Storm Forecast
Valid: Tue 03 Oct 2006 06:00 to Wed 04 Oct 2006 06:00 UTC
Issued: Tue 03 Oct 2006 11:21
Forecaster: VAN DER VELDE

SYNOPSIS

A large low pressure system affects Europe. A very baroclinic situation is present over France and the Alpine area, where a vigorous surface level trough with generally marginally unstable conditions under strong upper/mid level flow is forecast to stall over northern Italy, affecting much of the level 1 area.

DISCUSSION

...West side of Alps...

With marginal instability in place (morning SFLOCs/GFS MLCAPE of a few hundred J/kg),
a cold front and 500 hPa vorticity maximum pushes into the area during the afternoon and is the focus of severe convective developments. A strong shear environment with DLS of 20-30 m/s guarantees well-organized convection, a squall line possibly with supercells bearing large hail and severe gusts, while LLS of 10-20 m/s and low LCL heights (few hundred meters) support a chance of tornadoes. Locally a lot of rain may fall and cause flash floods.

...Southeast side of Alps...

Tricky situation, where most of the level 1 area is actually expected to be capped under warm air advection into the low. Very strong shear conditions are in place under the jetstream, so anything that develops can become a very severe storm. Have chosen to highlight the northeastern Italy area by a level 2, where GFS model expects persistent convective precipitation mostly after 18Z, apparently due to upslope flow.

As the depression enters the area rather late, 12Z northern Italy soundings are likely to be not yet affected by deep lift/potential instability release and may not be much unstable. However, models do not show a concentrated vorticity maximum for this area, making the primary trigger the strong southwestern flow against the mountain range.

Very strong kinematic environment creates potential for severe to extreme convective weather events. DLS soars at >25 m/s, with 0-3 km SREH progged by GFS in the range of 300-450 m2/s2, more than adequate for supercells with large/very large hail, while very low LCL heights along with 12-18 m/s LLS indicate strong potential for (violent) tornadoes. Severe outflow winds can accompany any storm with 20 m/s already present at 850 hPa.

...Denmark...

Low level instability increases later in the day and near-surface lapse rates are dry-super adiabatic, which under quiescent synoptic conditions with little wind potentially supports waterspouts. In the night a coastal front situation may develop (with inland clear skies)with enhanced convergence. Level 1 marks where 0-3 km CAPE is highest.

...Gulf of Biscay...

Another area of enhanced low-level instability along with superadiabatic near-surface lapse rates where chances of spouts seem to be enhanced.

...Balearic Islands to Sardegna...

Model convective precip signals develop in the second half of the night, especially north of the Balearic islands, with also good 10m wind convergence. Deep-layer shear and storm-relative helicity are strong enough for supercell storms. 06Z GFS scales down the precip chances again, but the area needs to be closely watched.