a split in the polar vortex and stratospheric warming are dominating weather chat and these are set to possibly bring colder conditions through January.  On the other hand, Phase 3-4 of the MJO (Madden Julian Oscillation) is usually associated with a positive North Atlantic Oscillation which brings milder westerlies to the UK.  So, it’s interesting times ahead, stay tuned and Happy Christmas!

Reigate weekend in brief: Saturday dry (but v wet North England: low risk of it extending our way); Sunday wet; Monday showers NE wind.

Update: forecast for weekend proved v tricky: Model rainfall forecasts struggled with the easterly cut off low set-up.  Saturday was pleasant and Sunday turned out dry and warm. Note: GFS rainfall seems not to cope well with meridional jet stream with cut-off low over UK scenario!

Wondering if the weather is a bit odd recently? Well, you may have it right… the weather is literally upside-down from what we would normally expect in the UK. We would normally expect LOW pressure systems to whizz over the country from west to east. Temperature normally decreases further north and we usually expect warm air to arrive from the south. This weekend turns this normal expectation upside-down! A WARM air mass is pressing down from the NE, having arrived on a long journey from the Mediterranean and Eastern Europe (where there is a heatwave). This warm air is due to meet the COOL polar air hovering over the UK and cause torrential rain over Saturday up North. Reigate will see none of this and will stay mostly dry on Saturday.  On Sunday, however, cooler air will push up from the South and cause heavy rain for the SE and Reigate. Warm air from the North?  Cool air from the South? Slightly unusual. 

warm frontWhere air masses of different temperatures meet the warmer air is forced to rise up and over the cool: a front.  The greater the difference in temperature between the air masses and the steeper the temperature gradient, the greater the uplift.  This causes condensation, clouds and rain.  Fronts usually approach the UK from the West.  Currently a huge HIGH pressure is blocking all westerly winds.  A warm front moving south from across the North Sea is most unusual!

The cause of all this is the development of a cut-off LOW over Europe. A very looping jetsream (meridional) has been drawing polar air down across the UK this week and this cold air has pooled up over Western Europe, hence all the convection and showers. Meanwhile, the jet stream anticsnorthward moving limb of the jetstream has been dragging warm air up over Eastern Europe (which has a heatwave).  This warm air is due to meet the cool air over the UK this weekend.
The good news is that Reigate is likely to have a reasonable Saturday out of all this. Sunday could be pretty wet though.
The cut-off LOW will hang around for early next week and the unsettled theme continues for us in Reigate until the end of next week when things could improve for Bank Holiday as HIGH pressure builds in from the Atlantic. This means the west and north are likely to see better weather than the SE for Bank Holiday, but that’s a long way off to be certain. Keep tuned!

Quick update Tuesday: This LOW is not now predicted to trouble UK Fri – Sat: it will not be the BLOCKBUSTER  we hoped might break through and bring milder conditions.  Instead, it’s a near miss to the South as the LOW skirts thru NFrance pushed aside by the Icelandic HIGH building once again to the North of UK. LOW to south WILL drag in more cold / cool easterlies through much of next week; so start of April will be cool but should be not quite so cold as now with daytime Tmax +6-7°C for Reigate.  Nights still frosty.  Mostly dry, poss of snow showers. No dramatic snow on the charts for Reigate at the moment. Point to note: Hekla volcano in S Iceland is seismically active: heightened eruption possibility. Hekla is not usually a show-stopper for air traffic and winds would blow ash away from Europe!

How and when will this record breaking cold weather end?  Models suggest the first week of April will still be cool but not quite as cold as now. The latest UKMO chart for the end of the week shows mild SW winds developing around a huge Atlantic depression and squeezing up against the cold air towards Southern England. The leading edge of fronts pushing up the Channel into southern England could bring snow for a time to the south, and possibly lots of it, as cold air undercuts the warm and forces it up.  The snag will be if these fronts stall for a while: the chart shows fronts parallel to isobars which usually means they are slow moving and could stick around, thus increasing any precipitation over areas underneath. Later models also show this LOW slipping to the south of the UK which will drag in more cool Easterlies for the start of April. Too early for details, could change… but this LOW may not be the blockbuster we all hope for to sweep away the cold!

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!