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Cold coming this week – how long will it last?

Cold coming this week – how long will it last?
November 18
13:02 2013
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Monday sees a transitionary day across the UK as we see the first hints of everyones* favourite season just around the corner (*N.B. – not necessarily true!). We see a band of rain moving South across the country with colder air already flooding in to the North of the UK, and here we will see scattered showers turning increasingly wintry as the day goes on.

Some complications in proceedings later Tuesday and in to Wednesday as some warmer air brings a band of rain in to the UK, with some snow on the leading edge in the North, and perhaps to higher ground further South during Wednesday.

All in all though the initial blast of cold air lasts through until Thursday. We then see high pressure starting to build in from the North-West of the UK, settling conditions down somewhat with the risk of overnight frost and fog along with, cloud cover permitting, some crisp, sunny days.

Our attention right now though is focussed a little further ahead. Our initial expectations for December were for low pressure to start taking charge of things across the UK, particularly through the second half of the month. There are though now just the first few hints that for the first half of December we may see something colder persisting. There’s a little bit of explaining to do here but bear with me, I will make this as simple as possible to understand.

The main reason during winters in the 1990′s and early 2000′s that we experienced milder and wetter than average winters was due to the strength and resilience of the Polar Vortex. The polar vortex is an area of intense low pressure systems that often encircle the polar regions during the winter months as natural cooling takes place in the Northern Hemisphere. An example of this can be seen below, with the black, purple and blue colours over the Arctic regions depicting a strong polar vortex:

Archive chart from January 1989 showing a strong Polar Vortex

This has the effect of bottling up all the colder air over the Arctic circle, whilst milder West or South-Westerly winds dominate conditions across the mid-latitudes, including the UK.

However what has happened in more recent winters from the past 5 years or so is that the polar vortex has been broken down at various times. This has been caused by a multitude of different and rather complicated factors, but the end result is something that looks more like this:

Archive chart showing a disrupted Polar Vortex from December 2010

What has happened above is that the polar vortex has been shattered in to pieces, with high pressure building across the Polar regions instead allowing the colder air from the Arctic to flood in to the mid-latitudes.

Could something similar be about to happen now? Well at this juncture we are not looking at anything like December 2010 from current forecast models. Certain forecasting models though are starting to hint that we could see a split in the polar vortex in around 8-10 days time. If this were to happen, and with high pressure close to the North-West of the UK, there is the potential for a Northerly flow to develop once again across the UK, but a more substantial one than the one about to be experienced this week.

Below is an example of one particular forecast model, the ECMWF, and how it splits the polar vortex in 10 days time. It goes from this:

To this:

Breaking the polar vortex in to two segments, and potentially¬†allowing the cold air to flood South towards the UK. This is still a developing situation and not all forecasting models are suggesting the same thing, but it’s an interesting one to watch and we thought it would be a useful case study for any amateur meteorologists out there to look for signs of colder weather.

Forecaster: Kris Surtees

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