#Reigate Boxing Day snow prospects?

December 26, 2014 — Leave a comment

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.

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