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December forecast

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Thundery wintry showers

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Like many forecasts, my forecast for December started off well then veered rather wide of the mark.
[quote]Changeable and generally cold, some snowfalls


During December 2012, the jet stream will be tracking from north-west to south-east over the eastern North Atlantic and Europe for most of the month, and this will enable a succession of northerly and north-easterly outbreaks to affect the British Isles. It won't be as intensely cold as December 2010 was, but it will be cold enough for snow at times, particularly over the north and east of Britain but less so in the west.

Following two cold bright days and then a milder interlude on the 3rd with some sunshine and a few showers in the west, another northerly outbreak will arrive on the 4th/5th December. A wintry mix of showery precipitation will spread southwards on the 4th, particularly affecting northern and central parts of England, though snow will mainly be confined to high ground. On the 5th most places will be cold, dry and sunny, but sleet and snow showers will affect eastern coastal counties with snow generally from Teesside northwards.

Between the 6th and 8th December another low pressure system will slide south-eastwards, and will bring an active belt of rain (preceded by a brief fall of snow in eastern Scotland and north-east England) on the 6th, which will aggravate any flooding problems left over from November's rain. The rain will clear away southwards early on the 7th, with some possible snow on its northern flank, but the wintry showers that will follow behind into eastern areas on the 7th/8th will generally produce rain/sleet at low levels and snow on hills.

Another depression will slide south-eastwards between the 9th and 12th and this low is associated with considerable uncertainty- the weather during the following week of the month will be strongly dependent on its precise track. A belt of rain and strong winds is expected, followed by another northerly/north-easterly outbreak with sunny intervals and wintry showers. Temperatures will be rather below average but not exceptionally so, and towards midmonth a north-easterly type is expected to prevail with high pressure extending from Iceland to Scandinavia. It will be generally dry and sunny in the west, while eastern areas will have some sunny intervals mixed with wintry showers, mostly falling as a sleety mix near the east coast but with snow inland.

Around the 15th-18th low pressure will start to attack from the south-west which will eventually result in milder air coming up from the south, but not before many of us see some sleet and snow on the northern flank of the weather systems. The Midlands and central and western parts of northern England will be most prone to snow, while eastern coastal areas will mostly see rain due to the winds off the comparatively warm North Sea.

The last third of the month is somewhat uncertain, as we will most likely see a burst of polar air coming down from the north around the 20th of the month, while depressions will continue to take a southerly track. Thus, a cold snap is likely shortly before Christmas, with north-eastern districts most prone to snow showers, while southern areas will be prone to belts of wintry precipitation associated with lows passing by to the south. It is hard to place much confidence on the chances of a white Christmas at this stage but the wintry spell may hang on for long enough to give some places a white Christmas, more likely the further north-east you are. A milder, changeable south to south-westerly type is expected to finally establish towards the end of the month.

Overview
Notably mild Decembers have been rare in recent years- the last one was way back in 1994 in the south, and 1988 in the north. This December won't be breaking that run, though nor will it be quite as cold as December 2010- temperatures will be about 2 to 2.5C down on the 1981-2010 average over most of the country, with a Central England Temperature of 2.4C expected. Much of northern and western Scotland and Northern Ireland and south-west England, however, will only be 1 to 2C short of average.

Rainfall during December 2012 will mostly be below the long-term average, though with considerable regional variation. Western Scotland and north-west England will have the largest shortages, of 50% or more, but some parts of eastern and southern England will have slightly above average rainfall, and heavy rainfall in the second week of the month may cause further flooding issues in south-west England. Averaged nationally the shortfall will be aruond 20-30%.

It will be a sunny December over most of northern and western Britain, with excesses of 50% or more over much of Ireland, western and northern Scotland, Wales, and western England. However, eastern and southern England, together with south-eastern Scotland, will only have slightly above-average sunshine. Averaged nationally sunshine will be about 30% above average.
[/quote]

The first 10 days of the forecast went pretty well in my opinion, but after that it went downhill. There was strong ensemble support for the link-up between the Siberian and Icelandic highs after the 10th which would have produced an east to north-easterly blast with sunshine and snow showers, perhaps a sleety mix near east-facing coasts and mostly dry in sheltered western areas. It would also have delayed the return of the Atlantic. However, in reality the Siberian high stayed put and the Icelandic high threw up a weak ridge down to Britain, giving a few dry cold sunny days and then a fast Atlantic breakdown.

As an aside, I remember a couple of comments talking of a fast breakdown being a case of greatest risk/greatest reward. Whenever I see that phrase it always seems that the "greatest reward" involves, at best from a snow lover's perspective, a limited area of the UK having a shot at a major snowstorm like the one that hit the south-west in February 1978 or the Midlands one on 8 December 1990, while the rest of us make do with a brief snow-to-rain event. Mid-December showed us the other side of the coin- the breakdown was so rapid that most of us just saw rain.

My forecast for around the 17th-20th fell into line with what actually happened, but then the trough in the eastern North Atlantic proved far more persistent than I had predicted (though I sensed that there was always a possibility of this- I just didn't consider it very likely). As a result there was no northerly pre-Christmas and a traditional mild west to south-westerly type increasingly became established towards the month's end.

As a result of the greater Atlantic influence, mean temperatures were a couple of degrees higher than I predicted, rainfall was markedly higher, and sunshine totals were lower, though the sunny first half more than counterbalanced the dull second half in most parts of the UK. In the end, the forecast from the 11th onwards was pretty inaccurate, though in my defence, most forecasts got heavily de-railed this month.
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