Storm Forecast
Valid: Mon 23 Nov 2009 06:00 to Tue 24 Nov 2009 06:00 UTC
Issued: Sun 22 Nov 2009 21:15
Forecaster: TUSCHY

A level 1 was issued for parts of United Kingdom, parts of the Netherlands, Belgium, Luxembourg and parts of Germany mainly for severe wind gusts and to a lesser extent for tornadoes.

SYNOPSIS

High pressure settles in over the NE-US/Newfoundland through this forecast, yielding a favorable pattern for a significant outbreak of true polar airmass far southwards over the N-Atlantic in conjunction with the strong cyclonic vortex north of Scotland. Downstream development is underway during the following 36h with falling geopotential heights S/SE of Greenland and positive height tendencies over far NW-Europe. Not much change for the European weather: warm and stable over the Mediterranean and the Black Sea, somewhat colder over NE-Europe and unsettled conditions over central/NW-Europe.

DISCUSSION

...Belgium, the Netherlands, Luxembourg and Germany ...

Aforementioned Atlantic trough causes a rapidly strengthening ridging over Ireland and UK throughout the period, with strong high-level jet winds developing along the eastern fringe of this ridge over Ireland and SW-UK as supergeostrophic winds spread southeastwards. This assists in the gradual development of a weak wave, which crosses UK during the late afternoon hours, Belgium, the Netherlands and W-Germany until roughly 21 UTC and the rest of Germany thereafter. A striking sign of this young wave is the persistent and well established phase shift of the respective trough axis between the surface wave and the one at mid-levels. Despite minor discrepancies in timing and strength, global models agree well among each other and latest modified IR loop of GOES-12 showes a constantly cooling cloud shield along this baroclinic zone offshore, where GFS and ECMWF initiate the wave.

Symptomatical of a wave at that time of year, the main story will evolve behind the sheared (20m/s LL shear) and broad warm sector, affecting those areas during the daytime hours with the main attention on the spot between the surface and mid-level wave. Nested model runs hint on the strongest moisture advection ahead of the mid-level wave and not in conjunction with the surface feature itself. Hence, mid-level cool-down atop of the LL moisture causes a fairly extensive area with up to 300 J/kg MLCAPE over Belgium/the Netherlands and NW-Germany, decreasing eastwards/southwards over Germany and during the night hours. The cold front, ill defined over the Low Countries, gradually shapes up as it crosses W-Germany and is the focus for main thunderstorm activity.

Shear is moderate in the deep layer up to 6km (15-20m/s) and strong in the lowest 0-1km / 0-3km with up to 20m/s with a sharp helicity gradient along the fronts. Next to shear, forcing is present as a vigorous vorticity lobe accompanies the developing upper trough.

Current scenario is a developing area of enhanced convection over the far eastern English Channel, Belgium and Netherlands during the evening and early night hours, as the developing cold front crosses those areas from the WNW. A forced line of showers/thunderstorms along the cold front will probably evolve with the strongest activity likely confined to the low probability thunderstorm area, where coldest EL temperatures are forecast. This line advances eastwards, crossing N-cntrl Germany during the night hours, reaching E-Germany in the morning hours. Weakening forcing, diminishing tongue of surface moisture and decaying CAPE ought to support a constant decrease over eastern Germany with a decreasing severe risk. The main risk will be isolated sever wind gusts (especially along bowing segments), as a 30 m/s-3km streak accompanies the cold front next to marginal hail with strongest storms. Prefrontal, stratiform rain shield and strong winds preclude the development of a more significant nocturnal, stable boundary layer despite the unfavorable timing of the front. LL SRH magnitude remains below 200 m^2/s^2 with highest values displaced to the east along the surface wave, so overall tornado risk is not excessively high, but an isolated event is possible, given low LCLs and strong wind fields in the lowest 3km.

A bit more complex over central/southern Germany during the night hours, as better LL moisture causes an increase of LL CAPE release. EL temperatures gradually warm and best forcing moves off towards the ENE, but a line of shallow convection could still accompany the cold front passage in that part of the level 1 area. Winds also increase in strength over S-Germany, so again, severe wind gusts and an isolated tornado event prompted us to expand the level area far southwards. Not uncommon is a potential temporal flare-up of convection over the Bavarian Forest with an isolated thunderstorm event possible during the morning hours.

... United Kingdom ...

The forecast remains more complex for parts of UK during the passage of the developing trough. Models saw-saw in respect of BL moisture content and attendant CAPE, but overall potential for stronger updrafts looks better and better for each run, so we decided to include a level 1 for severe wind gusts and isolated tornadoes, but no thunderstorm area was yet included. Highest chances for deep convection probably exist along the SW coast of UK and beneath the coldest mid-levels over Wales and eastwards.