Extended Forecast
Valid: Wed 22 Jun 2011 06:00 to Thu 23 Jun 2011 06:00 UTC
Issued: Mon 20 Jun 2011 22:04
Forecaster: TUSCHY

A level 1 was issued for NE-Spain, parts of France, Switzerland, S-Germany and N-Italy mainly for large hail, a few significant events possible and severe wind gusts.

A level 1 was issued for NE/central Germany mainly for severe to damaging wind gusts, large hail and a few tornado events, probably transitioning into a severe wind gust and heavy rainfall risk over W-Poland and the Czech Republic.

SYNOPSIS

A pronounced cold-core vortex is situated over Iceland, which steers numerous disturbances over parts of Europe. The most active feature will be an upper trough over UK/Scotland, which lifts to the northeast during the forecast and reaches the Baltic Sea during the night hours. Quiescent conditions exist along the Mediterranean and SE Europe, as mid-level heights gradually increase.

At the surface, an elongated surface boundary exists from central France all the way to Germany. A complex surface pressure scenario evolves from the North Sea to Norway/Sweden, as models develop numerous surface depressions with variable strength and placement. The one of main concern is currently forecast in fairly good agreement throughout the model field, including ECMWF and GFS. It is forecast to consolidate over N-Germany, moving rapidly to the northeast.


DISCUSSION

... Germany, W-Poland and the Czech Republic ...

Overall model consistency is adequate for mentioning a potential severe thunderstorm outbreak over parts of Germany, W-Poland and the Czech Republic, including severe to damaging wind gusts, large hail and a few tornado events.

ECMWF, CMC, NOGAPS and GFS all show a rapidly northeastward moving vorticity lobe, which crosses NW-Germany during the afternoon and Denmark during the early night hours from the SW. Favorable placement of a pronounced left exit region of a strong mid-level jet assists in an augmented surface pressure fall in the warm sector and hence the potential development of an extensive N-S aligned surface "pressure channel" over Germany, moving rapidly to the northeast. Hence, we expect various forcing foci, which allow scattered to widespread thunderstorm initiation within highlighted areas.

Same story as in the past when concentrating on thermodynamics with GFS being more bullish than ECMWF. Surface dewpoints in the mid-tens sound reasonable, given persistent influx of moist air from the E-Atlantic and the W-Mediterranean, so we will stick with ECMWF, expecting roughly 500 J/kg MLCAPE over N-Germany, increasing to 1000 J/kg further to the south and east (as first good EML of the season advects to S-Germany).

Kinematics look supportive for organized convection although exact strength heavily depends on the final development of the surface depression/low "pressure channel", especially with respect to any enhanced directional shear. WRF/GFS indicate an highly sheared environment, as 30 m/s speed max moves in from the SW, increasing 0-6 km DLS to roughly 25 m/s and somewhat weaker values in the lowest 3 km. If strength of surface features (seen in latest model data) remains in place, the unstable warm sector would see a favorable helical environment for supercells with an enhanced tornado risk to the north and large to isolated very large hail further to the south and east. However, the main hazard right now looks like to be bowing structures over central/N- Germany with concentrated shear within the lowest 3 km. Swaths of severe to damaging wind gusts would accompany that activity

Reasoning for not yet upgrading to a level 2 are the uncertainty about final CAPE over N/NE Germany, potential ongoing thunderstorm activity from the night before and also the final extent of the LL depression/wave over N-Germany (influencing LL speed/directional shear and the final tornado risk).

Probably numerous large thunderstorm clusters will affect W-Poland and the Czech Republic during the evening/night hours. Despite weakening CAPE, severe wind gusts and heavy rainfall are forecast well into the night.

... E-France to NE Spain and Switzerland to Austria...

Along a wavy surface front, initiation is forecast during the day. Despite the fact that progessive and intense upper wave over Benelux, NW Germany and Denmark misses those regions, another disturbance is forecast to enter the area of interest from the west during the forecast period. The ingredients are present for scattered severe thunderstorms, as 500 - 1000 J/kg MLCAPE and 20-25 m/s DLS overlap, next to locally strong veering throughout the lowest 3 km. Large to very large hail will be the main hazard next to severe wind gusts...the latter risk will be maximized if storms line-up due to strong LL shear/cold pool development. Parts of the level 1 may need an upgrade due to a significant large hail risk.