Forecast Update
Valid: Sun 17 May 2009 13:00 to Mon 18 May 2009 06:00 UTC
Issued: Sun 17 May 2009 13:22
Forecaster: TUSCHY

A level 1 was issued for Germany, Switzerland and extreme E-France mainly for large hail, severe wind gusts and tornadoes.

A level 1 was also issued for NE-France mainly for large hail and severe wind gusts.


SYNOPSIS

Please refer to the main outlook.

DISCUSSION

...Germany...

Latest hand analysis reveals two areas with dewpoints in excess of 13°C, one over SE-Germany and the other one over SW-Germany and extreme E-France. Dewpoints in excess of 11°C are present all the way up to NW-Germany and E-CNTRL Germany with substantial lower values over NE-Germany right now. Some mixing will still occur which could hamper further LL moisture increase. Strong diabatic heating and WAA brought surface temperatures in the lower twenties and temperatures will rise into the mid to upper twenties later the day over central/south Germany. LL WAA and cool air aloft should constantly help to steepen lapse rates during the next couple of hours and some low-end/moderate instability release is anticipated.

The main missing ingredient is a clear mid-/upper forcing mechanism as diffluent flow features just a few weak impulses, embedded in the SW-erly flow. Stronger forcing approaches from the west during the evening and night hours and enhances thunderstorm coverage.

... W/NW and N-Germany ...

Thunderstorm initiation over SW/W-Germany is imminent or already ongoing with strengthening TCU fields in the latest VIS loop now present. A convergence zone is present just east of Luxembourg with warm/moist air ahead of this line. Latest VADs indicate no real strengthening trend in the wind field with 20m/s at 4km and a straight hodograph at least throughout the lowest 5km. 12Z soundings over W-Germany show that not much modification is needed for initiation, especially as tongue of warm/moist air continues to advances northwards. Thunderstorms gain organization in a moderate CAPE/shear environment and severe wind gusts/large hail are the main hazard. An isolated tornado event can't be ruled out with lower LCLs and augmentend 0-1km shear.

... S-/E-Germany ...

Aforementioned WAA keeps atmosphere capped and agreement in the past few model runs is high that initiation won't occur until the evening/early night hours. Strong diabatic heating and an eastward moving convergence zone ought to help thunderstorms to initiate rapidly during the evening hours over Bavaria but also along the northern slopes of the NW-Alps. Steep lapse rates and up to 15m/s deep layer shear favor a large hail risk, especially during the more discrete phase of those storms. WRF indicates rapidy clustering of those storms and a loosely organized, northeastward moving MCS could evolve with an isolated large hail/severe wind gust threat.

Also during the afternoon/evening hours, thunderstorms increase over Switzerland and E-France, which move towards the northeast. As forcing at upper levels becomes stronger, a messy picture of numerous thunderstorm clusters is expected, which affect S-Germany during the night hours. An isolated large hail/severe wind gust event is still possible beside locally excessive rain amounts.

Two major inhibiting facts preclude the issuance of a level-2. The first one is the missing forcing, which could cause a messy picture of numerous thunderstorm clusters and the other one is the limited degree of BL moisture. Latest reports indicate that anticipated mixing of the thin, moist surface layer is underway, which hampers better instability release.

No major changes for the rest of the outlook area.