This coming week, Arctic air from Svalbard briefly encounters Tropical air from the Bahamas over Europe. Remarkably, how cold it gets here in the UK and Europe might depend on the story of a sub-tropical storm over the Atlantic just as much as the Arctic air trying to push south.
Pressure has built over the Arctic, nudging cold air uncertainly south into European mid-latitudes. The build in Polar pressure and relative fall in mid-latitude pressure is called a negative Arctic Oscillation as mentioned in previous posts. The pattern is already cool because the jetstream is to our south. The jetstream axis essentially divides the warm tropical air to the south from the cool polar air to the north.
However, the flow has been mostly westerly and even SW across Southern Britain as the jetstream is blowing purposefully from west to east across the Atlantic. While pressure remains relatively high over parts of Europe (e.g. Med and Spain), the coldest air has been unable to penetrate very far south. The jetstream chart above shows the unstable flow we have had this weekend bringing heavy thundery showers, some with hail, over Reigate. Check this pic of mammatus clouds this afternoon over Surrey, after thundery showers with hail.
It has been cool because the air has been circulating anticlockwise around a deep low over Scotland with a cold source region over Scandinavia. However, the airmass has been modified with a long track over the Atlantic. With such a strong zonal jetstream the more frigid Arctic air has not been able to penetrate far south into Europe, yet.
An unusual sub-tropical storm developed in the Bahamas last week over a very warm Gulf Stream. This low pressure will come to hover in the Mid-Atlantic this week and it might just hold the key to unlocking some more Arctic air over Europe.
A part of the sub-tropical system is forecast to cross into Europe mid-week courtesy of a trough disruption. This causes a part of the trough to break away and leave the parent “cut-off” in the mid ocean.
In this case, a small but vigorous “baby” LOW will ride off on the right entrance of the jetstream, a good location for deepening surface pressure, and enter Europe via the Bay of Biscay sometime on Wednesday.
Whilst this vigorous baby low will not impact the UK directly, it is set to lower pressure over Europe and, in its wake, will drag in Arctic air more purposefully SOUTH across more of the continent right to the Mediterranean. This wind will be significant and create the first proper wintry feel for about three years with wind chill on occasions down to -7C or lower.
Importantly, the trough disruption will also build pressure in the Atlantic to the north of the sequestered parent LOW. This is often the case in trough disruptions.
The increasing the pressure gradient in the Atlantic and lower pressure in Europe will push more Arctic air more purposefully south across the UK and into Europe. So the sub-tropical system has been key to unlocking the full Arctic blast!
Details about snow for Reigate will be updated this week. At this stage heavy snow settling here looks unlikely but some snow fall and wintry showers are certainly possible, especially later mid-week. It will get gradually colder through to the end of the week. Thereafter it is, of course, uncertain! The ECM has been performing better as a model than the GFS so, on that basis, continuing cold is more likely than a sudden return to mild westerlies, which the gfs tends to do too quickly. So assume it’ll stay cold into next weekend.