Storm Forecast
Valid: Mon 12 Aug 2013 06:00 to Tue 13 Aug 2013 06:00 UTC
Issued: Sun 11 Aug 2013 22:59
Forecaster: TUSCHY
A level 1 was issued for N-Spain mainly for an isolated large hail and strong to severe downburst event.
Two level 1 areas were issued for E/SE France, far NW Italy, parts of Switzerland and W-Austria mainly for an isolated large hail event.
A level 1 was issued for parts of W-Russia mainly for large hail.
SYNOPSIS
An extensive cyclonic vortex is placed over Norway and Sweden. Numerous smaller-scale impulses circle around that feature and cross parts of N/C Europe. Further south over SE Europe and the Mediterranean, numerous weak upper level troughs or cold-core lows affect a broad area with a regionally enhanced shower and thunderstorm activity.
DISCUSSION
... N-Spain, parts of E/SE France, SW Germany, parts of Switzerland and W-Austria ...
Numerous short-waves affect the area of interest over France and the W-Alps. Those features and a SE-ward sliding cold front insert enough forcing for isolated to scattered thunderstorm initiation during the afternoon and evening hours. Some better prefrontal moisture and mid-level lapse rates around 7K/km result in moderate MLCAPE values on the order of 400 to 800 J/kg. As a 25 m/s mid-level speed maximum approaches from the NW, DLS increases to aoa 20 m/s, which is enough for a few organized multicell events. Large hail and strong to isolated severe wind gusts will be the main risks. CAPE rapidly diminishes after sunset and so does the severe risk. We extended the 15-% lightning area far to the NE to include isolated nocturnal activity with a NE-ward moving MUCAPE plume. Nothing severe is expected with that activity. Those will be low-end level 1 areas and only an isolated severe risk is anticipated.
Over N-Spain, another weak impulse crosses the Pyrenées during the afternoon/evening hours from west to east. DLS will be weaker (10 to 15 m/s range), but MLCAPE probably exceeds 1000 J/kg. A few large hail and strong to severe downburst events are possible. Upscale growth into a few loosely organized nocturnal thunderstorm clusters is possible as they move E/SE into the more unstable air mass. Heavy rain, marginal hail and gusty winds will be the risk.
... Rest of the lightning areas ...
Daytime driven and non-severe thunderstorms are forecast beneath the base of the extensive upper level trough. Marginal hail and gusty winds accompany those storms, which gradually decay after sunset. Only offshore convection keeps going all night long. A few cold-core funnels/isolated tornadoes are possible with enhanced LL CAPE profiles.
Over S/C Sweden, SE Norway and S-Finland, SBCAPE probably approaches 800 J/kg during the afternoon hours. A few strong to severe pulse storms may occur with isolated large hail and heavy rain. The risk however remains too limited for any level area.
... Parts of W-Russia ...
A slowly eastward moving cold front serves as focus for scattered CI. Rich prefrontal moisture results in 500 to 1000 J/kg MLCAPE. Overlap with 15-20 m/s DLS exists, so numours organized multicells/isolated supercells with large hail and strong to isolated severe wind gusts are forecasts. This activity gradually exits the forecast area during the afternoon/evening hours.
