Storm Forecast
Valid: Fri 20 Dec 2013 06:00 to Sat 21 Dec 2013 06:00 UTC
Issued: Thu 19 Dec 2013 23:22
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for Ireland, parts of the UK and the North Sea for severe convective wind gusts.

A level 1 was issued for NW Algeria for excessive convective precipitation.

SYNOPSIS and DISCUSSION

A series of progressive troughs and ridges travels eastward within a strong frontal zone over the Northern half of Europe. Two short-wave troughs and their attendant frontal systems move from the Baltic Sea into Russia and from the North Sea and France to the Baltic States and Belarus, respectively. Beneath the base of the second trough, cold upper levels and QG lift on the cyclonic side of the jet stream will allow scattered thundery showers over parts of the Norwegian and North Sea in the 06 to 12z time frame. Due to limited instability and temporarily weak vertical wind shear, severe weather is not expected.

Further West, the warm sector of an outstanding Atlantic cyclone will overspread the British Isles with plentiful rain and 850 hPa winds up to 40 m/s. Scattered to widespread severe wind gusts are expected in the warm sector, where the pressure gradient is strongest, as well as along and behind the cold front, which will swiftly cross Ireland and Scotland after 18 UTC before it will gradually decelerate over England and the North Sea. It will mark the transition from rain to showers and isolated thunderstorms, which can easily mix down 850 hPa winds which are still between 25 and 30 m/s in the postfrontal air mass. A level 1 was drawn for those areas where an involvement of deep convection in the gust generation is expected. The Scottish West coast and the Hebrides were upgraded to a level 2, since they are favourably placed in the left exit region of a very strong jet streak. Hence deeper and more widespread convection, possibly in form of an organized line along the cold front, with an even higher risk of severe wind gusts is anticipated.
Additional subordinate risks are localized flooding (the convective contribution to the high rainfall totals will be small, though) and a brief tornado along the cold front, which is preceded by a tongue of very moist air (2m dewpoints up to 10°C) and strong directional wind shear.

The Southern half of Europe is influenced by a high pressure system which is especially pronounced at lower levels. However, cool upper-levels and various leftover convergence zones allow scattered showers and isolated thunderstorms across the Mediterranean Sea. A few waterspouts may occur with this activity.
Over Northwestern Algeria, a tongue of moist air is persistently advected onshore with 20 to 25 m/s Northeasterly winds at 850 hPa in the periphery of a cut-off low over Algeria. A level 1 for heavy precipitation with a risk of flash floods was issued here. Convection will contribute but will barely become deep enough to produce lightning, dampened by beginning QG subsidence.