Storm Forecast
Valid: Thu 15 Sep 2011 06:00 to Fri 16 Sep 2011 06:00 UTC
Issued: Wed 14 Sep 2011 15:03
Forecaster: GATZEN
A level 1 was issued for the southern Baltic Sea region mainly for severe wind gusts.
A level 1 was issued for the southern Alps mainly for large hail.
A level 1 was issued for the south-western Ukraine mainly for large hail.
SYNOPSIS
A polar trough spreads eastward over Scandinavia. At its southern periphery, a strong vort-max will eject into the Baltic States. While cool air masses dominate most of northern Europe, a frontal boundary stretches from central France to the southern Alps and into the northern Ukraine. Within the warm air mass further south, steep lapse rates are expected especially from the Atlas mountains to the Iberian Peninsula and over the southern Alps. Rich low-level moisture is located along the frontal boundary as well as over the south Mediterranean.
DISCUSSION
Southern Baltic Sea region
An intense short-wave trough will move rapidly eastward. A moist and well-mixed air mass with lapse rates near the moist adiabate is forecast to become unstable over the warm sea surface of the Baltic Sea. Showers and thunderstorms are possible in the morning hours that will spread eastward into the Baltic States during the day, where diurnal heating may lead to increasing instability. The storms will likely develop in a strongly-sheared environment as a 20 m/s low-level jet curves around the low at 850 hPa. Latest models indicate a low-level vertical wind shear of 12 m/s overlapping with the instability especially over the southern Baltic States. Low-level favorably veering profiles may evolve over the Baltic States downstream of a surface trough over the Baltic Sea during the day.
Current thinking is that numerous showers and thunderstorms will move across the Baltic Sea region that may evolve into small bows, capable of producing severe wind gusts. Farther east, the cells may merge into bowing lines, posing an increasing wind threat. Additionally, tornadoes are not ruled out. The main limiting factor is the weak low-level moisture that may reduce the low-level buoyancy. In the evening and night hours, the convective activity is forecast to decay over western Russia.
Southern Alps
CAPE will build over the southern Alps in response to diurnal heating given rather rich low-level moisture along the frontal boundary. Low-level convergence is forecast given easterly upslope winds along the eastern Alps. Thunderstorms are forecast over the southern Alps during the noon and afternoon that will decay after sunset. These storms may organize given favorable veering profiles at low levels. Latest models indicate about 100 mē/sē 0-3km SRH. The main threat with these cells will be isolated large hail. Excessive precipitation is not ruled out completely.
South-western Ukraine
Along the frontal boundary, a wave is forecast to develop over the south-western Ukraine. Given rich low-level moisture over western Romania and the southern Ukraine, diurnal heating will likely result in instability. Additionally, some QG forcing is likely as the base of the north-European trough approaches over the Ukraine.
While the instability will not overlap with the increasing vertical wind shear north of the frontal boundary, at least some overlap is likely to the east of the frontal wave. Thunderstorms that are forecast may organize, and supercells are not ruled out, capable of producing large hail and severe wind gusts.
