Storm Forecast
Valid: Mon 03 Sep 2007 06:00 to Tue 04 Sep 2007 06:00 UTC
Issued: Mon 03 Sep 2007 01:14
Forecaster: VAN DER VELDE

SYNOPSIS

A cool airmass invasion arrives over southern Scandinavia behind a second cold front. An earlier cold front with unstable air postfrontal air is present from the low over Finland all the way into the Alpine region. Cyclogenesis takes place in the lee of the Alps as QG forcing of the upper trough sinks southeastward and the warm airmass is lifted and destabilized. Most convective activity should occur late in the period.

A significant disturbance affects the western Black Sea region. High instability is present in the range of the strong southwesterly jet near an upper trough and warm low level flow from the north.


DISCUSSION

Turkey, Black Sea, Bulgarian, Romanian and Ukrainian coast:

Storm-relative winds and high boundary layer moisture content with moderate to strong CAPE values are capable of producing long-lived storms including supercells (SREH around 200 m2/s2, 0-6 km shear 25 m/s) with a chance of mainly (very) large hail, but also isolated severe gusts (locally dry mid levels due to Turkish elevated mixed layer) and a tornado/waterspout are possible: although low level shear is not so strong (mostly less than 8 m/s), LCLs are fairly low (<1000 m) and low-level CAPE over the western Black Sea not bad.
The shear, strong flow and any cold pool formation may sustain an MCS capable of producing more organized severe gusts.


Italy, Adriatic coast:

Some CAPE is forecast by GFS to develop in the lee of the Alps. The resulting amount is usually higher, so that it seems like a good chance exists some storms will develop. With deep layer shear just reaching moderate values (15 m/s 0-6, 1-8 km), the likely mode is strong multicells though probably not as severe as last week (now less instability and SREH). A large hail event or two should be possible.
Later, as the cold front moves in, activity will also reach farther south. Large rainfall amounts are expected for the surroundings of the northern Adriatic Sea due to persistent convection. Shear remains marginal to severe weather, some hail may occur and either tornadoes or waterspouts (the latter also farther into the Adriatic and at the west coast of Italy - but higher chances follow during the next days)


Kattegat, Baltic Sea:

As cold airmass advects over warm sea water, steep low level lapse rates are created and may trigger an isolated spout in areas of enhanced vertical vorticity. However, a level 1 has not been issued at time of writing, because forecast CAPE values appear rather modest.