Storm Forecast
Valid: Fri 01 May 2009 06:00 to Sat 02 May 2009 06:00 UTC
Issued: Thu 30 Apr 2009 16:59
Forecaster: TUSCHY
A level 1 was issued for parts of Slovenia, Croatia, Bosnia and Herzegovina, Serbia, Montenegro, Macedonia, parts of central Romania and NE Italy mainly for large hail and isolated severe wind gusts.
A level 1 was issued for parts of Austria and parts of Germany mainly for large hail.
A level 1 was issued for parts of NE Algeria and extreme N-Tunisia mainly for excessive rainfall.
SYNOPSIS
The pattern over Europe features a strong ridge over NW-Europe and an upper low over SE-Europe. In between those systems, cold air at mid-levels advects southwestwards, augmenting the thunderstorm risk over most parts of central Europe. Another outbreak of frigid air over extreme NE-Europe suppresses deep convection for at least the following 24h.
DISCUSSION
... Parts of Slovenia, Croatia, Bosnia and Herzegovina , Serbia, Montenegro, Macedonia, parts of central Romania and NE Italy ...
Latest soundings from Slovakia and Ukraine sample the airmass downstream of those areas well with very steep lapse rates to near 500hPa. GFS sticks to its solution for the past few runs that 2-4km lapse rates of 8-9K/km overspread those regions from the NE already during the morning hours with slightly lower values over Romania. ECMWF and GFS both forecast dewpoints at the surface running from 9-12°C in a concentrated swath just along the Dianaric Alps and those values look reasonable compared to current surface analysis. Strong insolation will push temperatures to well above 20°C, so beside weak capping/the nocturnal inversion, initiation will start during the late morning hours/noon.
Aforementioned environment results in MLCAPE release of 500 - 1500 J/kg. Steep lapse rates, combined with 10-15m/s DLS and some directional shear all point to a large hail risk. This risk is the highest during the early life time when storms are still more discrete. Rising LCLs during the afternoon hours and a dry subcloud layer also augment the risk of severe downbursts.
Betimes, thunderstorms cluster over the mountains and the main hazard then shifts to heavy rain and local flash flooding. Thunderstorms weaken after sunset.
The same over Italy but either shear or instability will be more conditional and hence isolated large hail will be the main risk.
... Parts of Austria and parts of Germany ...
A tongue of rich 0-1km mixed Theta-e runs from W-Germany southeastwards all the way to E-Austria. Furthermore, a pronounced wind shift zone from E-Germany and W-Czech Republic continues southwestwards with 0-2km deep convergence signals present mainly over SE-Germany and extreme NW-Austria. ECMWF remains more reluctant regarding surface dewpoint with GFS running a couple of degrees higher. We lean to the more agressive GFS solution due to a moist soil and strong evaporation.
As mid-levels stay cold, up to .5-1 KJ/kg MLCAPE are likely to evolve with highest values just along the SW-ward shifting convergence zone. Shear is weak at best, but nevertheless good instability release in the hail growth zone and steep lapse rates are available, so large hail will be possible, mainly over SE-/S-Germany and the level-1 area of Austria. Thunderstorms start to cluster betimes and predominantly northerly winds ought to push the storms slowly to the south/southwest.
Winds at all levels become very weak/variable over SW/W-Germany/E-France,Belgium and Luxembourg during noon/afternoon hours. Again, 500-1000 J/kg MLCAPE are forecast and mainly during the early/discrete thunderstorm stage, large hail is likely. However, thunderstorms tend to cluster rapidly in time and the environment is prime for locally excessive rain amounts...even training over the same spot can occur.
After sunset, thunderstorms decrease rapidly apart from SW/S-Germany, where a plume of strong MUCAPE persists well into the night. Winds back during the night and increase in strength, so the heavy rain risk increases along the N-Alps of W-Austria as cluster of showers/thunderstorms move to the south.
... NE-Algeria and extreme N-Tunisia ...
As strong high level streak curves in from the NW during the day, divergence at upper levels overspreads the level-1 area. Moist onshore flow persists during the forecast with cold mid-levels above that airmass. A cluster of showers/thunderstorms is forecast by GFS and AFWA-MM5 for the past few runs with intense rainfall amounts. A level-1 was issued, covering the region, where highest rainfall amounts will occur. Thunderstorms decrease during the night hours, but the precipitation keeps going well into the night.
... NW-Turkey /NE-Greece and parts of Bulgaria ...
An eastward sliding upper trough affects those regions during the day and widespread showers/thunderstorms are possible in a weakly capped airmass. Global models evolve small clusters with high precipitation rates but run-to-run consistency of those models is bad. There is a chance for local flash flooding beneath those storm clusters, especially over NW-Turkey and NE-Greece, but confidence is too low for issuing a level-1.
