Storm Forecast
Valid: Sat 10 Oct 2009 06:00 to Sun 11 Oct 2009 06:00 UTC
Issued: Sat 10 Oct 2009 05:44
Forecaster: SCHLENCZEK
A level 1 was issued for N Algeria and N Tunisia mainly for large hail and to a lesser extent for tornadoes.
A level 1 was issued for large parts of the Tyrrhenian Sea and central Italy mainly for excessive precipitation and waterspouts and to a lesser extent for large hail.
A level 1 was issued for the Balkans and SW Hungary mainly for large hail and excessive precipitation, and to a lesser extent for spouts.
SYNOPSIS
A large upper trough over central and southern Europe will be the focus for convective activity on Saturday. In a region with cool mid levels and quite high SSTs over the central Mediterranean, moderate instability will remain. Most convection should be associated with a low pressure system over NW Italy, moving southeastward during the period. Further upstream, another low pressure system over the North Sea will cross the Benelux countries and W-central Germany during the afternoon while weakening. In the vicinity of the upper shortwave trough, some low-end instability is forecast in a moderately warm and moist airmass. Other low pressure systems are located near Iceland and over NW Russia. Stable conditions are expected over E Europe and also over Iberia and SW France where high pressure prevails.
DISCUSSION
...Benelux countries, W-central Germany...
In the vicinity of a low pressure system, a few hundred J/kg CAPE will form during the day. Deep layer shear should stay between 10 and 15 m/s which will favor briefly organized multicell storms with small hail and gusty winds. Lightning activity should be rather low. Most CAPE should vanish in the late evening hours and thunderstorm activity will decrease rapidly.
...N Algeria / Tunisia...
Near the tip of an upper shortwave trough over the W Mediterranean, MLCAPEs in order of 1 - 1,5 J/kg are in place. During the afternoon, deep layer shear will increase to values around 15 - 20 m/s. Organized multicells and a few supercells are expected to form as SRH3 is slightly enhanced (150 - 200 mē/sē). The main threat should be large hail but a few tornadoes may also occur. The threat of severe weather will decrease in the evening hours when shear / SRH weakens.
...Tyrrhenian Sea, central Italy...
Ahead of the upper low, GFS predicts MLCAPE values around 1 kJ/kg over central Italy and the N Balkans. The airmass is weakly capped and an upper shortwave trough near Sardinia should provide enough lift for thunderstorm development as it moves eastward. Although deep layer shear should be less than 10 m/s, some thunderstorms may become severe. Rather weak winds at lower and mid levels will create good conditions for slowly moving multicells and clusters with a heavy rain / flash flood threat. Like on Friday, low level lapse rates should be close to 10 K/km and 0-3 km CAPE is forecast well above 100 J/kg between Corsica and Italy. This may allow some waterspouts between the Balearic Islands and central Italy as the wind field is relatively weak. Some isolated large hail may also be possible with the strongest cells.
...Hungary, Balkans...
Another shortwave trough affects the Adriatic Sea and the N Balkans during the afternoon. A relatively strong cap indicated by CIN around -200 J/kg will weaken during the afternoon and convective initiation is likely then. MLCAPE near 1 kJ/kg are in place and deep layer shear will increase in the afternoon hours to around 15 m/s. Most of the storms will be multicells but a few supercells are not ruled out. The strongest storms - especially supercells - may produce large hail and as some storms may persist in a region for some hours, excessive rainfall with flash floods is expected. Over the Adriatic Sea, waterspouts may form in a region with rather weak low level winds.
