Storm Forecast
Valid: Sat 15 May 2010 06:00 to Sun 16 May 2010 06:00 UTC
Issued: Sat 15 May 2010 04:07
Forecaster: VAN DER VELDE

A level 2 was issued for the southern Balkan for tornadoes and severe wind gusts, large hail and excessive precipitation.

A level 1 was issued for the northern Balkan mainly for isolated excessive precipitation / large hail.


SYNOPSIS

Much of Europe is under the influence of a large upper trough, in which surface lows are situated over Italy, southern Scandinavia and Shetland Islands. The relatively deep low over Italy and associated strong wind fields would not be out of place in the fall season. The cold front of this depression affects mostly the Balkan and is backed by a rather strong mid level potential vorticity anomaly. Moreover, a jet stream with faster than 60 m/s winds runs over the Balkan region, with winds as strong as 35 m/s being predicted at 850 and 700 hPa levels. Additionally, a modest to moderate amount of CAPE is forecast, and the combination bears potential for severe convective storms.

DISCUSSION

...S Balkan...

The forecast amount of CAPE by GFS (most conservative) and WRF models reaches up to 800 or 1000 J/kg along the Balkan coast at 09Z, and also over land during the afternoon, locally 1500 J/kg. Deep convergence in GFS at the coast is huge and likely indicates overreaction from the conveective scheme. The strong advection of water vapor and lifting over the elevated terrain provides focus for storms and both convective and stratiform precipitation contributions and in some places 50 mm or more rain could fall in few hours, mainly between 06Z and 12Z.

While CAPE does not seem very large in GFS, it does occur in areas with very large vertical wind shear and storm-relative helicity: 30-50 m/s (not knots!) deep layer shear (0-6 km) and upwards of 500 m²/s² SREH over 0-3 km. This is larger than currently over the Great Plains. The thinking is that the kinematic conditions can easily produce supercellular storms or a linear bow echo system along / ahead of the cold front in the Montenegro - Albania - N Greece - W Bulgaria region. The concern is that CAPE is too low (MLCAPE of only a few hundred J/kg, EL temperatures of just -20 to -40°C, only) and that storms may take longer to initiate or hold up against shearing apart. However, the upper forcing and low-level mesoscale lifting are strong and it may not be an issue at all.

The LCL and low-level shear parameter space (ML-LCL 400-800 m, 15-20 m/s 0-1 km shear vector magnitude, large region of 250-400 m²/s² SREH between 0-1 km!) is strongly in support of tornadogenesis, and such values can produce also strong tornadoes. If the main mode is linear, bow echo storms can occur with severe gusts near 30 m/s. An MCS may survive into the evening and move into Bulgaria or southern Romania.

Large hail is possible given such strong mesocyclone potential, but low LCLs (greater warm cloud depth) counteract this to some extent.


...N Balkan...

The area of S Hungary, N Croatia, N Bosnia, NW Serbia is in somewhat less strong wind shear conditions as described above. GFS is not so optimistic in terms of CAPE amount (presence: yes) but WRF models predict patches of up to 1500 J/kg CAPE. This seems to be associated with the warm front which more or less stalls in this region. The SREH seems to be predicted a bit north of the CAPE, but some interface region is quite possible. LCLs are also very low here, and forcing by the trough is strong, something that may come handy in case insolation is lacking. Discrete supercells are possible (forcing is not nearly as linear here) and have some chance of producing a tornado (despite weaker LL shear) or flash flood. The latter is more favored here because the resultant storm motion vectors are slow. Low LCLs work against large hail, but it may nevertheless be observed. The main threat appears surface-based or elevated multi- and supercells with flash flood threat.