Storm Forecast
Valid: Sun 19 Sep 2010 06:00 to Mon 20 Sep 2010 06:00 UTC
Issued: Sun 19 Sep 2010 05:58
Forecaster: SCHLENCZEK
A level 1 was issued for the western Balkan Peninsula mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts.
A level 1 was issued for S Iberia and N Morocco towards N Tunisia mainly for large hail and to a lesser extent for severe wind gusts.
A level 1 was issued for S Italy and Sicily mainly for large hail and severe wind gusts.
A level 1 was issued for the W Baltics and S Finland mainly for tornadoes.
SYNOPSIS
An upper low pressure system stretches from the Canary Islands towards the N Adriatic Sea. At its southern flank, a 35 m/s upper jet streak will provide strong deep layer shear and some embedded shortwaves may lead to sufficient forcing for convective initiation in a hot and unstable but strongly capped airmass. The three highlighted regions will get most instability and highest probability of initiation according to GFS 12Z.
Another low pressure system is centered over N Scandinavia and shifts slightly eastwards. Some low-end instability should be in place and showers / thunderstorms may develop.
DISCUSSION
...S Iberia, N Morocco towards N Tunisia...
In a hot and relatively dry airmass, some 800 - 1200 J/kg CAPE are forecast by GFS. Strong upper level winds provide some 20 - 25 m/s deep layer shear and 200 - 300 mē/sē SRH3 which may lead to high-based multicells and a few supercells, capable of producing isolated large hail and severe wind gusts. Towards Spain, instability should decrease but due to an upper vort-max, forcing should increase. Although hodographs show a wind profile favourable for supercells, only a level 1 is issued due to the strong capping inversion at mid altitudes which will reduce storm coverage.
...S Italy...
A similar situation is expected for S Italy and Sicily where some low to moderate CAPE is simulated in an environment with strong deep layer shear and locally enhanced SRH. Isolated multicells and supercells may produce large hail and severe wind gusts and may tend to merge into one or two MCSs with severe wind gusts as main threat.
...W Balkans...
The heavy rainfall threat will continue also on Sunday. Parts of Slovenia have already received 400 - 600 mm and some more 30 - 50 mm may follow. Further south, some 500 - 1000 J/kg CAPE overlap with 20 - 25 m/s DLS and some almost linear QG forcing from an upper vort-max. Current thinking is that a squall line / linear MCS may form and will move across the W Balkan Peninsula with isolated severe wind gusts and some marginally severe hail. A tornado is not completely ruled out but rather unlikely as the low level wind field is quite weak and SRH1 is only in order of 50 - 100 mē/sē.
...W Baltics, S Finland...
At the southern edge of the upper low, a few hundred J/kg CAPE may form in an almost saturated airmass. Low level shear in order of 10 m/s and some 150 - 250 mē/sē SRH1 may allow an isolated tornado in this region. Visby 00Z sounding reveals some instability, low level shear and a very low LFC. The tornado threat should diminish during the late evening / night hours when instability almost vanishes.
