22 Oct 2012

The first Arctic chill of autumn is on the way

If you read my blog last Friday, you’ll remember that I hinted there were signs of an Arctic blast arriving later this week, bringing a change to much colder weather than what we’ve experienced so far this autumn.

Having kept an eye on the weather charts through the weekend, the signal from weather computer models has remained consistent, with cold Arctic air covering all parts of the UK by the end of Friday.

As I mentioned earlier this month, autumn, like spring, is a transitional season. This means that a huge range of weather can be experienced during the month of October as warm and cold air battle it out for supremacy.

Therefore, the sudden change to something more akin to winter shouldn’t come as too much of a surprise.

So what’s causing the change?

The change is being caused by the path of the jet stream which normally meanders from west to east across the Atlantic – something in the meteorological world called a zonal flow.

This brings relative warmth and moisture in from the oceans in the form of low pressure, wind and rain.

However, later in the week, the jet stream is going to take a more north to south track in the vicinity of the UK – known as a meridional flow.

This results in areas of high pressure forming, called blocking highs, which effectively act as a barrier to the usual barrage of wet, windy and mild weather moving in from the Atlantic ocean.

The high pressure at the end of the week will build to the west of us and because winds blow around it in a clockwise direction, we’ll be in the path of a cold northerly wind.

Jet stream distributing heat

As well as determining where rain-bearing weather systems go, the jet stream also controls the distribution of heat around our planet.

This works in a fairly logical way. The key thing to remember is that the poles are cold and the equator is very warm. So when the jet stream dives southwards, it brings a scoop of colder polar air with it and when it heads northwards it brings a scoop of hotter sub-tropical air.

At the end of this week, the jet stream will dive southwards over the UK, bringing a scoop of cold air from the Arctic with it – hence the significant change to colder conditions.

What weather can be expected?

The first half of this week will continue largely cloudy and mild with a little rain, as well as fog overnight and during the mornings.

However, a cold front will sink southwards across Scotland later on Thursday, behind which will be the much colder weather. This will then continue southwards, reaching all parts of the UK by the end of Friday.

The Arctic air will bring a big drop in daytime temperatures from 13-17C at the start of this week, to 4-8C on Friday, accompanied by a marked breeze. At night, widespread frosts are expected, with temperatures falling as low as -4C.

On a positive note, the air will be drier, sweeping away the cloud and leading to an abundance of sunshine.

Whilst most places will be dry, a few wintry showers may affect north east Scotland and coastal areas of north east England.

Don’t forget you can get the latest forecast on the Channel 4 Weather website and you can send me weather pictures on Twitter – @liamdutton

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