Storm Forecast
Valid: Sat 23 Jun 2007 06:00 to Sun 24 Jun 2007 06:00 UTC
Issued: Fri 22 Jun 2007 17:49
Forecaster: DAHL

SYNOPSIS

Intense large-scale upper trough is located over central Europe and should continue to make eastward progress through the period. South and east of this trough, warm and unstable subtropical air mass is present, with a very deep and strong EML, which is expected to spread across the southern Balkans on Saturday. GFS seems to have a bad hande on the thermodynamic profiles, strongly underestimating the CAPE over the Mediterranean (ref., e.g., Friday's LIBR 12Z launch). Moderate to strong CAPE should exist over portions of eastern Europe, and given quite intense shear, another active severe weather day is anticipated.

DISCUSSION

... Balkan States ...

It seems quite likely that the EML will make it to the Balkan States until Saturday, providing this region with very steep lapse rates, but also with a strong cap. Surface dewpoints may not be as high as over the Mediterranean on Friday, but CAPE should still be on the order of 1000 to 1500 J/kg. Especially over the northern Balkans, the cap - and the lapse rates - should be weaker, but still sufficient for CAPEs on the order of 1000 J/kg.

Deep-layer shear is simulated to be in the 20 to 25 m/s range, with increasing low-level shear in the late afternoon hours, exceeding 10 m/s over the western Black-Sea region.

The main issue seems to be whether or not storms will initiate. The southern Balkans will be capped most strongly, and displaced farthest from the large-scale upward motion regime, so that chance of storm development will be decreasing towards the south. Towards the central and northern Balkans, chances increase, with ECMWF showing quite some precip over these regions, but rather early in the day. This is likely associated with an upper perturbation, which will cross the region in the first half of the day, being accompanied by a subtle SFC low. A subsequent vort max is simulated to support isolated storm development late in the afternoon by GFS, while ECM does not produce any more precip late in the afternoon/evening. Especially along the broad frontal boundary, thunderstorms should be able to develop, however, given mesoscale ascent with the frontal circulation.

If thunderstorm initiate, strong shear and instability will aid in rapid severe evolution. Among the likely modes are bow echoes capable of destructive winds and supercells, posing an additional threat for very large hail. It seems that the hail threat would be largest where the steep lapse rates are present and where the largest buoyancy in the mixed-phase region exists, that is, towards the south. Tornadoes may also become an issue, but likely in regions where the capping is minimal and where the low-level shear is maximized. This may occur along the broad frontal boundary as well as ahead of the mesoscale low reaching the W Black Sea in the evening.

Possible scenarios range from rather isolated convective development to rapid upscale growth into one ot more severe MCSs, depending on the exact cap strength and the degree and timing of the large-scale upward motion.

Despite the uncertainties, a robust severe thunderstorm threat exists, and a level two seems to be warranted. Will place it where current model guidance suggests maximization of LLS, but an upgrade may be necessary elsewhere and/or adjustment of the level-two region, depending on the details of the convective development on Saturday.