Storm Forecast
Valid: Wed 08 Apr 2009 06:00 to Thu 09 Apr 2009 06:00 UTC
Issued: Tue 07 Apr 2009 23:21
Forecaster: PUCIK

SYNOPSIS

Persistent tranquil conditions are expected over most of Central Europe as a large mid and upper level ridge stalls over the area, stretching from the Mediterranean. Ridge is expected to slightly retreat eastwards, giving its way to a mid-level impulse, affecting Great Britain by Wednesday 06Z and moving ENE- wards towards the Baltic Sea. This impulse is expected to weaken considerably during its way. A mid/upper level cutoff low, placed over the E Spain as of Tuesday 18Z will move SE-wards reaching N Algeria/Tunisia by Wednesday 21Z. This feature is already a focus for widespread thundestorm development with numerous lightning strikes reported from the Balearic Sea. At the surface, a series of low pressure systems will cover NW Europe. A large high, centered over NE Europe will be extending across Central Europe to the Mediterranean. Few shallow low pressure areas have developed close to the Spanish coast in the region of the jet-stream, surrounding the base of above-mentioned cut-off low. Frontal zone will be running from Scandinavia, across Germany into the Mediterranean and N Africa, advancing eastward during the forecast period especially over the S Mediterranean region.

DISCUSSION

...Central Europe, Balkans, Italy...

A daytime heating is expected to destabilize the boundary layer, resulting in several hundred J/kg of MLCAPE release by 12/15 Z. This conditionally unstable airmass will be placed ahead of the cold front. With almost non-existent synoptic scale support for the convection ( apart from the regions of E Germany / W Poland which will be affected by the impulse by the evening hours), models have considerable problems with pinpointing the location of the thundestorm initiation, with each run being quite different about this issue.

Clearly, thunderstorms will be initiated by the local topographic circulations and their boundaries. In very weak deep layer shear, thundestorms will be of single or multicell cluster nature with almost no severe potential. Some sub-severe hail might occur with the storms, due to the overall cold profiles of the troposphere. Diurnal thunderstorm activity will quickly dimnish after sunset.

The wind shear will strenghten in the evening hours across the northern half of Germany and Poland, but this will coincide with the general decrease in the instability rooted in the boundary layer, so no severe weather is expected also there. This impulse, however, might sustain some of the storms into the late evening or even night hours and especially 18Z GFS run advertises storms tapping elevated instability and persisting till the next morning. It seems that this late evening/night thunderstorm activity will occur just ahead of the advancing cold front. Some uncertainity exists regarding the real coverage of the storms in Poland area, but sferics data support that electrified convection is ongoing ahead of the front ( as of Tuesday 23Z).

...Tunisia....

Although the models show only modest instability build-up over the region, probably only around 500 J/kg of MLCAPE or so, favourable wind shear profiles are anticipated with DLS between 20 - 25 m/s and shear in the lowest 3 km up to 20 m/s. Moreover, as the shallow low is predicted to form with center over S Tunisia, strong backing of the low level wind field is anticipated. This will lead to the high SREH values, reaching 300 J/kg in 3 km layer. Therefore, storms are expected to be well organized multicells or supercells, probably oriented in a squall line, as suggested by the strong convergence and along the front.

Thunderstorms will be initiated by the arrival of the mid-level impulse and the cold front with activity beggining around 12Z. Large hail might occur with the stronger storms, especially with the supercells. Moreover, strong perpendicular component of wind flow to the boundary and moderate values of DCAPE suggest that severe wind gusts are possible within the squall line. Severe threat should quickly dimnish after 21Z.