December 2011 Forecast

[Thundery wintry showers]

Here was my forecast for December 2011:

Changeable, mild midmonth, colder early and late

The stratosphere is currently very cold, and there is a strong polar vortex up at high latitudes, promoting low pressure to the north of the British Isles. Neither of those look likely to chaange during the course of December, so it's reasonable to suspect that Atlantic-driven weather will persist for most, if not all, of the month.

Despite this, the first third of December is set to feature close to average temperatures for most of us, and perhaps falling a little below in the north. The reasons for this are low pressure over Scandinavia and a north-westerly regime bringing Arctic air southwards. The 2nd December will start off dry and bright with some frost but wind and rain will spread east to most parts by late evening. This rain will then be slow to clear southern areas on the 3rd, while elsewhere the 3rd and 4th December will have sunshine and blustery showers. Colder air will be digging down from the north-west so much of Scotland and Northern Ireland will be seeing sleet and snow in the heavier showers. By the 5th December sleet and snow showers are possible to low levels as far south as the Midlands, and on high ground further south, though don't expect any significant accumulations unless you live on high ground in the north. Hail will also be widespread in the west.

Between the 6th and 8th it will turn rather less cold as a trough moves across to the north, introducing more windy showery weather, the showers merging into a longer spell of rain for a time. It will remain cold enough for snow in the Scottish Highlands which should have excellent skiing conditions by the end of the first week, but lowland areas, even in Scotland, will mostly see rain and hail.

After a brief north-westerly incursion around the 9th-11th, again perhaps bringing sleet and snow showers as far south as the Midlands, an abrupt change of pattern is expected for mid-December as high pressure extends across to the south. The weather will start off bright and frosty, but towards midmonth a very mild west to south-westerly regime is likely, with a lot of cloud and rain for much of Scotland, Ireland, north Wales and north-west England. Southern and eastern England will be mostly dry with a fair amount of sunshine while intervening areas will be mostly dry, cloudy and windy.

During the last third of December we are likely to see depressions take a more southerly track, introducing a wet and windy pattern to most parts, and towards the end of the month the fairly cold north-westerly type may return, though there is limited confidence on this. Thus, around the Christmas period, it is likely to be mostly showery with occasional longer outbreaks of rain, and some wet snowfalls are possible but widespread snow cover looks unlikely, so only northern hills stand much of a chance of having their third snow-covered Christmas in a row.

Overview

December 2011 will be quite a mild month in the south with temperatures about 1.5C above the 1971-2000 average, but in Scotland temperatures will be within 0.5C of the average. My punt for the Central England Temperature is 5.8C.

Rainfall will generally be above normal except in parts of southern and eastern England, where shortages of up to 30% are possible, and eastern Scotland will have close to average rainfall. Most of the western third of Britain will have percentages between 150 and 200.

Despite the rain, sunshine totals will be near or above normal in most regions. Western Scotland and Northern Ireland and some western coastal districts elsewhere may have shortages of up to 20%, but the east of both England and Scotland can expect excesses of around 50%.

Looking at the above forecast, the first third of the month went much as I predicted, although I may have understated the potential for severe gales. I then got the middle and last thirds of the month the wrong way around. I suggested that a milder anticyclonic influence would dominate around midmonth with colder zonality returning in the last third, when in reality the milder anticyclonic type got delayed until after the 18th. As a result, the monthly stats came out very similar to what I predicted.

Thus, I'm not sure how to assess the accuracy of this forecast- for those after a general indication of what December as a whole would be like, it was almost spot on, but for those who wanted to know what the patterns during the last two-thirds of December would be, it was rather wider of the mark.

[leicsnow]

When (and before) you issued the forecast, I think a lot of people had the idea in their head that we would see a pattern change to colder weather at the end of the month, just as many are suggesting for mid to late Jan now. Spot on when you say the anticyclone got delayed, it explains why you got the forecast somewhat off the mark.

When are you issuing a new monthly outlook?