Storm Forecast
Valid: Fri 05 Jun 2009 06:00 to Sat 06 Jun 2009 06:00 UTC
Issued: Thu 04 Jun 2009 14:38
Forecaster: TUSCHY

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


SYNOPSIS

The main feature of interest is a strengthening trough over the Bay of Biscay, which moves eastwards and a constantly consolidating surface depression along its eastern fringe. A warm and increasingly unstable airmass overspreads the W/CNTRL Mediterranean although most areas remain capped. Cool conditions prevail over the rest of Europe with enhanced convection possible beneath cold mid-levels.

DISCUSSION

... SW-France ...

The pattern is characterized by a steady drop of pressure over the Bay of Biscay, W-France and N-Spain as a positive tilted trough draws near from the west. Global and mesoscale models agree well in timing but also in the development of an elongated and gradually strengthening surface depression over S/SE France in the evening/night hours. Placed under the left exit region of a mid/upper jet over the western Mediterranean, strong upper divergence and weak capping ought to promote scattered to widespread initiation, mainly over SW-France, starting already during the morning hours.

As far as instability goes, latest surface dewpoint analysis over SW-France exhibits 12-16°C with locally higher values but latest soundings show a thin, moist boundary layer with plenty of dry air above for some mixing. Right now, we therefore lean more to the ECMWF solution with dewpoints in the lower to mid tens instead of GFS, which had mid to upper tens in the past few runs. The instability forecast is less aggressive than GFS with roughly 400-800J/kg MLCAPE and locally higher peaks.

Strongest convective activity ought to be confined to the coldest mid-levels, so deep layer shear is in the range of 15m/s with stronger shear to the east and south.

A combination of a weakly capped airmass, strong upper divergence and a rapid cool down of the mid-levels support widespread and rapid thunderstorm development and a cluster of thunderstorms is forecast during the afternoon hours over SW-France, moving gradually northeastwards. Large hail, strong to severe wind gusts and an isolated tornado event (augmented LL CAPE) are likely mainly during the more discrete phase of the storms, but the main hazard will then shift to flash flooding beneath the cluster of storms. Thunderstorm activity diminishes during the night hours as instability vanishes, but heavy rain could affect SE-France well into the night hours (no level-1 for that threat due to the more stratiform nature next to a few, embedded thunderstorms).

... NE Spain ...

Strongest pressure fall is expected over NE-Spain as upper trough approaches from the west but also due to strong diabatic heating and GFS/ECMWF agree in the development of a moisture convergence zone, which crosses NE Spain during the late afternoon/evening hours from the west. The airmass is better capped compared to SW-France but stengthening LL convergence and the approaching upper trough from the west should trigger thunderstorms over NE Spain during the afternoon/evening hours. Steeper mid-level lapse rates and a moist boundary layer yield adequate instability release with peaks near 1000 J/kg MLCAPE . DLS is strong with 20-30m/s and large hail is a distinct possibility with those storms with isolated very large hail not ruled out. Another hazard will be the severe wind gust risk as storms, which evolve along the moisture convergence zone, could ingest well mixed and dry air (sampled with latest soundings of Madrid). LL CAPE release increases in the evening hours, so despite weak LL shear, an isolated tornado event can't be excluded. A level-2 may be needed in later outlooks, especially if more CAPE will be released than currently anticipated.

... Coastal areas of W-Italy , N-Italy and N-Adriatic Sea ...

After 00Z, the environment changes for the better and thunderstorm probabilities increase. The surface depression over S-France approaches during that time frame from the west and a stout mid-level PVA maximum crosses the region fromt he WSW, so plenty of forcing is present. Onshore, moisture recovery is on the progress but still too slim for widespread surface based activity and therefore we expect thunderstorms to be mainly elevated until 06Z, before diabatic heating starts. Enough MUCAPE is present in a strongly sheared environment for rotating storms (DLS 30-40m/s, SRH3 300m^2/s^2), so large hail and strong to severe wind gusts accompany those storms. Any surface based activity would be in a favorable environment also for LL mesocyclone development with SRH1 near 200m^2/s^2 and LLS up to 15m/s and therefore an augmented tornado risk would exist. This possibility will be evaluated in the upcoming model runs and an update may be needed.

It is doubtful that thunderstorms are able to evolve along the W-CNTRL coast of Italy as forcing is weaker and the low/mid troposphere still quite warm. We went with a blend of ECMWF and GFS/WRF and dismissed NGP due to the persistence of the first ones, but we will monitor the area.

We also included S-Switzerland into the level-1 as isolated, elevated thunderstorms are possible with an isolated large hail threat, given intense shear.

... NW/W-Spain, Portugal but also the Balkans ...

An isolated large hail event is possible as diurnal driven thunderstorm activity affects those areas, but shear is too weak for a more widespread risk.