Storm Forecast
Valid: Sun 18 Sep 2011 06:00 to Mon 19 Sep 2011 06:00 UTC
Issued: Sat 17 Sep 2011 21:46
Forecaster: SCHLENCZEK
A level 2 was issued for NE Italy, Slovenia and W Croatia mainly for widespread excessive precipitation and large hail and to a lesser extent for severe wind gusts and tornadoes.
A level 2 was issued for NW-central Italy mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.
A level 1 was issued for large portions of the W and central Mediterranean mainly for large hail and severe wind gusts and to a lesser extent for excessive precipitation and tornadoes.
A level 1 was issued for the eastern Alpine region and Dinaric Mountains mainly for excessive precipitation and large hail.
A level 1 was issued for W Hungary, parts of Austria, E Czech Rep., Slovak Rep. and S Poland mainly for large hail and severe wind gusts.
A level 1 was issued for the coastal regions of Belgium, the Netherlands and NW Germany mainly for excessive precipitation and tornadoes.
SYNOPSIS
An upper trough, initially centered over the western North Sea, affects western and central Europe on Sunday. Its southern tip will cross the Alpine region during the afternoon which will lead to excessive convective rainfall in N Italy and other countries along the Adriatic coast. Primarily, Slovena and Croatia should have the highest potential for widespread excessive rainfall.
DISCUSSION
...NE Italy, Slovenia, W Croatia...
WAA ahead of the approaching upper low and high BL moisture lead to moderate instability with MLCAPE around 1000 J/kg over the area of interest. Deep layer shear will increase during the day with maximum values between 20 and 25 m/s. Multicells and / or large clusters / MCSes are likely to be the dominant convective mode at this setup. Large hail and severe wind gusts may occur in the whole level 2 region and also in the surrounding level 1 region, but the main threat should be excessive precipitaton, especially in the mountains.
WRF shows strong veering in the lowest 1.5 km during the late afternoon / evening hours over NE Italy and nearby regions ahead of the trough which should favour the development of supercells and a subsequent tornado risk for this area.
The strong southeasterly / southerly LL flow ahead of the developing surface low should lead to an intensification of convective precipitation especially over Slovenia during the afternoon / evening hours.
...Mediterranean region...
Similar conditions as described for the other LVL2 region are also predicted for the Mediterranean region, but CIN is somewhat higher which should favour isolated storms. Any storm that initiates in this environment should be capable of producing large hail and / or severe wind gusts. The northwestern and western parts of central Italy may additionally get very intense convective precipitation. As the cold front moves across the Mediterranean, the focus of activity moves to the southeast, reaching extreme S Italy and Sicily on Monday morning.
...parts of Austria, Slovak and Czech Rep., Hungary, Poland...
Even though CAPE is limited to a few hundred J/kg, moderate values of 0-6 km deep layer shear should lead to some severe multicellular systems with large hail and severe wind gusts as main threat. Peak activity should be expected in the late evening / night hours.
...North Sea region, coastal area of the Netherlands, NW Germany and Belgium...
As the upper cold core of the low pressure system moves across the rather warm North Sea, low LCL / LFC heights and steep lapse rates at lower and mid levels are expected. Showers and thunderstorms, which evolve in this environment, may easily produce a funnel / short-lived tornado. Weak winds at upper and mid levels and high values of effective PWAT may also highlight a threat of excessive precipitation for the coastal regions.
