UK’s unsettled weather and the jet stream

21 10 2013

The UK is set to see unsettled weather throughout this week as heavy rain and windy conditions are expected to affect many areas, whilst temperatures will remain mild for the time of year.

We talk about the jet stream quite a bit in the UK because it has such a big influence on our weather, and this week is no exception as it’s playing a leading role in determining the unsettled outlook.

What is the jet stream?

The jet stream is a band of fast moving westerly winds high up in the atmosphere which circle around the pole in the northern hemisphere. It can feature winds of up to 200 knots (230 mph) or more, and these winds tend to guide wet and windy weather systems which come in off the Atlantic.

The jet moves around a fair bit and its position can have a big impact on weather here in the UK depending on where it is.

If the Jet is over the UK or just to the south, we tend to get a lot of wet and windy conditions as it brings weather systems straight to us. If the jet is to the north of us, it guides that changeable weather away to the north to leave the UK with more settled conditions.

What’s the jet stream doing now?

Unsurprisingly given the outlook for this week, the jet is positioned more or less directly over the UK – but it’s the detail of its track which is important.

As you can see from the picture below, the jet currently swoops south from western Canada – moving over the Atlantic before taking a sharp turn north to head over the UK.

Forecast chart showing  expected position of the jet stream at 1pm on Tuesday 22 October

Forecast chart showing expected position of the jet stream at 1pm on Tuesday 22 October

This means relatively cool air is being dragged south then over the Atlantic, where warmer seas heat the air from below. This causes the air to warm and rise – creating instability and generating cloud and rain.

By the time weather systems reach they UK they have picked up a lot of rain and relatively warm air, bringing us the wet but mild conditions we are currently seeing.

What’s the weather outlook?

Currently unsettled weather looks set to impact the UK through the week, with heavy rain affecting many areas at times.

There may be more settled conditions on Thursday, and perhaps again on Saturday, but looking further ahead into the start of next week the outlook is for unsettled weather to continue.

You can stay up to date with what to expect with our detailed forecasts out to 5-days and our weather warnings, as well as a general view of what we expect out to 30 days.

You can find out more about the jet stream in our YouTube video.


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4 responses

21 10 2013
Tony

The weekly forecast is therefore “wet, wet wet” but “at least it will be mild”. That said, I seriously hope people stay safe and when venturing out and about, they drive carefully.

21 10 2013
derekbeattieimages

Good to see the real reasons behind the weather and not the alarmist posts coming from an ever increasing number of so called weather sites springing up on the web.

22 10 2013
nuwurld

Great report Met. I was initially dismayed at the written content but, with the additional info in the video, the report is clear and concise. So pleased you’ve correctly shown the “Jet” being above the “maximum thermal gradient” at warm/cold air boundaries, and shifting with these as they compete.

I would also like to add that the position of the “Jet” marks the “zero energy balance” point between the excess equatorial thermal energy and the polar deficit. As a climatic indicator, the “Jet’s” mean latitude tells us the relative energy balance of the hemisphere, with the sum of both hemispheres indicating global trends. Highly modulated streams trending equatorial also bring more cloud cover into otherwise surplus energy regions reducing insolation, and acting as amplification of solar reduction.

26 10 2013
Lord Beaverbrook

‘excess equatorial thermal energy and the polar deficit’, ‘amplification of solar reduction’….. heh, heh, heh. Thanks for the chortle.

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