Storm Forecast
Valid: Fri 03 Jul 2009 06:00 to Sat 04 Jul 2009 06:00 UTC
Issued: Thu 02 Jul 2009 11:15
Forecaster: TUSCHY
A level 1 was issued for E-France, E-UK, Belgium and Luxembourg mainly for excessive rainfall.
A level 1 was issued for E-Germany, parts of the Czech Republic and N-Austria mainly for excessive rainfall.
A level 1 was issued for the Pyrenees and surrounding areas mainly for large hail.
A level 1 was issued for parts of Italy mainly for large hail and excessive rainfall.
A level 1 was issued for parts of Sweden mainly for large hail and strong to severe wind gusts.
SYNOPSIS
Warm and humid conditions prevail over central/southern Europe with widespread thunderstorm activity anticipated. An approaching upper low from the E-Atlantic pushes a weak frontal system well inland, but this front decays somewhere over E-France/W-Germany during the night hours. CAA over NE-Europe suppresses deep convection.
DISCUSSION
... E-CNTRL UK, E/NE-France, extreme W-Germany, Luxembourg, Belgium and parts of the Netherlands ...
Words like 'forcing', 'front' and 'dynamic' can finally be involved again in the forecast. A constantly weakening cold front from W-France draws near during the afternoon hours and probably dissolves somewhere over the highlighted area. Shear at all levels remains weak although a marginal increase in speed shear is possible at mid-levels. The main advantage with such a feature is that moisture pooling along the convergence zone will assist in a concentrated swath of 800-1500J/kg MLCAPE as mid-level lapse rates steepen somewhat. Excessive rain due to slow storm movement will be the main hazard, but an isolated large hail report can't be ruled out, given better instability release and a slim increase in wind speeds throughout the troposphere. However the main effect of this set-up will be the longevity of thunderstorms mainly over Belgium, the Netherlands, Luxembourg and W-Germany, which will continue well into the night, probably until next morning. Isolated tornadoes are possible during the peak time heating with GFS showing LL CAPE of roughly 200J/kg.
In general the same for E-/CNTRL-UK, where stronger shear ought to offset weaker buoyancy, so multicell storms are possible. Isolated flash flooding and large hail are the main hazard. Prolonged period of divergence at high levels, weak surface 'trough' and the slow eastward progression of the surface front could locally yield a favorable environment for isolated spots to see repeated thunderstorm development both in the prefrontal airmass and during the passage of the front, which could locally increase flash flood risks.
... SE-Germany, parts of the Czech Republic and parts of Austria ...
Boundary layer moisture increases as weak wind shift zones approaches from the NE. MLCAPE in excess of 1kJ/kg is likely with very slow storm motions, so excessive rain is the main hazard. If thunderstorms are able to line up due to local cold pool development, a loosely organized line of storms could reach N-Austria, where the local topography could again increase the flash flood risk.
... Hungary , the Balkans and parts of Italy ...
No change to the past few days. Rich moisture in the boundary layer, moist profiles and very weak DLS cause slow moving storms, which are again prolific rain producers. An isolated flash flood risk exists but no level area is included for now.
The same for parts of Italy although slighly better shear and steeper lapse rates increase the large hail risk. Due to the combined hail/flash flood hazard, a level-1 may be warranted.
... Pyrenees and surrounding areas ...
The tip of a northward sprawling EML affects the Pyrenees where the appraoch of a weakening cold front from the S-Bay of Biscay ought to keep BL moisture high enough for robust MLCAPE release, 1KJ/kg or higher. Both speed and directional shear (20m/s and up to 200m^2/s^2 SRH respectively) foster a severe hail risk. Given magnitude of shear and instability but also the more isolated convective precipitation signals in the models the possibility increases that even an isolated extreme hail event can occur. No level-2 was issued due to the limited coverage, but an update/upgrade may be performed later-on.
... S-Sweden ...
Despite the constant detachment between instability/shear over most parts of Sweden, 10-15m/s DLS and roughly 500J/kg MLCAPE are enough for multicell storms with an isolated large hail/severe wind gust threat. The latter one due to 15m/s at the lowest 3km. Augmented LL shear is restricted to the tip of the instability tongue (over the Baltic Sea), where only low-end MUCAPE is available, so tornado risk should be low. However, latest runs persistently evolved a CAPE maximum just north/northeast of Stockholm and if this verifies, an isolated tornado event due to better LL shear is possible. A level-1 was issued for the hail/wind gust risk.
