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Weather updates from site founder Brian Gaze that keep you in the know.
Only 5 days until Christmas but the weather prospects remain uncertain. The reason is that the UK is expected to be close to the border between cold and mild air masses. That leads to the possibility of snow in places, but also wet conditions.
The chart below is generated using data from the UK Met Office global model and is valid for 00:00 GMT, Saturday December 25th. Christmas Day! The blue and purple shading to the north and east of the UK shows a very cold air mass. The green and orange to the south and west a mild one.
Different computer models have variations on this theme, but the big picture is similar. The problem is that relatively small variations have a big difference on the weather we experience in the UK. An adjustment of 100 miles or so could be the difference between a mild wet day and a cold snowy one.
Christmas Day forecast pressure and air mass temperatures
I think it is possible to say that the further north and east you go the greater the chance of it being cold. Therefore, it is also more likely to be a white Christmas, but there are still scenarios where snow could fall in parts of southern Britain.
Interestingly, today's Weather Analogue Index tracker has 1970 as the best match (see WAI 20th December 2021). That was apparently a famous White Christmas in much of southern Britain.
The MOGREPS 00z ensemble temperature plot for London shows a good deal of spread on December 25th. Most of the individual runs are suggesting average to mild conditions, but a number are going cold.
When will the Christmas Day weather prospects be resolved? Things should be a lot clearer by tomorrow, but it could be as late as Wednesday or even Thursday if the border between cold and mild air masses keeps wriggling around in the computer model output.
So at the moment I would suggest snow is most likely in the northern half of the UK on December 25th. However, we've not yet reached the stage where a Christmas miracle is needed for it to happen in the south.
If the Christmas day weather is still an unknown is it possible to say anything useful about the days which follow? Yes it is, because as I said the big picture is more clearly defined than the details. Things could change of course, but at the moment is looks as though there is a reasonably high chance of colder air moving southwards across all parts of the UK.
The GEFS ensemble data table below shows 850hPa temperatures. The purple shading is sued to show the percentage of runs forecasting a cold air mass on a given day. On this update over half of them are purple on 27/12, 28/12 and 29/12. Comparable tables for locations further north show even more purple.
Therefore, a cold snap of a few days is favoured, so it could be feeling very seasonal if you're hoping to walk off the turkey and mince pies in the days between Christmas and the New Year. Nonetheless, a significant number of the GEFS runs are going for a milder outcome and today's ECM (a very highly regarded computer model) 00z run is also less keen on a cold spell.
In the longer term the amount of purple decreases. The greens and yellows which indicate a mild air mass become dominant once again. Therefore, at least at the 850hPa level the expectation is that it will turn mild again. If high pressure remains close to the UK and brings calm conditions it could remain colder at the ground level. However, a lot of the individual runs show a brisk southwesterly flow which would ensure mild conditions aat the ground level too.
Even now the Christmas weather prospects are uncertain. In general terms the likelihood of a white Christmas increases as you head northeastwards. Nonetheless, it may be another 2 or 3 days before the details firm up.
In the days which follow Christmas there is a signal for cold air to push southwards UK for a time, but that is not certain. Looking towards and beyond the New Year the current indications from the GEFS ensemble are for it to become milder again.
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