Storm Forecast
Valid: Tue 22 Apr 2008 06:00 to Wed 23 Apr 2008 06:00 UTC
Issued: Mon 21 Apr 2008 23:07
Forecaster: VAN DER VELDE

SYNOPSIS

A dipolar pressure configuration is situated over Europe, with Scandinavia under a blocking high while a low pressure system affects mostly Italy and the Balkan, as well as more central parts of Europe. Unstable airmass can lead to isolated to scattered thunderstorms with the focus appearing to be Romania, northern Bulgaria and the nearly stationary cold front stretching from northern Austria into Slovakia.
The vertical shear is weak, except for the eastern parts of the Balkan, where a strong upper jetstream lies... with the left exit region of a 60 m/s jetstreak (300 hPa) situated near NW Bulgaria and southern Romania (in GFS model 12Z) during the afternoon. Low level convergence however runs ahead of this zone, being over eastern Romania at 15Z.


DISCUSSION

...Romania and northern Bulgaria....

GFS model simulates weakly capped instability with 2000m cloud base height at 12Z. Several hundred J/kg MLCAPE should be available from a deep source layer. However, Monday 12Z soundings in the area are quite dry and barely have any CAPE, large scale lifting is needed to create more substantial CAPE.

The picture shown by various bulk shear magnitudes is supportive of supercellular convection, but the 0-3 km shear and storm-relative helicity are offset to the east of the most likely triggering zone indicated by low-level convergence. If anything, this environment may favour splitting cells.

The larger scale forcing imposed by the jetstreak could yield a MCS going over S to E Romania and Moldova, while N-Bulgaria may see more of a preference for isolated cells that also have more SREH available for their rotation, and slightly enhanced (8 m/s) 0-1 km shear marginally conducive to tornadoes but relatively high LCL (1500m) is a limiting factor.

Large hail can be produced by supercells and large multicells, as well as isolated severe gusts. The relatively dry inflow air will likely limit hail size but evaporative cooling could enhance gusts. The anticipated MCS could become the main producer of strong to severe gusts. An upgrade to level 2 may be considered according to convective mode.


...Adriatic Sea...

A convergence line is indicated by GFS to cross the Adriatic Sea along with some instability, most prominent near Albania at 18Z. Its proximity to the jet and enhanced SREH and low level shear may yield an organized storm or MCS capable of isolated large hail... but no confidence in a threat level for this small area.

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