Storm Forecast
Valid: Sat 17 Jul 2010 06:00 to Sun 18 Jul 2010 06:00 UTC
Issued: Sat 17 Jul 2010 04:43
Forecaster: VAN DER VELDE

A level 2 was issued for SE Sweden mainly for severe convective gusts.
A level 1 was issued for S Sweden for large hail, gusts and isolated tornados.
A level 2 was issued for W Poland, Czech Republic, E/SE Germany, N Austria for large hail and severe convective wind gusts.
A level 2 was issued for northern Italy mainly for severe convective wind gusts, and isolated large hail and excessive rain.
A level 1 was issued for N Netherlands, N Germany, E Denmark mainly for a chance of isolated large hail.

SYNOPSIS

A mid-level trough with its axis from W North Sea to S France at 12Z moves eastward. A cold front stretches from Sweden southward into W Poland towards N Switzerland, with a large body of moderate MLCAPE to its east and south. A secondary cold front behind it, especially in mid levels, will move over Germany. Lower MLCAPE and lower EL heights are forecast over N Germany, Netherlands, North Sea and Denmark during the afternoon. The quasi-geostrophic lifting associated with the trough is located in two maxima, one moving over Netherlands and eastern North Sea, the other over central France to the Alpine area. Weaker lift is present in a band between these regions. A shortwave within the trough moves over S Sweden. Sinking QG motions are initially forecast to be present over N Germany until 15Z.

DISCUSSION

...southern Sweden...

Streamlines clearly show that a very unstable airmass indicated by the Lindenberg 00Z sounding (1200 J/kg CAPE) is advecting northwards into Sweden. GFS MLCAPE values reach even higher during the afternoon and are particularly high in the lowest 3 km, with low LCL. As deep layer shear values reach 20 m/s or more, SREH locally 150-200 m²/s² , supercell are possible with chances of gusts, large hail and perhaps a tornado given LCLs lower than 800m. But low level shear (0-1 km) is marginal for this, locally 8 m/s. Storms could be of discrete nature given weak linear forcing.
During the afternoon, southeastern Sweden has chances of MCS development with bow echoes at the linear convergence zone associated with the cold front, the shortwave trough aloft and the regional MSL pressure minimum. In this zone SREH is increased to values up to 300 m²/s² with strong low level shear as well. This will likely lead to a squall line with bow echoes, producing severe wind gusts and locally large hail.

...W Poland, Czech Republic, E/SE Germany, N Austria...

Up to 300 m²/s² SREH is found just west of the surface cold front in the border area of Germany and Poland. CAPE is highest east of the line, and slightly elevated to the west. However, it is likely that storms will be able to tap into the streamwise vorticity and start rotating. Hodographs indicate strong mid-level storm-relative winds. These factors and the abundant MLCAPE up to 2000 J/kg are favorable for strong cells with large/very large hail and severe wind gusts, which are enhanced by evaporational cooling indicated by delta theta-e > 20° and LCLs (east of the front) of 2500 m with steep low level lapse rates.

...northern Italy...

Large CAPE values >1600 J/kg already present in 00Z soundings, going over 2000 J/kg in GFS during the afternoon, and dry upper layers pose a threat of severe wind gusts. Large hail is also possible, once shear increases to marginally moderate values from the west. More importantly, and the reason for level 2, is the trough coming out of France passing the region during evening and night, setting off widespread storms with primary chance of severe wind gusts. Large hail and excessive precipitation can also occur.

...S France...

Chances of initiation are slight, but a storm developing in the forecast large circular hodographs can become a supercell with large hail. But strong entrainment will likely win with so little CAPE.

...N Netherlands, N Germany, E Denmark...

With help of lifting motions of the trough, instability can just be sufficient during early-mid afternoon and develop relatively low topped thunderstorms, which under influence of strong (25-30 m/s) deep layer shear and some SREH might develop some rotation. Isolated marginally large hail or isolated severe gusts could result.