Storm Forecast
Valid: Thu 30 Aug 2007 06:00 to Fri 31 Aug 2007 06:00 UTC
Issued: Wed 29 Aug 2007 21:24
Forecaster: GATZEN

SYNOPSIS

At the southwestern flank of northeast European long-wave trough, a short-wave trough extends from southern Germany to Iberian Peninsula and will propagate eastward during the period. Ahead of this feature, a weak surface low will move across Italy, providing easterly winds at the northern/eastern flank. Very moist low-level air mass is expected in the range of this low, while cool/dry air mass advects into France, and west Mediterranean Sea. Stable air masses are also present over Iberian Peninsula, west, and central Europe, as well as over parts of east Europe.

DISCUSSION

Northern Italy region/northern Adriatic

Very moist boundary-layer is expected to remain over northern Italy region, as weak surface low will lead to easterly surface winds during the day. Latest soundings indicate rather deep low-level moisture over most places. Aloft, strong upper jet streak now over west Mediterranean is expected to spread across Italy, providing strong southwesterly winds advecting an EML into central, and probably northern Italy. Where the EML will overlap with the high moisture, high CAPE is expected to build. Limiting factors are remaining low-level clouds of overnights thunderstorms and associated weak diurnal heating. Latest model output suggests that widespread thunderstorms will form as early as late morning, probably due to QG forcing at the left entry region of the upper jet streak. Expect numerous outflow boundaries and very moist low-level air mass, this scenario seems to be quite likely. The result will be widespread convection in the region with strongest QG forcing. Given strong vertical wind shear and favorable veering profiles, well-organized multicells and embedded mesocyclones are forecast, capable of producing large hail/isolated severe gusts. Tornadoes are not ruled out given locally strong low-level buoyancy and low LCL heights, but weak LLS is a limiting factor. Convective activity is forecast to reach the peak intensity during the afternoon. Convection will spread northeastward into Austria/northern Balkans. In the evening hours, CAA will set in in the wake of the low, and convection will likely weaken. An upgrade to a higher level may be warranted when strong diurnal heating and/or isolated supercells will develop in the region eith best vertical wind shear.

Further south, strong instability is expected in the range of the surface low center, given very moist boundary-layer and EML at mid levels. Although quite strong capping inversion is likely, expect initiation during the day. Thunderstorms that form will likely benefit from strong DLS underneath the jet axis, and highly organized thunderstorms are forecast. Supercells that form may produce very large hail and severe wind gusts. Limiting factor is rather weak low-level convergence and thunderstorms may be rather isolated. Additionally, storms that move across the Adriatic may weaken due to rather weak low-level buoyancy. Decide to issue a level 1 as chance for very large hail seems to be quite limited at this time.

Northern Black Sea region

In the range of propagating short-wave trough, QG forcing is expected over the Black Sea region. At low levels, warm and rather moist air mass is present over the Black Sea, and diurnal heating will likely be sufficient for some CAPE in the range of a weak convergence zone from Bulgaria to eastern Ukraine. Thunderstorms will likely go on over this region. Given rather strong DLS, a few severe thunderstorms are expected, with large hail the main threat.