Storm Forecast
Valid: Sat 20 Jun 2009 06:00 to Sun 21 Jun 2009 06:00 UTC
Issued: Sat 20 Jun 2009 00:31
Forecaster: SCHLENCZEK

A level 1 was issued for central Italy and the Balkans mainly for excessive precipitation and severe gusts.

A level 1 was issued for parts of Romania and W Ukraine mainly for severe gusts and tornadoes.

A level 1 was issued for E Spain mainly for severe gusts and large hail.

A level 1 was issued for N Germany, Denmark and S Sweden mainly for tornadoes.

SYNOPSIS

Cyclogenesis is expected in the extreme eastern flank of an upper trough over Scandinavia with a 990 hPa surface low travelling northward towards the Arctic Sea. Over central and southeastern Europe, geopotential heights are forecast to rise but unstable conditions with possibly severe thunderstorms will continue over Italy and the N Balkans.

Most of the thunderstorms should develop along a frontal boundary that stretches from extreme NW Spain via S France and N Italy towards extreme E Austria and SE Poland to Belarus. This cold front should move southward during the period, reaching the W Mediterranean on Sunday morning.

DISCUSSION

...central Italy, Balkans...

In the late morning / early afternoon, an upper vort-max will cross northern Italy, moving northeastward. Compared to the situation on Friday, instability and shear over N-central Italy have decreased but there should be still MLCAPEs around 1 kJ/kg in a weakly capped environment with 15 m/s deep layer shear. Especially over Croatia, strong LL convergence is forecast and thunderstorms may merge into a large cluster / MCS with some possible severe gusts but excessive rainfall as main threat. GFS and WRF show precip signals exceeding 50mm / 6 hr over a broad region which requires a level 1.

...W Romania, SW Ukraine...

In the vicinity of the frontal boundary, some hundred J/kg CAPE should be created during the day. Shear at lower levels will remain strong with about 15 m/s 0-3 km bulk shear and 10 m/s LLS which should favor organized storms. In Romania, severe gusts and isolated large hail should be the main threat. Further to the northeast, LCL heights will stay quite low and an isolated tornado should be possible. After sunset, storm coverage and severity should gradually decrease.

...E Spain...

Although there are very weak signals of precipitation in the models, the southern tip of an approaching upper vort-max over S France might provide enough lift for convective initiation in an environment with a few hundred J/kg of CAPE in a very warm and dry airmass. Deep layer shear in order of 15 m/s should be sufficient for some organized multicells and / or a short-lived supercell which may be accompanied by severe gusts and large hail.

...S Sweden, Denmark, N Germany...

Polar airmass showers / thunderstorms are forecast in a region with cold upper air over S Scandinavia and parts of Germany. Diurnal heating will lead to some hundred J/kg CAPE and widespread initiation is likely with the weakly capped airmass. Low LCL heights, weak LL winds and strong LL buoyancy may result in some funnels / brief tornadoes. Not much activity is expected after sunset as CAPE vanishes.