Storm Forecast
Valid: Wed 22 Apr 2009 06:00 to Thu 23 Apr 2009 06:00 UTC
Issued: Tue 21 Apr 2009 23:27
Forecaster: PUCIK

SYNOPSIS

Generally, at midlevels, most of Europe is under the pool of low geopotential heights, but many complex features can be resolved in this situation. A broad cyclonic vortex will persist through the forecast period centered over the E Mediterranean. A strong jet-stream will surround its base, with its axis stretching from Sahara desert to the SE Mediterranean and Turkey, slowly shifting eastwards. Another important feature will be a cut-off low, which is expected to move from Denmark to S Germany by the end of forecast period. Otherwise, a ridge is forecast to stretch from Spain to the southern Scandinavia.

At the surface, a large low pressure system will dominate the central and E Meditteranean with two centres anticipated - one over Sicily and another one developing over Turkey. A trough will stretch from this system into the Central Europe, reaching to N Poland as a cold front will approach this area. Most of W and E Europe will be under the high, yielding stable conditions.

DISCUSSION

...Belt from N Germany to Czech republic...

ECMWF and GFS are both quite consistent in the development and the course of a cut off low, which will affect this region. Stronger mid-level flow will surround it with low placed in its left exit region. As the low is characterized by a pool of cold airmass , higher mid-level temperature gradients are anticipated in its core. QG forcing as well as the day time heating should result in the destabilization with several hundreds J/kg of MLCAPE expected, mostly forming over the NE part of Germany. With low level convergence ahead of the advancing cold front, thunderstorms will form and are expected to spread southwards with respect to the movement of the mid-level low. As wind shear will be very weak, with DLS around 10 m/s and instability only in lower values, the chances for severe weather will be very low. However, due to the cold tropospheric profiles, sub-severe hail might occur with the stronger cells. Surface-based instability will quickly dimnish after sunset, but strong forcing and the presence of elevated instability will probably allow for the storms going into Czech republic during the night hours.

...Turkey...

Placed under the left exit region of the jet-stream combined with the arrival of the significant mid-level impulse will result in the strong upward motion. Ahead of the surface low, an advection of warm and moist airmass is expected with the instability building up quickly even in the morning hours. MLCAPEs might reach over 800 J/kg. Wind shear will be quite favourable, with DLS being over 20 m/s and shear in the lowest 0-3 kms in the range of 15 - 20 m/s. Moreover, backing flow ahead of the surface low will lead to enhanced SREH values over 200 J/kg. These factors will probably be supportive of very well organized storms, including supercells. Limiting factor will be rapid storm clustering due to the widespread thunderstorm development so that we do not expect isolated and long lived storms. Nevertheless, marginally severe hail can occur with the stronger cells, especially with the brief supercells. Moreover, strong perpendicular component of the flow to the boundary suggests that some bowing segments might form and isolated severe gusts are not ruled out. Combination of these threats warrants a Level 1 for this region.