The most wintry set-up of the “winter” has decided to arrive at the start of Spring! High pressure over the Pole is still keen to push out polar air into mid-latitudes as the meteorological Spring starts tomorrow. The Arctic Oscillation shows this tendency as it has been dipping negative, showing relatively high pressure over the Pole and lower pressure in mid-latitudes. The belated rise in Polar pressure has been attributed to the stratospheric warming which occurred around a month ago.
Unfortunately, for most of late winter the jetstream has remained strong enough to push a predominantly Atlantic flow into the UK and breach any blocks attempting to drag in sustained cold air. This is shown by the slight but persistently positive North Atlantic Oscillation below: this set-up spoilt any chance of proper cold this half of the winter.
However, this week the wind is expected to turn more northerly as pressure lowers first over the North Sea and then the Channel / N France. By the weekend the UK will be in a cold Arctic flow courtesy of the same LOW lingering over Europe and a blocking Atlantic ridge. Between them and the jetstream they will do a good job of pulling down a cold Arctic flow this weekend. Spot the cold anomalies in the chart below. Lots of lying snow is not likely but some wintry precipitation is possible at times here, especially a marginal chance on Friday am. Things change though, so check weather professionals like the MetOffice to make any weather related decisions.
Here is the story of how we get to some belated cold by this weekend. Tuesday sees an end to the cool clear HIGH that brought Spring-like sunshine to Reigate and Tmax 9C. A warm front will sweep in tomorrow morning and bring rain for most of the day for the SE. It will be breezy, though relatively mild in the warm sector shown below, Tmax 10C.
Things cool off into Wednesday as an active blustery cold front ushers in colder polar maritime air through the morning. Any snow is restricted to the NW of the UK.
The flow swings increasingly to the north west during the day with the possibility of showers later here, some quite heavy, maybe even with the outside chance of hail and thunder thrown in. Tmax 6-7C. Update: The video below shows how that showery trough passed through Reigate during Wednesday:
Thursday is the crux to building a set-up capable of producing any snowfall at all for the SE. A LOW is forecast to develop from a system off Greenland and cross the Atlantic smartly from the NW.
This low, with rapidly occluding warm sector, will deepen slightly and meet the cool pool sitting over the UK from Wednesday’s Polar attempt. The LOW is expected to arrive late Thursday and track south east overnight into Friday bringing in a cool NE flow capable of wintry precipitation for a while on Friday morning.
The latest charts suggest the track into N France will result in NE winds which could bring snow for a time in the SE on Friday morning.
It is all rather marginal for us in the SE and Reigate though. A rain / sleet event with a possible wintry mix at times is more likely and nothing much is expected to settle.
By Saturday and into the weekend a cold northerly / NE flow sets as the “Greenland” LOW settles over Europe. Wintry showers could develop across the SE, especially in any distrubances in the Arctic flow. However, pressure is likely to be on the rise as the Atlantic ridge creeps in from the west under an increasingly anticyclonic jetstream.
How long any early Spring cold lasts is not certain, the Atlantic ridge looks like toppling over to bring in warmer conditions later next week.