Storm Forecast
Valid: Fri 01 Nov 2013 06:00 to Sat 02 Nov 2013 06:00 UTC
Issued: Thu 31 Oct 2013 23:03
Forecaster: TUSCHY

A level 1 was issued for SE Sardinia mainly for a waterspout and excessive rainfall risk.

SYNOPSIS and DISCUSSION

Strong westerlies persist over most of Europe. A decaying upper low over the C-Mediterranean sparks a few slow moving storms with some potential for localized heavy rainfall amounts. Supportive conditions deteriorate during the overnight hours with a lowering severe risk. Expect a few training storms over far SW Italy/Sicily (probably ongoing from the night before), but too isolated for a level area. A confined level 1 was added for SE Sardinia mainly for a waterspout/excessive rainfall risk. Favorable placement beneath the mid-level vortex increased confidence in more widespread training thunderstorm activity.

A structuring warm front over the N-Netherlands and extreme NW Germany lifts north until noon. Reversed shear profiles along that front and some MUCAPE support repeated shower/isolated thunderstorm development with potential training activity. Heavy rain and strong wind gusts will be the main hazard although this risk spreads offshore betimes.

A shallow low/mid-tropospheric wave affects S-UK until 00Z and the S-North Sea thereafter. Despite a gradual decoupling of the tongue of subtropical air from the S/SW, strong differential temperature advection assists in a plume of MUCAPE, spreading east with that feature. Expect isolated to scattered storms - elevated in nature - to move east during the evening and overnight hours, mainly bound to the warm front/dry slot interface. Despite a more stable stratification of the lower troposphere, stronger downdrafts might bring 20 m/s H85 winds down to the surface, so strong to isolated severe wind gusts will be the main hazard.
Attendant cold front becomes W-E aligned over far NW Portugal/Spain, so onshore moving showers/thunderstorms will produce heavy to excessive rain. Effective PWs in the lower to mid 30s (mm) enhance the rainfall risk. The late onset of convectively enhanced precipiation (00Z onwards) kept us from issuing a level 1.

Numerous weaker mid-level impulses/low-tropospheric convergence zones/troughs cross S-Norway/Sweden and Finland from SW to NE during the day. An unseasonably moist LL air mass spreads east within a neutral stratified environment, so some low-end SBCAPE and 10-12 m/s LL shear ovcerlap ... especially over S/C-Sweden during peak heating. LCLs remain low with weak directional shear in the BL so an isolated tornado event is possible. The main risk however will be strong to isolated severe wind gusts. A decoupling of strongest shear and best CAPE precluded a level 1 area for now.

A stronger/more dynamic shallow warm core depression undergoes rapid intensification just west of Ireland at 21 Z onwards. Intense gradient flow along its southern side remains just west of Ireland. Of concern will be eastward spreading dry slot (00Z ), which could induce enhanced convection along the backside of the cold front/occlusion. Severe gusts due to enhanced downward impulse might be the result, but low-end MUCAPE kept confidence below a level-1 area. Back-bent occlusion with a 35 m/s 850 hPa streak remains just west of Ireland until 06Z.