Forecast Update
Valid: Fri 05 Jun 2009 09:00 to Sat 06 Jun 2009 06:00 UTC
Issued: Fri 05 Jun 2009 09:20
Forecaster: TUSCHY

A level 2 was issued for NE-Spain and extreme S-France mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for NE-Spain, S-France and N-Italy for large hail, severe wind gusts, excessive rain and tornadoes.


SYNOPSIS

Please refer to the main outlook.

DISCUSSION

... NE-Spain and extreme S-France ...

Early morning convection, triggered by a NE-ward moving short-wave, is about to exit the area. Clearing over SE-Spain works its way to the north as dry slot overspreads SE/E-Spain. New convection already starts along the sharp moisture convergenze over SE-Spain beneath dry high-level air. Latest synop data reveals temperatures in the upper tens/lower twenties with dewpoints in the mid tens and modified with cooler temperatures below 700hPa compared to what 00Z soundings showed, a favorable environment is present for weakly capped, deep convection. Another vorticity maximum draws near from central Spain between 12Z-15Z, so already ongoing thunderstorm activity will keep going and even increase betimes.

GFS still indicates a rapid increase of mid-level lapse rates over Valencia during the late afternoon hours just atop the gradually eastward moving, sharp moisture convergence zone. This scenario looks reasonable in respect to the approaching dry slot and strong mid-level lift and therefore, a concentrated instability tongue with MLCAPE in excess of 1000 J/kg along the convergence zone evolves during the afternoon hours (e.g. Valencia northwards). Deep layer shear up to 30m/s, 15m/s at lowest 3km and forecast very steep mid-level lapse rates indicate a significant risk for large hail with very large hail possible in more discrete thunderstorms. LL CAPE release is augmented along the convergence zone, so thunderstorms also have an isolated tornado risk next to severe wind gusts. There remains one major uncertainty left with this outlook as strong NVA and subsidence follows the passage of the vorticity maximum after 15Z, which could inhibit further development, despite favorable conditions. We therefore keep parts of Valenica in a high-end level-1 but any developing thunderstorm could pose aforementiond severe weather risks.

Thunderstorms over extreme SE-France also experience DLS in excess of 20-25m/s, so large hail and severe wind gusts are possible, too. We also went with a level-2 due to the high storm coverage. Thunderstorms decrease after midnight.

... N-Italy ...

GFS/WRF and ECMWF still agree in mostly capped conditions over central Italy although latest GFS runs came in more aggressive with some convective precipitation signals well to the south. Again, any developing thunderstorm over Corse/Sardegna will be elevated due to steep mid-level lapse rates and warm low levels. Shear would be more than adequate for large hail, but expected thunderstorm coverage does not justify a level area for now.

Cooler mid/low-levels over N-Italy and surface dewpoints already in the mid/upper tens yield a favorable environment for surface based convection. Despite some nocturnal cooling, confidence increased that surface based convection will occur and we therefore augment tornado probabilities due to strong LL directional/speed shear but we stick with the level-1. This tornado risk also covers extreme SE-France/extreme SW-Switzerland. Severe wind gusts and isolated large hail accompany stronger storms.

Some modifications were done for the rest of the outlook.