Published on 30 Jan 2013

A cold February? To topple or not to topple, that is the question

As January draws to a close, it will no doubt be remembered for the two week cold spell that saw much of the country blanketed in snow, with temperatures struggling to get above freezing all day.

The cold spell was caused by a stratospheric warming event that took place 30-50km up in the atmosphere, with its effects gradually percolating down to affect our weather at the surface.

As a result of these effects feeding downwards in the atmosphere, a blocking high pressure developed which stopped the usual west to east movement of low pressure systems and put much of Europe, including the UK, in a big freeze.

Whilst the jet stream has roared back to life this week, with a return to mild, wet and windy weather, there are signs that in the coming weeks, there’ll be a trend to something colder again.

The jet stream rules the skies

Any change to colder weather will be dependent on the position and speed of the jet stream – the fast moving ribbon of air high up in the atmosphere that determines the weather that we experience at the surface.

At the moment, it is taking a normal path of rapidly moving from west to east across the Atlantic Ocean, with a gently snaking motion – meandering around from day to day.

With the jet stream taking this path, low pressure systems develop frequently and vigorously over the Atlantic, heading towards us bringing wind and rain.

However, gradually during the next week, there will be a trend for the jet stream to snake around more, meandering sharply in a north to south motion over the Atlantic, rather than west to east.

This will result in a reduction in the number of low pressures that form and see a trend towards large areas of high pressure building to the west of the UK, bringing spells of quieter, but colder weather as a north to north westerly wind sets in.

To topple or not to topple?

The key to how cold it will get across the UK will depend on the size and persistence of the area of high pressure that builds in the mid-Atlantic, which in turn will be determined by the jet stream.

In order for a high pressure system to build and persist, there needs to be a strong northwards surge of warm air on its western side – known as ridge amplification.

If this isn’t maintained, then the high pressure topples like a wall, allowing low pressure to return, with the whole process having to start all over again.

During the next 7-10 days, this is exactly what will happen to the west of the UK, with our weather alternating between settled and unsettled, as well as mild and cold.

However, towards the middle of February, there are hints that the high pressure in the mid-Atlantic could become stronger and persist for longer.

This would put the UK in the path of a cold plunge of air from the Arctic, with an increasing risk of below average temperatures, as well as snow, frost and ice – especially in the north.

As ever, things can change given that it’s still two weeks away, but I’ll keep you updated here on my blog and on Twitter – @liamdutton

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