Storm Forecast
Valid: Sat 04 Aug 2007 06:00 to Sun 05 Aug 2007 06:00 UTC
Issued: Sat 04 Aug 2007 03:45
Forecaster: VAN DER VELDE

SYNOPSIS

At 18Z Saturday, a blocking high pressure area at lower levels spans a broad region from Spain to the Baltic states, keeping the southwesterly flow at distance from the continent, over the British Isles and Norway. Upper level lows are centered over Hungary and western Russia, while later in the night a upper cut-off low approaches the Iberian Peninsula. The cold front of the previous day has stalled over the Balkan, where vorticity maxima around the upper low provide sufficient lift for widespread thunderstorms in a tongue and bubble of steep mid level lapse rates.

Destabilisation takes place at the west flank of a mid level ridge over the western Iberian Peninsula, where a thermal low pulls inland some Atlantic moisture. Very high LCLs and very steep mid level lapse rates are in place (some localised microburst threat) The late arrival of the trough would allow increasing numbers of thunderstorms from evening onwards.

Mid level air over Scandinavia should warm at the approach of a temperature ridge, which would suppress deep convection.


DISCUSSION

...Balkan...

The main zone of convection seems divided into four areas:

Hungary, later southward into former Yugoslavia:

a bubble of steep lapse rates near the core of the low and surface convergence are likely to cause many showers and thunderstorms, in relatively cool airmass behind the cold front. The setup may allow for funnel clouds or an isolated landspout, as well as sub-severe size hail. Most shear borders the area of instability, and some storms might profit from that with better organisation.

W Ukraine into Romania:

Weak instability triggered by mountains and upper support at the east flank of the low, SREH is forecast to be well enhanced over northern Romania to over 200 m2/s2, but deep layer shear appears to be a bit modest... still some cells may develop a rotating updraft and create a large hail threat.

N Greece, Macedonia, SW Bulgaria:

Here instability is larger, and deep layer shear around 15-20 m/s, but not with much veering: SREH is only locally higher than 100 m2/s2. Some split-storms may be observed, with a local chance of large hail.

SW Black Sea:

The best shear conditions of the map with strong storm-relative winds created by northeasterly low level winds and southwesterly mid level winds, 20-30 m/s 0-6/1-8 km shear, 100-200 m2/s2 SREH... but a late show: 21Z and onwards into the morning according to GFS instability parameters. There would be a band of QG lift from the Balkan system overspreading this area and weak low-level convergence. Any storm may develop rotation and thus large hail threat, severe gusts threat (dry middle levels), but tornadoes are not likely due to weak low level shear. The storms may quickly form a much stationary MCS. There is only one minus against this scenario: GFS does hardly produce convective precipitation.


...W Russia...

At the inner side of the curling front around the low, instability of some 1000 J/kg is still forecast to be present, but decreasing during the day. Some SREH and okay 0-1 km shear are predicted in this area, but deep layer shear is rather weak. It does seem that the combination of relatively high CAPE and locally moderate shear may yield a few large hail events.