Storm Forecast
Valid: Sun 17 May 2009 06:00 to Mon 18 May 2009 06:00 UTC
Issued: Sat 16 May 2009 18:43
Forecaster: TUSCHY
A level 1 was issued for E-France, Switzerland and parts of Germany mainly for severe wind gusts, tornadoes and large hail.
A level 1 was issued for N-France and Belgium mainly for severe wind gusts and large hail.
A level 1 was issued for Ukraine mainly for severe wind gusts and large hail.
SYNOPSIS
A broad vortex is placed over NW-Europe with some WAA downstream over central Europe. Another but much weaker upper trough is situated south of Greece and just slow forward propagation to the east is expected. NE-Europe gets affected by cool and stable air while hot and stable conditions prevail over the southern Iberian Peninsula. A SW-NE aligned surface boundary over SE-France and Germany serves as focus for thunderstorm initiation as it moves constantly to the east.
DISCUSSION
... E-France, Switzerland and parts of Germany ...
The forecast area is situated downstream of an extensive cyclonic circulation and just along the upstream flank of a slowly eastward moving ridge. The main upper forcing arrives late in the evening/night over Belgium/Netherlands and NW-Germany and is therefore well displaced to the west of the energetic airmass to its south/southeast. AFWA-WRF, GFS and also ECWMF have some PVA signals over S/SE-Germany during the evening and night hours, which comes in from E-France and moves off to the northeast with some strengthening trends but it is dubious how big the role of any convective feedback problem in this strengthening trend is. Nevertheless, streamlines curve more cyclonic and upper divergence increases, so initiation looks reasonable even without a clear forcing mechanism. At lower levels a SW-NE aligned convergence zone is situated over W/NW-Germany during the day and increases its forward speed to the east during the evening/night hours as aforementioned vorticity lobe arrives from Belgium/Netherlands. This convergence zone/cold front crosses Germany during the night hours and serves as focus for thunderstorm initiation.
GFS/ECMWF were not far-off with their dewpoint forecasts yesterday and we decided to go with a compromise of both models for today's forecast. Strong moisture advection and some evapotranspiration will indeed push surface dewpoints to well above 10°C but it likely will just be a thin surface layer with dry air above and mixing should keep dewpoints below those more extreme GFS peaks. Strong WAA at lower levels and some cooling above increases mid-level lapse rates during the day, so MCLAPE around 500 J/kg or more will likely evolve over S/SE-Germany during the day with decreasing values to the west and north.
Shear is moderate with 0-6km bulk shear of 10-15m/s over SE/S-Germany and 20-25m/s over SW/CNTRL/N Germany. 3km speed shear near 20m/s is noteworthy, too. Directional shear remains weak during the day with some concerns arising over E-Germany, where LL wind field could back somewhat if a weak surface depression evolves. Another area of concern is the tip of the moisture tongue, situated over N-Germany during the late morning to afternoon hours. GFS has convective precipitation in this area with strong LLS/DLS so moisture advection to the north has to be monitored closely.
Thunderstorms evolve quite early along the convergence zone over NW/W-Germany and move rapidly to the NE. If 12Z GFS is on the right track regarding strength of the moisture advection, the environment would be favorable for severe thunderstorms with an attendant tornado, severe wind gust and large hail risk all the way to N/NE-Germany mainly between 12-18Z. Further to the south, thunderstorms also increase rapidly over Switzerland and E-France and a cluster of storms moves NE-wards during the evening/night hours. Forecast soundings in this environment show a well mixed layer up to 700hPa and dry air, which could cause a stronger cold pool formation and an MCS development over south CNTRL/E-Germany with an attendant severe wind gust/large hail threat. More discrete storms over S/SE and E-Germany could produce very large hail, too. During the night, thunderstorms decrease from the SW but keep going over E-/NE-Germany until 06Z. Excessive rainfall is likely with those thunderstorms, especially with the night-time thunderstorm cluster. However, no pronounced LLJ and drier air in the deep warm cloud layer don't hint on any more significant/widespread flash flood event.
Parts of that large level-1 may see an upgrade, namely a swath from roughly Nuremberg/Aachen to Hamburg/Berlin, but we first want to see if new model runs/synop data support such a robust moisture tongue. In fact, if GFS is on the right track, a fairly widespread severe thunderstorm outbreak is possible.
... Italy, Alps, the Balkans and SE-Europe ...
Diurnal driven thunderstorms produce isolated large hail and strong to severe wind gusts but coverage remains below a level-1 threshold. The most robust large hail risk arises over Bulgaria, Macedonia and N-Albania, where best instability release is expected.
... Ireland, UK and Scotland ...
Placed under the base of the extensive vortex, daytime driven thunderstorms are possible over a wide area. However, we just highlighted those regions, where thunderstorms are most likely due to coldest mid-levels, convergence signals and surface heating. A short-wave crosses SE-UK during the afternoon hours with better surface moisture and cool air aloft. Despite models showing now CAPE, forecast soundings indicate that not much modification is needed for initiation in a strongly sheared environment. If initiation occurs, an isolated severe wind gust/tornado will be possible. Onshore convection decreases after sunset while offshore convection continues well into the night.
... Extreme N-France and parts of Belgium ...
A short time frame for organized thunderstorm arises over that region from about 14-19Z. Boundary layer moisture remains satisfying as mid-levels cool down rapidly. Some low-end instability is forecast and ECWMF/GFS showed initiation for the past few runs. Deep convection is possible and DLS near 20m/s combined with 15-20m/s at 850hPa point to a severe wind gust/isolated large hail threat. Thunderstorms decay after sunset.
... Ukraine ...
An eastward sliding cold front serves as focus for scattered thunderstorms during the day. Mid-level jet arrives from the west with winds increase to 25m/s at 500hPa and so does the DLS, which augments to near 20m/s. The jet chops the front in a right angle and a forward propagating MCS likely evolves with an attendant severe wind gust and isolated large hail risk. During the evening hours, this MCS will exit the forecast area to the east. As the LLJ remains quite weak during the forecast period, no widespread wind damage is expected, which would require a level-2, but degree of moisture and instability but also sounding reports are monitored for a more serious wind damage risk.
