Forecast Update
Valid: Fri 30 Apr 2010 11:00 to Sat 01 May 2010 06:00 UTC
Issued: Fri 30 Apr 2010 10:41
Forecaster: TUSCHY
A level 1 was issued for Morocco, extreme NW Algeria and most parts of Spain mainly for large hail and strong wind gusts.
A level 1 was issued for extreme SW-France mainly for isolated excessive rainfall.
A level 1 was issued for extreme SE-Germany and NW-Austria mainly for isolated large hail.
SYNOPSIS
Please refer to the outlook, issued at Thu 29 Apr 2010 18:20 UTC.
DISCUSSION
... Scotland ...
Upgraded parts of the low-prob. thunderstorm area to a 50% area, to reflect increasing confidence in more widespread thunderstorm initiation, probably building southwards during the late afternoon hours. DLS increases over N-UK to near 20m/s, mainly speed shear. This may cause an isolated large hail report with better organized storms, as WBZ lowers. Positive tilt of the upper trough precludes better and more robust instability build-up and therefore no level 1 was introduced due to the limited coverage of anticipated hail reports. Thunderstorms gradually decay after sunset.
... SE-Germany, parts of Austria and E-Czech Republic ...
Current scenario was more or less an outlier yesterday but more and more model runs now confirm this trend, so confidence increased adequately for upgrading to a 50-% thunderstorm area. GFS/WRF and BOLAM are the more aggressive ones, indicating rapid development along and south of a cold front over SE-Germany, but also along the Alps itself. Latest synop data reveals rapid BL modification with dewpoints now at or above 10 °C beneath somewhat steeper lapse rates. This constellation often assists in thunderstorm development during the early afternoon hours along the Austria/Switzerland border with a movement to the northeast. Shear is low to modest with 10-15 m/s throughout the lowest 6km with a negligible directional shear component. Therefore, upscale growth into one or more clusters is forecast over SE-Germany, moving east/northeastwards thereafter. The main risk will be strong wind gusts, marginal hail and locally heavy rainfall amounts. Similar events in the past proved to produce a good chance of better organized storms over extreme SE-Germany despite marginal shear and weak forcing. Given abundant CAPE support for this scenario, we went ahead and introduced a level 1 for large hail, despite the more isolated nature of this hazard.
Cluster of showers/thunderstorms tend to weaken over NE-Austria/S-Czech Republic during the night, as thermodynamics deteriorate.
... Spain and Morocco ...
The main reason for updating was the significant expansion of the level 1 to the east. Despite decreasing confidence for initiation over SE Spain and still good model spread in QPF forecasts, we went ahead and upgraded all of SE/E-Spain to a level 1. GFS indicates isolated initiation over that area with steep mid-level lapse rates and 15-20m/s DLS. Confidence is high for hail reports in the overlap of level 1 and 50-% thunderstorm areas, with more isolated events forecast in-between (e.g. SE-Spain). During the night, storms move offshore.
