Will the 'Beast from the East' return?
As expected, the much anticipated colder weather is filtering southwards across the country today and wintry showers are on the cards for many overnight.
Tomorrow night, temperatures are expected to drop as low as -12C across sheltered parts of Scotland and -4C in rural spots along the M4 corridor with daytime temperatures for most in the low single figures.
By the time we get to next week, there is growing confidence that winds, like last year, becoming easterly in direction - and THAT is when we really start to dig into the more impactful cold weather.
Whilst it's still too soon to make a call on whether we're facing a return of the Beast From The East, we are certainly moving into a drawn-out spell of exceptionally cold weather and it's well worth staying up to date with the latest on this developing situation.
Tonight: A cold night with widespread frost and icy patches. Windy, especially in the north with gales. Some showers, with possible snow to lower levels in Scotland, northern England and Wales.
Thursday: Starting frosty, with icy patches. There will be wintry showers, mainly in the north and east. Otherwise, a sunny but cold day, with winds easing for most.
Outlook for Friday to Sunday: Into this weekend it will be cold, with overnight frost. Outbreaks of rain and hill snow will move southeastwards at times
UK Outlook for January 20 to 29: Sunday will be cold across the UK, but for many it will be dry; brightest in the southeast. Some light rain or sleet is possible in the southwest at first; then a band of rain, sleet and snow is likely to spread across the northwest later, moving erratically southeast. Overnight frost and freezing fog patches are likely into Monday. Cold weather is likely to continue through mid-week, remaining unsettled and at times windy with fronts spreading southeast across the UK bringing rain, sleet and snow. Snow is most likely over north and eastern areas and over high ground. Similar cold conditions are likely to remain in place until the end of the period, interspersed with brighter showery spells, although these could also be wintry. Overnight frosts will remain likely.
So what takes us out of "normal winter" and into the realms of something rather more widespread and long-ranging? It's our old friend Sudden Stratospheric Warming (SSW) - the upper atmosphere event that brought us last year's Beast From The East.
A quick SSW internet search reveals tons of highly scientific, extremely complicated explanations of this event. It really is "deep science" that you don't need to know, in order to place an extra log delivery or dust off your thermals. It's not rocket science, but it IS complicated, and arguably one doesn't need to understand how an engine works in order to drive to the shops.
But just in case you're a budding Metling or, more likely, it's the clincher question in a pub quiz, here's all you need to know about SSW and how it's likely to affect us in the coming few weeks, in it's most simple and basic form:
There are four layers to our atmosphere - the Troposhere (where our weather happens), the Stratosphere (where SSW happens) and then the Mesophere and at the top the Thermosphere. It's the bottom two layers that we're most interested in here.
Prior to a SSW event, the Stratospheric winds that circulate around the Arctic temporarily weaken, and sometimes even reverse. This in turn triggers a spell of rapid compression within the Stratosphere (roughly 10km above the polar region of the northern hemisphere) - and that compression creates a sudden rise in temperatures (the warming part of SSW).
Enter the jet stream - a current of fast moving wind (around 200mph) which flows between the Troposphere and the Stratosphere. Think of it like a conveyor belt for weather systems that runs west to east, and most commonly brings our weather in from the Atlantic.
That sudden and dramatic change in temperature higher up in the atmosphere slowly starts to affect the jet stream that sits just beneath it. It weakens, and then starts to reverse it. And by reversing it, this of course, means that the general flow of weather in the next layer below (the Trophoshere) starts to turn from a westerly to an easterly direction.
This reversal of the jet stream also helps to create and strengthen a huge area of high pressure that sits across Scandinavia and Russia throughout the winter - and with the flow of our weather starting to turn from west (milder, wetter Atlantic) to east (colder Scandi/Russian) we essentially get caught in the far western periphery of a Siberian winter.
That's the crux of it: a SSW event results in us getting weather from the freezing east, not the usual weather from the milder west.
This process of the jet stream reversing, boosting and expanding that huge Scandi-Russian high pressure is happening at the moment. Exactly whether we will, once again, be in the firing line of another Beast will soon become clear.
Good luck in the pub quiz!