5 Nov 2014

Jet stream hinting at what early winter may bring

With December just a few weeks away and thoughts of winter straying into our minds, I’ve already had lots of people on Twitter asking if we’re going to get any snow this year.

Whilst the inevitable answer is that it is simply too early to tell, it is possible to look at the pattern of the jet stream over the next few weeks and get a flavour as to what the weather might throw at us.

During the past week or so, I’ve been keeping a close eye on the pattern of the mid-latitude jet stream – the driver of the weather that we experience at the surface.


Interestingly, it has been offering a fairly consistent signal as to what characteristics the jet stream will take through the rest of November and into early December.

Jet stream weakening and meandering

During the next 10 days, there is a sign that the mid-latitude jet stream will start to weaken and meander significantly northwards and southwards at times.

The jet stream normally takes a general west to east path across the Atlantic – something in the meteorological world called a zonal flow.

jetstream_21stNov_WZ_wpThis brings relative warmth and moisture in from the oceans in the form of low pressure, wind and rain, as we’ll see during much of next week.

However, around 18-20 November, there are hints that the jet stream is going to take a more south to north track in the vicinity of the UK – known as a meridional flow.

This can result 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.

What’s causing the jet stream to do this?

The jet stream is driven by the temperature contrast between the poles and the equator. The greater the contrast in temperature, the faster the jet stream tends to move, as well as taking a more direct west to east path.

However, during the next 10 days, there are signs of the stratosphere gradually warming over Asia by around 20-30C, about 15 miles up in our atmosphere.

Warming of the stratosphere often causes a weakening of the mid-latitude jet stream, which then causes it to meander further north and south than normal, as described a little earlier.

Now this warming doesn’t match the scale of sudden stratospheric warming that was considered a big player in the run of cold and snowy winters the UK experienced a few years ago.

Nevertheless, it hints that we may progressively see a trend to a weather pattern offering something colder as we move towards December.

Does this mean snow is on the way?

At this stage it is simply too early to tell whether or not the resulting weather pattern will bring the UK snowfall.

london_snow_g_wpSuch weather patterns can often put the UK and much of north west Europe under a big area of high pressure, which would equate to dry and cold with frost and fog, rather than snow.

Given how finely balanced the nature of snowfall is here in the UK, attempting to forecast its likelihood so far ahead is almost pointless.

Even so, this change in weather pattern has given us meteorological folk some food for thought and will have us glued to the weather charts in the coming weeks.

All of what is written above comes with the usual caveat that things can change, although I’ll keep you up to date here in my blog and on Twitter – @liamdutton

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