Storm Forecast
Valid: Wed 08 Aug 2007 06:00 to Thu 09 Aug 2007 06:00 UTC
Issued: Tue 07 Aug 2007 20:04
Forecaster: GATZEN

SYNOPSIS

High present over northeast Europe remains during the period. To the west, sharp trough cuts off into northern Italy. Low geopotantial is also present from Alpine region and Germany to the Black Sea. While cool air mass is present over west Europe as well as over northeast Europe, a broad region with warm and locally unstable air mass is present east of a cold front from northern Italy to eastern Germany and western Scandinavia.

DISCUSSION

Western Mediterranean

At the southern flank of the developing cut-off low, a very strong upper jet streak moves into
north-western Mediterranean Sea. Embedded vort-maxima will spread across the region during the period, providing strong DCVA. At the surface, a cold front propagates south-eastward into north-western Mediterranean. To the south of this frontal boundary, warm air mass characterized by rich low-level moisture and CAPE in the order of 500 J/kg is present. Due to low level forcing as well as temporarily QG forcing, convective events are likely along the cold front during the day. Given strong deep layer vertical wind shear in the range of the jet streak (20 m/s initially, increasing to 30 m/s at the end of the period), thunderstorms are expected to organize, and well developed multicells are forecast as well as a few supercells given favorable low-level veering profiles. Large hail, severe wind gusts and a few tornadoes are not ruled out with the stronger storms. Limiting factor is uncertainty of QG forcing at the anticyclonic flank of the jet streak, and fields of instability and forcing may not overlap. However, given rather warm water surface and nice moisture at low levels, as well as low-level forcing along the cold front, expect that chance for thunderstorms is relatively high. Thunderstorms will likely weaken in the evening hours as QG forcing will weaken in the wake of the main vort-max/trough axis.

Northern Italy, western Alpine region

In the range of propagating vort-max/trough axis overspreading the main frontal boundary,
strong QG forcing will affect a very moist boundary layer/instable air mass. Showers and thunderstorms will likely form and merge into clusters, capable of producing high precipitation, and flash flooding. Best chances for organized convection/MCSs will be over the south-western portions initially, where deep layer vertical wind shear is forecast to increase to more than 20 m/s ahead of the trough axis. Severe wind gusts will be the most significant threat. During the day, region of DLS shifts to the east across northern Italy. While convection will be more isolated there, a few supercells may form, capable of producing large hail and severe wind gusts.