Storm Forecast
Valid: Sat 06 Jun 2009 06:00 to Sun 07 Jun 2009 06:00 UTC
Issued: Sat 06 Jun 2009 00:11
Forecaster: DAHL

A level 2 was issued across N Italy ... E Austria ... N Balkans into S Poland mainly for tornadoes and severe wind gusts.

A level 1 surrounding the level 2 was issued across N and central Italy ... S Germany ... Alpine region ... N Balkans into S Poland mainly for tornadoes and severe wind gusts, and large hail.

SYNOPSIS

Complex pattern is developing over Europe, and the convective scenario is quite uncertain at the moment. Over portions of east-central Europe into the N Mediterranean, the situation may quickly evolve into a major severe thunderstorm outbreak.

The upper-air maps are mainly featuring an intense SWly upper flow with several imbedded vort maxima, most of which are transient in nature, rapidly lifting northeastwards into the Baltics. These features maintain and temporarily augment a SFC-low complex over western/central Europe.
One of the vort maxima imbedded in the SWly flow will lift across Germany, maintaining a wave cyclone centered over western Germany on Saturday afternoon. A warm front is anticipated to stretch from the low's center eastward into N Ukraine and the trailing cold front should stretch into the W Mediterranean at 12Z on Saturday.
Another vort max will dig into the northern Mediterranean late on Saturday/early Sunday morning.

DISCUSSION

... N Italy ... E Austria ... N Balkans ... E Czech Republic ... Slovakia ...

Main question this period will be whether or not instability can develop amidst extensive stratiform cloudiness/precipitation that is simulated across most of the warm-sector air mass.

Shear profiles will be quite intense with 25 m/s DLS and 10 m/s LLS, increasing to more than 15 m/s across the N Balkans into S Poland late in the day. GFS 18Z simulates quite extreme LLS after 18Z, locally exceeding 20 m/s. SRH1 will be maximized along the warm-frontal boundary, but even in the warm sector air, 200 J/kg should be common.

Precipitation and cloudiness should result in somewhat cool but nearly saturated boundary-layer air. The fact that the Saharan EML will remain S of the region of interest suggests that minimal capping will be in place, so that low-level kinematic and thermodynamic profiles should become very conducive to tornadogenesis. Numerical guidance suggests that CAPE will be present despite the existence of cloudiness/precip.

Currently, it seems that convective development will be quite messy, with a mixture of elevated/imbedded cells, and new development where breaks in the clouds occur. Of much concern is the strong low-level shear. Any SFC-based storm will have much potential of evolving into a tornadic supercell. Despite the anticipated weak instability, individual cells may become quite long-lived and an F3 tornado seems well within the scope of possibilities. In addition, severe wind gusts and marginally severe hail may occur.

An update will likely be necessary on Saturday, including a possible upgrade to LVL3, should widespread convection occur in the strongly-sheared regime over N Italy, the N balkans and E-central Europe.

... central Italy ...

Farther south, a sturdy cap is expected to obstruct any attempt of convective development during the day, but vigorous DCVA-forcing is expected ahead of the trough that moves into the Mediterranean late Saturday night/early Sunday morning. Though the strong cap should limit the coverage, an isolated storm or two may occur late in the night. Given strong ambient shear, supercells may be possible, posing mainly a large-hail and severe-wind threat.