Storm Forecast
Valid: Fri 19 Sep 2008 06:00 to Sat 20 Sep 2008 06:00 UTC
Issued: Thu 18 Sep 2008 18:01
Forecaster: TUSCHY

SYNOPSIS

Ridging persists over N-/NW-Europe with mid-/upper ridging supported by strong WAA of strengthening Greenland/E-Canada polar vortex and surface high pressure being the result of the southward moving cold continental airmass over N/CNTRL Europe. Weak pressure gradients prevail , which sharpen somewhat over E/SE Europe next to a westward building upper low. Another disturbance is placed west of Portugal with intense WAA downstream of this feature. Weakening/southward moving mid-level trough enters the central Mediterranean and focus for scattered thunderstorms shifts to this feature.

Upper lows, which meander for days W/SW of Portugal, become more interesting for any sub/tropical transition betimes. Latest WV loops do not show any sign of organisation for any of the two upper lows ( ~ 22°N/40°W and ~ 30°N/20°W). The latter one should be already in its shearing stage, but attendant well evolved warm conveyor belt continues to pump warm/moist air northwards, so mid-level lapse rates over Portugal/Spain continue to weaken during the forecast period.

DISCUSSION

... W-CNTRL Mediterranean ...

Weakening mid-level trough with a still pronounced thermal trough at least at 500hPa cross the area from the NNW from the midday hours onwards. Latest MCSST scan measured SSTs in the range of 20-23°C over the Tyrrhenian Sea, decreasing to well below 15°C south of France. As mid-level trough crosses the area, rapid destabilization of the atmosphere should occur with widespread 500 J/kg MLCAPE release. Despite not much cool-down at lowest 2km, temperature gradient between the sea and the lower atmosphere should acquire enough strength for robust LL CAPE release and hence the chance for waterspouts was introduced in the outlook, mainly over the Tyrrhenian Sea/south of Italy, where SST increases constantly. Background shear is not that weak with 15m/s, which could be a more negative factor regarding spout evolution.

Approaching the shear zone west of Sardinia, conditions could become more favorable for organized convection but the bulk of activity should stay east of this zone. A relative coarse level area was issued due to the combination of organized convection mainly in the western/southern part of the level area (isolated tornado/large hail and severe wind gust risk) and the increasing chance for waterspouts further to the east.