Storm Forecast
Valid: Wed 29 Apr 2009 06:00 to Thu 30 Apr 2009 06:00 UTC
Issued: Tue 28 Apr 2009 23:13
Forecaster: PUCIK

SYNOPSIS

An interesting pattern shapes up across Europe, as a large trough at mid levels will stretch from British Isles, across France, Italy, Balkans into the region of Black Sea. A mid-level low is embedded in the trough and is expected to slowly fill up during its progress to the southeast towards N Italy. In the wake of this low showers are expected to form across France, some of them being weakly electrified. Southern flank of the low is surrounded by a belt of strong flow. At the same time, a ridge will stretch from Russia, across N parts of Central Europe into Scandinavia and NE Atlantic. At the surface, dominant feature will be a high pressure system over Scandinavia and Africa . Between them, a string of low pressure systems will exist, with the most prominent one developing in the region of N Italy / Germany / Czech republic, slowly moving eastwards and filling at the same time. Several ill-defined and weak frontal zones will accompany the lows with the main one stretching from Balkans to Germany.

DISCUSSION

...Italy...

A strong QG forcing is expected ahead of the progressing mid-level low, which will result in the favourable upward motion. In combination with the day time heating and the advection of moister airmass inland, destabilization is expected with several hundred J/kg of MLCAPE expected to form by 12Z. GFS is much more agressive concerning the instability release than ECMWF, which exhibits lower values with peaks over the Adriatic Sea. GFS suggests more than 1000 J/kg of MLCAPE over N Italy. Thunderstorms will easily initiate in the favourable synoptic setup and the activity will shift eastwards during the forecast period. Due to the fact, that models are inconsistent about the instability issues over N Italy and the region being under the low values of wind shear, no risk level is anticipated. Marginally large hail is possible with the stronger cells, if GFS model is correct and moderate instability values will be in place along with the low 0°C isotherm.

To the south, lower values of MLCAPE are being simulated, but still, storms are anticipated also over Central Italy. This region will be situated under the belt of stronger flow on the southern edge of the mid-level low, DLS values migh reach more than 20 m/s, with shear in the lowest 3km in the range of 10 - 15 m/s. Furthermore, slightly enhanced SREH values are forecast due to the slight veering of winds thanks to the proximity of surface low. With this setup, well-organised multicells might form with a brief, low-topped supercell not ruled out either. Slight threat of large hail seems plausible and a Level 1 is issued for the area. Thunderstorm activity should gradually weaken after 18Z and move eastwards.


...Balkans to Hungary...

A belt of instability will probably form on the frontal zone lying in this region. Weak vertical motion is also simulated above the region thanks to the approaching mid-level low. Thunderstorms will probably stay confined to the frontal region, moving slowly eastwards with it. Mostly weak flow aloft will lead to the weak value of wind shear and storms should stay poorly organized, probably into multicell clusters. Moreover, severe threat will also be reduced by the meager values of instability, so no severe threat is forecast.