Monday 28th December 2020

We are at the beginning of a cold spell, and it could potentially be a memorable spell. Though I tend to find that cold spells only tend to become memorable when there is snow involved. And there are several chances of snow this week – but only chances, all of them far from guaranteed.

This will be a difficult forecast, though as the cold weather itself was forecasted weeks in advance, the details in terms of snow possibilities will be really tricky – and close to nowcasting events.

Two main difficulties – exactly where small features bringing potential snow (or rain) will track, and as I just alluded to, whether precipitation does fall as rain, sleet, snow or a mixture – things are very marginal in terms of temperatures, dew points, etc – and will remain so this week. Though perhaps colder next week, but more on next week later.

Thanks to Lisa for the wintry photograph.

Tonight will be mostly cloudy, a bit of mist or patchy fog possible but most will avoid this, and a slight frost in places by dawn, around 0’C or just under.

For Tuesday we are in this cold northerly flow with low pressure close to our east.

It will be mostly cloudy – one or two scattered wintry showers during early morning, around a 30% chance of catching one, then another cluster of wintry showers heading down from the north in the afternoon – though these look more likely to be to our west as things stand. A little brightness at times, and catch a morning shower and there could be a temporary slight covering of snow. Cold and breezy, 4’C. Clear spells overnight, though some cloud later in the night – down to around -1’C and frosty for most.

Wednesday morning will be cold with hazy sunshine. An area of rain with snow on its northern flank will slip down from the north-west in the afternoon – the exact track is very uncertain, but if we are on the northern edge there should be some snow falling – this is currently the slightly more likely outcome. Even if it is a bit further north than currently projected, and therefore is rain when it arrives, there is a good chance that it will turn to snow. Either outcome could see 1-5cm of snow accumulating.

If it is quite a bit further north than currently projected, then it will be rain – it could also be further south (second most likely of the four outcomes) and we remain dry but cold.

Around 4’C by day and -2’C overnight. Ice possible.

So Wednesday’s potential snow was barely showing on the models yesterday – we are looking at 48 to 72 timeframes in terms of developing features so uncertainty really is high from Thursday onwards, in terms of potential snow.

New Year’s Eve remains cold, if anything colder – especially if there is snow on the ground. Sunny spells, cloud at times (especially afternoon), the odd scattered wintry shower. 3’C and breezy. There are suggestions of an area of snow spreading down from the north or north-east either evening or overnight, but this is just an early suggestion. Around 0’C overnight if snowing – or down to around -3’C if clear. Ice very possible.

New Year’s Day remains cold with sunny spells and chances of wintry showers – though lower chances than previous days as it currently stands. 3’C, though perhaps a tad lower if there is lying snow. Uncertain overnight – either cloudy with a bit of rain/snow or frosty.

Saturday is still cold – I hope you will be getting used to it at this point. A fairly slack NNE flow with high pressure fairly close to the west.

Details uncertain, but arguably more likely dry than not, and around 4’C.

Sunday similar, though a slightly increased chance of wintry showers. It remains cold – whatever!

Next week will be cold, perhaps verging on the very cold – though I guess the definition between cold and very cold is woolly, I don’t have a strict definition in mind – maybe 0’C by day would be very cold?

The most likely evolution is for either an easterly or north-easterly, and further snow showers or spells of general snow would be likely at times. I cannot rule out rain instead, but snow more likely.

A somewhat less likely evolution would see high pressure close to or over the UK, bringing cold, settled conditions – with harsh overnight frosts and freezing fog possible.

Where do we go from here?

Well I forecasted the cold spell for the end of December, and to last into the very first part of January, as part of my winter weather forecast that I wrote at the beginning of December. I then expected a return to mild and wet conditions.

I am now doubtful as to whether we will return to mild and wet conditions in January – the tropospheric polar vortex that brings our mild and wet winters is completely disrupted, the Atlantic jetstream is far to our south, La Niña and other background signals still favour a continuation of the cold too – if anything background signals favour an intensification of the cold in January.

And this is before the sudden stratospheric warming (SSW) event that is now likely, somewhere between 5th and 12th January, which would give a good chance of a significant cold spell to follow, normally starting within 2-4 weeks of the SSW event, though I’ve seen some people more knowledgeable than me suggesting that lead time would be even shorter.

I do expect an attempt to assert milder conditions from the south-west during January. The earliest possible attempt would be around 7th January – unlikely but possible. I’d suggest the attempt more likely in the period between 10th January to 14th January, but any attempt at mild conditions from low pressure bumping into cold weather would at least start with heavy snow.

And even if mild does win for a while, I fully expect further cold conditions to return either late January or into February, assuming the SSW falls in our favour for cold.

I don’t want to get ahead of myself – there are plenty of snow chances ahead, not just this week but in the week or two that follows, and if none of them actually happen you’ll be wondering what I was on about – and perhaps just think it was a bit colder than normal.

But there certainly is now a chance that this could be one of those more memorable winters.

Leave a Reply

Your email address will not be published. Required fields are marked *