Storm Forecast
Valid: Thu 27 Sep 2007 06:00 to Fri 28 Sep 2007 06:00 UTC
Issued: Wed 26 Sep 2007 16:10
Forecaster: GATZEN

SYNOPSIS

A well-developed cut-off low is expected to remain quasi-stationary over central Europe. Its center slowly moves from southern France to central Alps during the period. A very strong upper jet curves around the periphery of this trough, spreading across west Mediterranean. At low levels, a frontal boundary is expected from southwest Mediterranean Sea to southern Italy, central Balkans, and southern Poland. While the front remains quasi-stationary over most of the area, WAA is forecast over Poland/east Germany at the eastern flank of the cut-off low.

DISCUSSION

Southern Adriatic

Main frontal boundary will be present from southern Italy to southern Balkans during the period. While low-level warm and quite moist air mass remains east of the front, upper height falls are likely during the period ahead of a strong southwesterly jet. Steep mid-level lapse rates are expected and rather high instability is likely to develop. Forcing will be limited during the period underneath the anticyclonic exit of the upper-level jet, but a weak upper trough and diffluent mid-level flow may support some lift. Low-level forcing will be strongest in the range of the frontal boundary and associated frontal waves as well as in regions with upslope flow over the Balkans. Thunderstorms are forecast to develop especially over the Balkan regions. During the period, winds from the south-west will strengthen as indicated by latest model output, and even low-level winds are forecast to increase to more than 15 m/s. Given favorable veering with height, some thunderstorms are expected to organize into supercells. Rich low-level moisture and increasing low-level buoyancy and SRH are expected to contribute to an increasing tornado threat in the afternoon/evening hours. Large hail and severe wind gusts are also forecast. An upgrade to level 2 may be required when strong instability and QG forcing will overlap during the period, and widespread convection is expected. ATTM, decide to issue a level 1 due to the more isolated severe weather threat.

Italy/Adriatic/western Mediterranean

In the range of the cut-off, steep low-level lapse-rates and sufficient buoyancy are expected over the warm sea surface. Thunderstorms are forecast as some QG forcing is likely underneath the cyclonic flan of the jet streak as well as in the range of WAA regimes in the broad frontal boundary. This convection will likely organize given strong vertical wind shear. Expect a couple of thunderstorms capable of producing locally severe wind gusts and probably a few tornado/large hail reports. Greatest threat exists over the Adriatic, where low-level buoyancy and SRH are strongest.

Balkans

Ahead of the strong jet streak, models indicate increasing instability in the range of the WAA regime. This seems to be reasonable as rather rich moisture is advected in low levels, and steepening lapse rates ahead of the propagating southerly jet streak are forecast in the afternoon/evening hours. Given strong forcing, strong convection is expected to develop. Given strong vertical wind shear and favorable veering (100 J/kg 0-1 m SRH), rotating updrafts are likely. Greatest uncertainty exists about the low-level buoyancy as boundary-layer warming may be limited. Convection that roots to the boundary-layer will be capable of producing tornadoes and large hail. Thunderstorms may spread into Poland during the night and are expected to be more elevated, but an isolated tornado/hail report is not excluded.