Storm Forecast
Valid: Sat 28 Nov 2009 06:00 to Sun 29 Nov 2009 06:00 UTC
Issued: Fri 27 Nov 2009 22:41
Forecaster: TUSCHY

A level 1 was issued for N-Bay of Biscay, extreme NW-France and the English Channel mainly for severe wind gusts and to a lesser extent for isolated tornadoes.

A level 1 was issued for S-UK mainly for tornadoes and severe wind gusts

A level 1 was issued for extreme NW-Spain and N-Portugal mainly for severe wind gusts and to a lesser extent for excessive rainfall.

SYNOPSIS

Cyclonic vortex still in place over NW/N-Europe. Next disturbance starts to affect Ireland and United-Kingdom with a weakening one entering the North Sea. Another upper wave over the Mediterranean moves off towards the east but causes showers/thunderstorms over the central Mediterranean. The weather conditions are cool and stable over NE-Europe.

DISCUSSION

... The English Channel, S-UK ...

To start with the main point: Global models had a significant stop-go with the depression of interest in the past few runs, but a more consistent scenario can be seen in the last 06/12 UTC runs. There are hints of a substantial risk of tornadoes over S/SE-UK next to severe wind gusts, unfolding after sunset.

Latest airplane reports, Met9 wind and shear analyses at upper levels and GEM analysis all show the strengthening jet over the N-Atlantic and support the idea of a 80-90m/s streak, curving southwards along the upstream flank of the developing trough over W-Europe. How fast this jet digs southwards also dictates the timing and progression of the evolving depression S of Ireland. GEM, ECMWF and GME 12 UTC runs remain the outliers with a slower forward motion of this depression, whereas the rest agrees in timing and strength, bringing a ~ 979hPa (GFS, NOGAPS, UKMO) depression onshore over far SW-UK at roughly 00 UTC. It is not unknown that models have problems in seizing the wind field of jets over the N-Atlantic and the tendency to slow down the eastward progress of any developing surface depression (upper jet is slower in southward translation). We decided to go with a compromise between ECMWF and GFS, as the solution of the European model looks to be too bullish in occluding the depression before entering the English Channel, whereas GFS brings a broad warm sector over most parts of S/SE-UK during the night.

Current thinking is, that depression will enter the Channel later compared to what GFS forecasts, but we still expect a warm sector to affect S/SE-UK until 06UTC. Increasing pressure gradients push the warm front well onshore with a moist (Td 6-9°C) warm sector over extreme S-UK. A combination of a potent vorticity lobe, crossing UK from W to E during the night, a rapidly approaching dry slot from the SW and a gradual cool down of the mid-levels creates an environment, supportive for deep convection. Especially in case of the GFS 12 UTC solution, scattered thunderstorms are possible in an environment, very supportive for tornado activity, especially along the warm front itself. Severe wind gusts are also forecast, especially as the cold front draws near from the SW with up to 30m/s at 850hPa and better LL CAPE. An upgrade may be needed, if the GFS scenario verifies, but for now, we stick with a level 1, reflecting the slower scenario/ later timing of the warm front in this forecast.

Extreme NW-France is also under the gun for enhanced convection during the evening hours with a severe wind gust and isolated tornado risk. However, the cold front, responsible for the bulk of convection, outruns better dynamical/thermodynamical conditions and hence, a gradual demise (temporarily) of convection is forecast over the Bay of Biscay during the early night hours.

It also has to be noted that the coast of N-France and Belgium also has to be monitored in case of a more eastward motion of this depression (GFS 06 UTC run), but confidence in this scenario is too low for a level 1.

Just for the sake of completness: The 18 UTC run (GFS) just came in, with a somewhat slower solution, but still similar to 12 UTC. BL moisture is worse compared to 12 UTC with very slim CAPE onshore, so this will be the crucial point for the final tornado risk.

... Extreme NW-Spain and Portugal ...

The cold front passage (18-00 UTC) will be the main focus for thunderstorms, despite a gradual decrease in the activity. Wind field is adequate for severe wind gusts and good moisture influx from the Atlantic also increases locally high precipitation amounts. Postfrontal subsidence will be short-lasting with a gradual increase, as CAA deepens. The wind field is still adequate for strong to severe wind gusts and the combined risk of both events made a level 1 necessary. Deep convection will enter the Bay from the west during the morning hours with strong wind gusts possible.

... Central Mediterranean ...

Flat upper trough translates eastwards. Thunderstorms are forecast, probably most concentrated along the coastal areas (E-Tyrrhenian Sea and E-Adriatic Sea). Isolated large hail (especially over central Italy) and strong wind gusts are possible, next to locally heavy rain, but this risk is not worth a level area. Thunderstorms abate from west to east during the forecast.