A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.
Temperatures are due to take a dip below the seasonal norm. It’s still a way off so details will change but here’s the current idea moving ahead.
The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole. As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK. As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)
The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative. The AO is a measure of air pressure over the Pole relative to mid-latitudes. It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.
12Z GFS ENS maintains the theme for a more pronounced Greenland/Polar block and here’s some further info. pic.twitter.com/wpXJxM6rLD
— Matthew Hugo (@MattHugo81) January 6, 2016
When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes. The chart below shows the 500mb mean heights for 8-10 days time. Spot the anomalously high pressure over the Pole relative to the mid-latitudes.
The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia). This is a long-term indicator for potential pressure rises in this region.
In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole. The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!
As well as a push of cooler Arctic air, pressure is also due to rise over the UK. With increased pressure we can thankfully expect a drier period.
A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars. On the other hand, frost, fog and icy conditions might also be expected too. In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies. However, this is uncertain so stay tuned for more regular updates on twitter. Of course, seek professional forecasts for decision making purposes.
update 06/01 **looking potentially more prolonged now. cold snap turning into a spell.
January 2016 cold snap
2015-16 winter forecasts have long considered the possibility of cold weather in the latter stages from Jan thru to feb. This was explored in a post here