EMAIL EXCLUSIVE: John Hammond’s Month Ahead Wet or white through the weather window?
All change! The weather’s about to get a whole lot more seasonal as chillier air seeps across much of the UK in the days ahead. Not quite as seasonal as this time last year, when deep snow blanketed the heart of the UK, but some of us will see a bit of the white stuff. However, as we’ve been flagging up for some time, there will continue to be skirmishes between ‘Mild’ and ‘Cold’ over the next few weeks, so don’t take this shift as the last one before Christmas. There are good reasons to suggest there’ll be a more decisive winner of the battle through the first few weeks of 2019, but that’s for a later blog. Let’s concentrate on the festive period – it’s going to keep us on our toes…
A week of wintry hazards ahead
Wetter period as Christmas nears
Increasing chance of severe cold into January
MONDAY 10TH DECEMBER – SUNDAY 16TH DECEMBER
Proper winter weather
After a blustery and occasionally wet weekend, colder and drier weather will be arriving from the north by Monday. Cloud and higher temperatures may linger across westernmost areas, while eastern coasts will be prone to the odd wintry shower. But that leaves most of us looking forward to some bright, crisp days and frosty nights.
An atmospheric roadblock will be in place across Scandinavia, preventing the usual passage of weather systems from the Atlantic… and that’s where the fun really starts!
We look to the west again. Will the jet stream have sufficient ‘oomph’ to dislodge the block of cold air in residence? Computer forecasts are all over the place with this one, but it looks like the answer will be “Yes” (eventually). Just how is another matter, though, and will be all important to what falls out of the sky. I expect rain to arrive across many western areas by midweek. Its progress eastwards will be erratic. There is a significant chance of disruptive snowfall as this moister air moves further east and north across the UK. This process may take several days before milder air takes control.
Fractions of degrees may make all the difference between a wet day and a white one!
MONDAY 17TH DECEMBER – SUNDAY 23RD DECEMBER
Mild air fights back
The week before Christmas looks ‘messy’! It’s likely that, temporarily at least, milder air may have reached most, if not all of the UK. Periods of wet and windy weather are likely, with southwestern areas, closest to the core of the jet stream, most prone to disruptive stormy weather.
However, fundamentally, that leaves much of the country to the north of the jet stream, within inherently chilly air. This being so, temperatures may be marginal enough for rain to turn to wet snow in places. Fractions of degrees may make all the difference between a wet day and a white one!
Come what may, whether you’re Christmas shopping, carol-singing or going to any of the many outdoor festive events taking place, layers will be needed to complement your Santa hat.
It’s worth reminding ourselves, though, that the weather charts for the next few days show little resemblance to how they were predicted to look just a day or so ago by computers. So meteorologists should be wary of putting much confidence in the latest computer output as we go into late December. The goalposts may move again.
So we approach Christmas Day on a knife-edge of uncertainty.
Green or White Christmas? It’s simply too far off!
MONDAY 24TH DECEMBER – SUNDAY 6TH JANUARY
Signs of major change
You’ll have inferred, then, that I am not yet going to commit: Green or White Christmas? It’s simply too far off!
However, there are several reasons within the set-up of the atmosphere this winter that suggest the cold air will be back sooner rather than later. El Nino is one; the cycle of spots on the sun is another even more distant influencer.
Additionally, in the last few weeks I’ve spoken of signs that the usually strong stratospheric westerly winds that circle the Arctic (know as the Polar Vortex) may weaken markedly as December ends. This signal is becoming more ominous. It’s important because it introduces the possibility of a ‘Sudden Stratospheric Warming’ in which the Polar Vortex goes fully into reverse. Ultimately, it was this which led to the severely cold spell that struck the UK late last winter.
From start to finish, this process takes several days, if not weeks, before such a reversal works its way down to ground level. But on balance, there is an increasing chance that cold air will return from the north or the east as we go later through the Christmas period, and more particularly into the New Year.
Exciting times ahead – for the child and the weather enthusiast in all of us!