Storm Forecast
Valid: Fri 23 Jul 2010 06:00 to Sat 24 Jul 2010 06:00 UTC
Issued: Thu 22 Jul 2010 23:44
Forecaster: TUSCHY
A level 2 was issued for N-Italy and parts of Slovenia mainly for large hail (a few significant events possible), severe wind gusts and tornadoes (a strong one possible).
A level 1 surrounds the level 2 mainly for isolated large hail, severe wind gusts and excessive rainfall.
A level 1 was issued for Lithuania, Latvia and Estonia mainly for tornadoes, severe wind gusts and large hail.
A level 1 was added for France and Benelux mainly for an isolated funnel/tornado report and large hail event.
SYNOPSIS
A quasi-stationary upper trough remains in place over W-Europe. This feature results in unsettled conditions over a vast area with widespread showers/thunderstorms. Quiescent conditions are forecast for the rest of Europe.
DISCUSSION
Highly meridional aligned upper trough over France remains in place during the forecast period with no significant change in strength and position. However, its tilt modifies during the forecast from 'neutral' to slightly 'negative'. This scenario helps to push a surface front gradually to the south/east along the Alps. Various different thunderstorm modi are expected over central Europe, as described below:
... France and Benelux...
Placed beneath cold mid-/upper thermal trough (500 hPa temperatures drop to at or below -20°C) with moist BL present, so moderate SBCAPE build-up is forecast with diurnal cycle. Shear beneath the trough axis is weak, so lowest 3km will be the focus for storm organisation. BL is moist, weakly capped with good diabatic heating forecast, so modest 0-3 km CAPE and low LCLs point to an augmented funnel/tornado risk. Low WBZ heights also indicate an isolated large hail risk. This hazard diminishes after sunset.
... N-Italy and Slovenia...
The stage is set for numerous organized multicells/supercells, as upper trough and SE-ward moving surface boundary result in abundant lift for scattered initiation around noon onwards. Overlap of 1500 J/kg MLCAPE and 20-25 m/s 0-6km bulk shear next to enhanced directional shear result in well organized mid-level rotation within long-lived updrafts. Hence, a large hail risk exists, a few significant events well possible. Also, strong to severe wind gusts accompany those thunderstorms. No real hint on any enhanced tornado risk during the early afternoon hours, although an isolated event will be possible over NE-Italy next to the N/NE Adriatic Sea, where BL moisture content remains very high. LL shear really ramps up during the evening hours with an increasing tornado risk mainly over NE Italy and Slovenia and we would not exclude a strong tornado event along the NE coast of the Adriatic Sea, where SRH1 values peak out. Thunderstorms keep going during the night, while affecting Slovenia and Croatia....probably as far east as Hungary.
...Austria, S-Germany and the Czech Republic ...
Ambiguous signals are present in model pool, especially for Austria. Overall, environmental set-up will be supportive for scattered to widespread shower/thunderstorm initiation beneath highly diffluent upper streamline pattern. Sheared flow (northerly component in lowest levels with strong southerly/southwesterly jet atop) will result in numerous more or less organized clusters of showers and thunderstorms over S-Germany, moving to the east/northeast (probably more or less stratiform in nature). As storms tap into better BL moisture over Austria, a better organized cluster of thunderstorms may evolve, affecting especially E-Austria during the afternoon/evening hours with strong to severe wind gusts and isolated large hail. Further north, locally excessive rainfall affects parts of SE-Germany and the Czech Republic during the evening/night hours.
Mesoscale model data indicate rapid pressure rise over S-Germany behind eastward moving surface cold front, which could be a signal of a rapidly eastward racing MCS over extreme SE-Germany into N/NE Austria. 18Z run supports this idea with suspicious signals like a strong pressure nose just north of the Alps, an organized convective system in mesoscale model data, a tight thermal gradient along the eastward moving cold front and an unstable prefrontal air mass. We want to wait for confirmation in next model runs, but if this scenario verifies, an upgrade will become necessary due to a more widespread severe wind gusts risk.
... From Poland to Estonia ...
A developing wave/depression over Lithuania to Estonia will be a nearly quasi-stationary feature during the forecast period. Shear along the occlusion/warm front will be strong with favorable environmental conditions for low-/mid-level mesocyclones. Any thunderstorm, which moves along those boundaries will gain rapid organisation with an attendant tornado, large hail and severe wind gust risk. An high-end level 1 was introduced du to the uncertainty, how widespread thunderstorm initiation will be. Further to the east in the warm sector , a few multicells are forecast with strong wind gusts and large hail. Betimes, numerous MCSes evolve over Poland, the Baltic Sea and S-Finland with a strong wind gust/locally excessive rainfall risk.
... Spain ...
Only a few QPF signals are forecast, as locally better BL moisture advects inland mainly along the E-coast of Spain. Not much forcing is present, but mid-levels cool down slightly, so not much lift needed for a few thunderstorms (probably enhanced by regional topographic effects). DLS of more than 20 m/s and steep mid-level lapse rates will be available for storm organisation of any thunderstorm, which manages to evolve. Large hail will be the main hazard. If an isolated thunderstorm develops along the E/SE coast of Spain, even a significant hail event can't be ruled out. An update will be issued, if initiation looks more probable. 18Z run of GFS now has weak QPF signals over E/SE-Spain, which has to be monitored closely.
