EMAIL EXCLUSIVE: John Hammond’s Month Ahead Will the weather join the festive fun?
I’ve been writing my ‘Month-Ahead’ blogs for over a year now; and I have to admit that this is one of the hardest I’ve written. It only takes a few cold days for anticipation to build up like a giant snowdrift: “Is this the beginning of a long, cold winter?” And then, of course, there’s Christmas on the horizon… Well, I hate to disappoint, but neither of those two questions will be answered categorically here. If only it was that easy. The atmosphere is in a peculiar state, making the next few weeks challenging enough to predict – more so than usual.
Atlantic winds attempt to bring milder end to the month
Cold air likely to return through early December
Automated forecasts unreliable
MONDAY 26TH NOVEMBER – SUNDAY 2ND DECEMBER
Cold v Mild
The week starts with a renewed push of raw air from the northeast. Cloudy skies and sleety showers will clear to bring sharp overnight frosts in places.
Computer forecasts for the end of the month now look very different now from the picture they portrayed a few days ago. Overwhelmingly, they indicate that a revved-up jet stream will sweep our cold block of high pressure away. In its place, a milder, wetter and windier few days will be ushered in to end the month. Fed by these computer models, weather apps (and many a human forecaster) are confidently following suit. Indeed, some large weather agencies are expressing unusually high confidence that mild weather will then dominate well into mid-December. I urge more caution.
Even in the short-term there are reasons why the forecast for the week ahead may change again. The likely development of a sub-tropical disturbance in the mid-Atlantic is one of them. That alone may push the jet stream (and computer forecasts) out of kilter.
It does look likely that Atlantic fronts will spread rain and wind erratically eastwards by midweek. Progress will be faltering, though, as the fronts come up against the resilient block of cold air to our east – a recipe for some snowfall? This looks most likely across some northern and eastern areas – especially, but perhaps not exclusively, the higher hills and mountains. There may be impacts.
Things are on a knife-edge. On balance, I think that westerly winds will have displaced the coldest air away from many southern areas, ushering the advent of a damp and blustery weekend of Christmas shopping. Further north, it’s much less certain.
…I suggest we keep an eye on the rear-view mirror!
MONDAY 3RD DECEMBER – SUNDAY 9TH DECEMBER
Beware the “confidence” of some forecasters
The rejuvenated jet stream may bring a few days of very typical December weather through the first few days of the month. Typical not festive. Blustery winds will bring a mixture of rain and shine, with the wettest weather across the north and west. Significant snowfall will probably be restricted to the higher hills and mountains, although frost is still a possibility more widely when overnight winds fall light. But I suspect artificial ice-rinks will be a more reliable place for slip-sliding. Here’s just a few of the many pre-Christmas events on offer.
Looking further into December, there are a number of reasons why I feel that the push of milder air from the Atlantic may be temporary. Oscillating waves of ascending and descending air, as far away as Australasia, can be linked to the fits and starts of our jet stream closer to home. These so-called ‘teleconnections’ are something which computer models struggle to handle.
Energy also propagates between the top and bottom of the atmosphere and vice versa. There are stirrings in the stratosphere which suggest a weakening of westerly winds in the weeks to come. How and when this works its way down to earth is a moot point, but it may have bigger implications later in the month.
So while we look to the west early in the month, I suggest we keep an eye on the rear-view mirror!
Only a fool would predict a White Christmas at this range…
MONDAY 10TH DECEMBER – SUNDAY 23RD DECEMBER
Cold back in time for Christmas?
One way or another, then, it’s with great caution that I peer towards mid-month. Forecasts for this period, and the hopes and fears of millions of people, will fluctuate wildly through this period. Please don’t take automated forecasts on face value, despite their veneer of precision.
I expect the jet stream to gradually and erratically become deflected further southwards across the Atlantic as intensely cold air begins to build and spread out from the Arctic.
There is an increasing chance that as December wears on, high pressure will begin to push much colder air back towards us from the north and east. So after a short interlude in which wind and rain take centre-stage, the chance of frost and significant snowfall will increase. Only a fool would predict a White Christmas at this range, but it may be more likely this year than most.
CAUTION: ALTERNATIVE OUTCOME: Such is the current fickle state of the atmosphere, it may well be that events in the next few days change the direction of the longer term weather evolution described above. In this scenario, the jet stream fails to dislodge the block of high pressure, and much colder easterlies return as early as next weekend.
So beware soothsayers. Trust me – accurate forecasting at long range will be especially hard in the weeks ahead. The next few days will be both fascinating and crucial to what happens as December arrives. We’ll update you as things evolve. In the meantime, Here’s a reminder of our Early Winter Forecast.