Storm Forecast
Valid: Wed 14 Apr 2010 06:00 to Thu 15 Apr 2010 06:00 UTC
Issued: Tue 13 Apr 2010 21:40
Forecaster: PUCIK

A level 1 is issued for the belt from South Hungary to Central Serbia mainly for large hail.

A level 1 is issued for Southwestern Iberia mainly for excessive precipitation and to the lesser extent for tornadoes.

A level 1 is issued for Eastern Algiers and Tunisia mainly for large hail and severe wind gusts.

SYNOPSIS

A very complex synoptic pattern has established over Europe with a large high pressure system over Atlantic ridging into Southern Scandinavia. Under the prevailing meridional - northerly flow over Europe a locally modified polar to arctic airmass will cover most of the continent. This fact hinders the presence of abundant moisture content and also the chance of significant latent instability built-up. A system of cut off lows will be positioned to the south of the ridge with individual centers to the west of Iberia, over the Western Czech Republic, Southern France and Northern Italy as of Wednesday 06 UTC. At the surface a large high to the north/ low to the south couplet will exist over the Eastern Atlantic, with low affecting Iberia. Rest of Europe will be under the influence of shallow surface pressure systems. Main frontal zone will lie over the Southern Mediterranean.

DISCUSSION

... Hungary to Serbia ...

With a cut off low in the middle troposphere filled with cold airmass, marginal instability is forecast over the region with MLCAPEs slightly over 500 J/kg. ECMWF is even less optimistic in this case, with only local spots of instability. Forcing should be favorable thanks to the approach of sharply defined trough and associated DCVA and therefore storm initiation should not be problematic. A strong mid-level flow is forecast to approach the region during the day, increasing the deep layer shear values to the range of 15-25 m/s, which are favorable values for well organised convection. Shear in the 0-3 km layer might locally reach 15 m/s. In such setup, despite the lack of well defined instability, a well organised multicell or a low-topped brief supercell might occur, with an attendant threat of marginally severe hail. Coverage of the severe weather, if model predictions of CAPE verify, might be sufficient for a low end Level 1.

... Southwestern Ibera ...

With the deep surface low positioned to the west of the region, a prevailing southery flow will advect high Theta-E airmass over the region, with abundant moisture content and high relative humidities in the lower troposphere. With the constant advection of unstable airmass from the ocean and coastal convergence, several waves of thunderstorms are possible over the area during the forecast period, especially from 18 UTC onward. High precipitation efficiency might be achieved by storms thanks to the high RH and low LCLs. Detrimental factor might be the lack of mid or upper level forcing and also absence of strong low level jet. Locally excessive rainfall might occur, however. Strong low level wind shear is also forecast, but the lack of well organised storms due to the weak deep layer shear might supress a chance for tornadogenesis. A low end Level 1 for the aforementioned threats seems to be warranted at the moment.

... Tunisia ...

Thunderstorm initiation is questionable over this area, with ECMWF being more optimistic than GFS and breaking out precipitation more inland. With sufficent MLCAPE values to support deep moist convection, strong wind shear over 25 m/s in 0-6 km layer might suffice for a well organised storms, including supercells. Provided supercells manage to form, isolated large hail or severe wind gusts might occur. Therefore, Level 1 is issued for the region with the most likely initiation.