Storm Forecast
Valid: Mon 27 Sep 2010 06:00 to Tue 28 Sep 2010 06:00 UTC
Issued: Sun 26 Sep 2010 16:14
Forecaster: DAHL

A level 1 was issued across the W Ukraine and S Belarus for
severe wind gusts, large hail, tornadoes, and excessive rain.

A level 1 was issued across N Italy and the N Balkans mainly for excessive rain and tornadoes.

A level 1 was issued across Tunesia and parts of the south-central Medieterranean Sea mainly for damaging winds, large hail and tornadoes.

SYNOPSIS

Main feature this period is an extensive upper low over central Europe that exhibits several vorticity maxima at its periphery. These maintain a large-scale cyclonic circulation at the SFC, which likewise exhibits perturbations at its periphery. To the east of this system, warm/moist air is advected northwards into the eastern parts of Europe. Accordingly the main (rather stationary) low-level baroclinic zone curves roughly from Poland across the Ukraine and the central Mediterranean into Morocco.

DISCUSSION

... W Ukraine ... S Belarus ...

Some instability should exist in the warm/moist plume E of the main upper low, and GFS advertises 20 m/s DLS and around 7 m/s LLS over the western Ukraine and S Belarus during the early afternoon. At that time, however, the upper-level forcing seems to be rather weak, consistent with a lack of widespread simulated precipitation. Still, scattered thunderstorms are likely to form, and given the favorable kinematic support, these should evolve into well-organized multicells and/or supercells. Severe threats will include large hail, damaging wind gusts, and also a tornado or two. More widespread precip should overspread the region later in the day when strong DCVA overspreads the region. However, model guidance suggests that this activity will be largely non-convective in nature.

... Tunesia into south-central Mediterranean Sea ...

Scattered thunderstorms should form over the northern Maghreb states, aided by orography/diurnal heating and DCVA-related ascent. Best instability is forecast over and east of Tunesia where moisture appears to be deepest. 25 m/s DLS will be in place and towards the evening, LLS will increase beyond 10 m/s. This suggests that well organized storms, including tornadic supercells may occur. It seems that the coverage will be somewhat limited however, so that a LVL1 threat will be sufficient for now. However, should the forcing/coverage prove to be larger than what is currently anticipated, an upgrade to LVL2 may be required.

... N Italy into N Balkans ...

Scattered thunderstorms are anticipated within a strip from NW Italy into the N Balkans in modified polar air. This convection will be just north of a region of 20 m/s DLS, putting it in a region of maybe 10 - 15 m/s DLS. This suggests that weakly organized multicells may result. More importantly, however, steep low-level lapse rates and low LCL heights will likely favor non-mesocyclonic tornadoes, especially over the N Adriatic Sea and the Ligurian Sea. Moreover, locally excessive precip may occur. Farther S over the W Balkans, isolated storms could also form, which will benefit from 20 m/s DLS. Isolated severe wind gusts, marginally severe hail, and maybe a brief tornado will be possible with this activity. Overall, a LVL1 seems to be marginally warranted.

... SW Mediterranean Sea ... E Aegean Sea ...

A chance for waterspouts will exist over the western Mediterranean Sea late Sunday night when convection should form in response to increasing DCVA-related lift. Another focus for waterspouts will exist over the E Aegean Sea early in the period. Given large uncertainty about the coverage of the water spouts, a LVL1 is not introduced at this time for these areas.