Storm Forecast
Valid: Thu 07 Apr 2011 06:00 to Fri 08 Apr 2011 06:00 UTC
Issued: Wed 06 Apr 2011 21:08
Forecaster: GATZEN
A level 1 was issued for south-eastern Poland, eastern Slovakia, north-eastern Hungary, south-western Ukraine, and north-western Romania mainly for large hail, severe wind gusts, and tornadoes.
SYNOPSIS
High geopotential is centred over south-western Europe, whereas low geopotential is located farther east and north-east. Together with an intense short-wave trough, a strong north-westerly mid-level jet moves from the North Sea to northern Romania during the period. At low levels, a cold front initially over the North Sea will propagate quickly south-eastwards reaching the central Balkans and northern Black Sea at the end of the period. Ahead of this cold front, a tongue of warm air will spread eastward on Thursday. Cool and slightly unstable air will remain over south-eastern Europe.
DISCUSSION
Southern Poland, south-western Ukraine, Slovakia, Hungary, north-western Romania, Czech Republic, northern Austria, south-eastern Germany
Ahead of the cold front, latest model output suggests warm air advection from the west that is associated with a tongue of low-level moisture spreading eastward. Latest Kuemmersbruck sounding over southern Germany indicates this rich boundary-layer moisture that exceeds 6 g/kg mixing ratio in the lowest kilometre. Additionally, a region of weak low-level convergence is expected ahead of the cold front that travels eastward into south-eastern Poland until 15Z. A critical ingredient will be the poor lapse rates. The best potential of CAPE is forecast over south-eastern Poland, where the low-level warm air advection will overlap with mid-level cooling due to the approaching trough. This favourably overlap will move southward in the afternoon hours.
Current thinking is that showers will likely develop at numerous places ahead and along the front, but the weak instability will limit the potential of thunderstorms. The best potential is forecast from south-eastern Poland to north-western Romania in the afternoon and evening. However, storms that form may become severe as favourably veering profiles and relatively strong bulk shear of about 15 m/s in the lowest 3 km is present. Rotating and long-lived updrafts are forecast to develop, and isolated storms may become supercells especially in the level 1 area. Although the weak lapse rates do not support large hail, supercells are forecast to be capable of producing large hail as well as severe wind gusts. Tornadoes are not ruled out in the strongly-sheared environment (15 m/s 0-1km bulk shear).
In the evening/night hours, strong cold air advection from the north will rapidly spread south-eastward, and storms will likely decay.
Sweden, Norway
At the cyclonically sheared flank of the strong mid-level jet streak, a well-mixed air mass is expected to become more unstable in response to diurnal heating. A short-wave trough axis crosses the area from the north, and a low-level convergence is expected to move south-eastward. Main limiting factor is the lack of low-level moisture in the polar air mass. Expect that some showers and thunderstorms can develop, these will be likely organized. The main threat may be strong to severe wind gusts given the large downdraft CAPE in the dry and well-mixed air mass, but the threat seems to be too low for a level 1. Storms will decay in the evening hours when the trough moves away.
Southern Turkey
The base of a trough is present over south-eastern Europe. Over Turkey, diurnal heating will likely assist convergence over the mountains that leads to upslope flow along the southern coasts. Associated moisture advection is forecast to build some instability. Weak vertical wind shear is present, and storms will likely be unorganized. Severe weather is not expected except for local intense precipitation given the slow motion of the storms. In the evening, storms are forecast to weaken due to low-level stabilization and decreasing low-level forcing.
