Storm Forecast
Valid: Sun 07 Oct 2018 06:00 to Mon 08 Oct 2018 06:00 UTC
Issued: Sat 06 Oct 2018 18:34
Forecaster: TUSCHY

A level 1 was issued across parts of the W-Mediterranean mainly for excessive rain and an isolated tornado/large hail and severe wind gust threat.

SYNOPSIS

Slightly disturbed high-index pattern continues over N-Europe. A deepening cyclone over the Baltic States pushes east ahead of an amplifying ridge over Norway/Sweden. Despite an evolving warm sector in a strongly foreced and dynamic environment, mixed out BL moisture beneath weak mid-level lapse rates preclude any probabilities for organized DMC activity.
Otherwise a decaying upper low over N-Italy merges with a southbound digging cyclonic vortex over S-France and forms a zonally aligned region with low thickness and rather cool mid-levels. No surprise that offshore regions and coastal areas with an enhanced vertical temperature gradient remain the main focus for thunderstorm activity. The French vortex weakens during the forecast and so does the mid/upper-level wind field. Attendant LL depression - although weakening - pushes a cold front to the SE over the W Mediterranean, which stalls just west of Corsica/Sardegna during the forecast.

Elsewhere, quiet convective conditions prevail.

DISCUSSION

... W/CNTRL Mediterranean and surrounding highlighted areas ...


Over the CNTRL Mediterranean, weak shear/instability overlap with diffuse forcing and hence scattered areas with augmented thunderstorm chances exist. Slow moving thunderstorms pose an heavy rainfall threat but the general risk remains too spotty for any level area. The region to the south (S-Italy) becomes more stable during the day with weak ridging aloft.

The W-Mediterranean becomes the main focus for more organized thunderstorms as the weakening vortex approaches from NW. In general a bad overlap of ingredients is forecast, excluding a confined corridor along the SE-ward sliding cold front (which slows down during the day and gets caught up by the French vortex with better shear). 20 m/s DLS and 1 kJ/kg MLCAPE overlap with enhanced LL shear (speed and directional). The Balearic Islands and offshore areas further east could see an isolated supercell with all kind of hazards including a tornado and severe wind gust threat (20-25 m/s 0-3 km shear and enhanced LL directional shear). This riks seems to be maximized during the noon/afternoon period as enhanced synoptic-scale forcing approaches from the W/NW. Due to the isolated nature only a level 1 area was issued.

This level 1 was expanded E to Corsica and Sardegna, where the cold front stalls during the day. Resulting repeated thunderstorm development could impact the same area with an augmented flash flood risk. However drier air at 500 hPa and skinny CAPE profiles combined with weak 850 hPa flow should keep this threat in check.

Later during the night, scattered thunderstorms continue to fire S of the Balearic Islands although models diverge with the extent of thunderstorm coverage. DLS in the 20 m/s range and 500-1000 J/kg SBCAPE continue to pose a risk of organized multicells with isolated large hail and strong wind gusts. Hence this area was added to that broad level 1.