EMAIL EXCLUSIVE: John Hammond’s Month Ahead – You think this is cold..?
“Bitter” might be pushing it, but at last, we’ve a seasonal chill! Temperatures are certainly low enough for snow, as some of us will soon see this weekend. However, between now and early next week, the White Christmas hopes of millions of children (of all ages!) will first be lifted in anticipation, only to be crushed as temperatures rise once more in the days ahead. Premature dejection? Read on…
Milder westerlies return
Colder air comes back by Christmas week
Increasing chance of severe cold in January
MONDAY 17TH DECEMBER – SUNDAY 23RD DECEMBER
From cold to mild (and back again?)
After the disruptive rain, freezing rain, snow and wind of the weekend, the following few days don’t look short of further eventful weather.
As temperatures rise, we will be looking to the west. A large area of low pressure will trundle in our direction from mid-Atlantic, bringing some fairly wet and windy weather, especially towards mid-week. One way or another, you’d be well advised to check the forecast before heading out for any pre-Christmas events.
To say that what happens next is not nailed down is an understatement! Some computer models suggest this low pressure system will be swiftly followed on the jet stream’s conveyor belt by another – continuing the mild, wet and windy theme through to the weekend.
Others have a different idea. In this minority scenario, the jet stream is once more stopped in its tracks. Indeed a few computer forecasts suggest a resurgent block of cold continental air will flood westwards again by the weekend.
On balance this seems extreme. However, it looks increasingly apparent to me that our normal supply of mild Atlantic winds will be temporary. With the jet stream again becoming more distorted and meandering in its path, it’s only a matter of time before colder winds from the north or east return. The question is: “When”?’ Perhaps not as quickly as some snow enthusiasts would like, but it looks likely before Christmas.
I do not expect extremes of cold … this side of New Year.
MONDAY 24TH DECEMBER – SUNDAY 30TH DECEMBER
More battles ahead
I’ve been on the radio quite a lot this week; and of course in every interview has come the question: “Will it be a White Christmas?”. The correct answer is that it’s impossible to say at this range. However, to all those who ask me, I have suggested that it is more likely this year than most. As we’ve been saying since we issued our Early Winter Forecast back in October, the atmosphere is more predisposed to ‘atmospheric blocking’ than usual.
Hard as the computer models try to restore the normal dominance of Atlantic weather systems (picking up on ripples of jet stream energy stemming from the other side of the world), they change their tune and run out of steam as the time and place approaches. How’s that for mixing my metaphors?
The upshot is that the UK ends up being in the buffer zone where mild air meets cold. The type of ‘battleground’ scenario we’re seeing played out this weekend could well recur through the last few days of 2018 – a nightmare for longer-range forecasters.
The north and east (closest to the cold block) will be more likely to see the coldest and potentially snowiest weather. Further southwest (nearest to the Atlantic influence), milder, wetter spells are likely more of the time.
And that, folks, is as explicit as I can get through Christmas week. Anything more precise that you may hear, see or read in terms of detail – and yes, that includes your app – is delusional at this range.
One point I would make is that whatever comes our way, I do not expect extremes of cold (of the type we saw last Winter) this side of New Year.
… forecasters are now talking about the ‘elephant in the room’.
MONDAY 31ST DECEMBER – SUNDAY 13TH JANUARY
Some serious signs
Last week I raised the potential for a ‘Sudden Stratospheric Warming’. Ultimately, it was this which led to the severely cold spell that struck the UK late last winter.
Computer models are growing increasingly bullish about the prospect that the Polar Vortex will weaken quite markedly in the weeks to come. This ‘ring-fence’ of strong winds at the top of the atmosphere helps to contain frigid air within the Arctic.
Weakening is one thing, but if the circulation of the vortex disintegrates entirely, the stratosphere can suddenly warm. As the thermodynamics effectively go into reverse, so a surge of easterly winds can overwhelm the northern hemisphere. From start to finish, this process takes several days, if not weeks, before such a reversal works its way down to ground level.
This is not a done deal, and at this range, remains a relatively low probability. But on balance, there is an increasing chance that more serious cold weather could transpire. Indeed, forecasters are now talking about the ‘elephant in the room’ (or should that be polar bear?).
Good reason, then, to keep our powder dry with the phrase “bitter” for the time being. It might come in handy later in the season…