From the Met Office News Blog this morning:


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New post on Met Office News Blog

Could sudden stratospheric warming bring a cold start to spring?
by Met Office Press Office

Our atmospheric scientists are predicting a dramatic change in high altitude
winds 50km above the ground and the imminent occurrence of an event known as a
sudden stratospheric warming (SSW) in early March.
Professor Adam Scaife, Head of Monthly to Decadal Prediction explains: "Sudden
stratospheric warming events occur high up in the atmosphere and involve a
complete reversal of the high altitude polar jet stream - they can even affect
weather at the surface, and for the UK a sudden stratospheric warming increases
the risk of wintry weather."
The phenomenon begins with a wave-like disturbance which travels up into the
high-altitude jet stream. Scaife said: "This disturbance can grow to a point
where it turns over and breaks, just like a wave on a beach."
Normally the jet stream flows from west to east with some north and south
oscillation, but the force from this high altitude disturbance pushes against
the jet stream until the winds actually reverse and flow from east to west
instead. Air then falls into the Arctic and is compressed so that it starts to
warm: the temperature can rise by as much as 50C in just a few days.
Professor Scaife added: "This reversal of high altitude winds can also burrow
down into the lower stratosphere. Once it is within reach of weather systems in
the lower atmosphere the Atlantic jet stream often weakens and moves south. This
allows cold air from the east into northern Europe and the UK."
Sudden stratospheric warming events occur on average every couple of years and
our long-range forecasts have consistently suggested an increased risk of sudden
stratospheric warming towards the end of this winter. The last big event was in
early 2013 and was followed by a cold end to winter. Although the impact of the
current event is unlikely to be as severe, it increases the risk of cold north
easterlies and wintry weather for the UK over the next few weeks.
Does this mean we'll see snow at Easter?
You may have seen in the media that we will see snow at Easter. At this stage it
is too early to provide details about what the weather will bring for Easter.
Beyond a week ahead we can't say what will happen on specific days, but we can
give an idea of what type of weather we can expect. Keep up to date with the
latest 30-day forecast using our forecast pages.
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Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.
http://peakdistrictweather.org