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Weather news and views from Brian Gaze.
Posted Wed 9th January 2019 19:22
Milder conditions will return during the coming days but it probably won't last for too long. Computer models continue to point towards the second half of January being colder.
The GEFS ensemble model continues to favour colder weather later this month. Given the winter we have had so far that really wouldn't be too difficult! Also, colder does not necessarily mean a Siberian freeze and winter wonderland. So how is it looking?
I've annotated this evening's London GEFS 12 plot below. Th key points are:
1) In the short term it becomes quite mild
2) It turns colder around January 15th but milder air probably returns a couple of days later
3) By January 21st it turns colder again and temperatures stay below the average until the end of the run on the 25th January
How cold could it be? During the February 2018 Beast From The East spell, 850hPa temperatures approached -15C. Recent GEFS updates have contained a few runs where -10C is reached and the odd straggler going lower. However, most of the runs are coming in at between -5C and -10C.
The other point to note is that a lot of the runs are not consistently cold, therefore we could have a situation where mild air is pushing up across the southern half of the UK. A pattern like that on paper suggests the possibility of significant snowfall close to the boundary between air masses.
At this stage it is far too early to think about the details, but if pushed I would say northern snow lovers are most likely to be kept happy. However, if (like me) you're in the south don't give up!
Another way of looking at the GEFS data is with the postage stamp plots. The one below shows forecast 850hPa temperatures from each of the runs on Tuesday 22nd January. Blues indicate -10C or lower, yellows are 0C or higher. The greens are between 0C and -10C. Remember the 850hPa level is approximately 1500m above sea level.
On the postage stamp plot above the key things to note are:
1) A number of runs show very cold air over or close to the UK
2) A number of runs show mild/ish air in the vicinity
3) The majority of runs show cold air over the UK
What all of the above says to me is that colder spells of weather are likely. However the cold may be punctuated by milder periods, especially in the southern half of the UK. The synoptic evolution - if correct - suggests the chance of significant snowfall in the (small) area where the cold and mild air masses meet.
Colder weather continues to look probable during the second half of January. The length and potency of potential cold spells is open to question, with the ensembles suggesting upper air temperatures will not approach the values we saw in February 2018. Nonetheless there is reason to think there could be some snow around, especially in the northern half of the UK best placed for it.
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