Forecast Update
Valid: Fri 14 Aug 2009 10:00 to Sat 15 Aug 2009 06:00 UTC
Issued: Fri 14 Aug 2009 10:14
Forecaster: TUSCHY

A level 2 was issued for extreme NE Algeria and N-Tunisia mainly for severe wind gusts and large hail.

SYNOPSIS

Please refer to the main outlook.

DISCUSSION

.... NE Algeria and N-Tunisia ...

An upgrade was performed over parts of extreme NE Algeria and N-Tunisia. 00Z soundings along the coast show the overlap between the moist and warm marine layer with the well mixed and dry mid-/upper-level airmass. Despite some temporarily weakening, this set-up remains present throughout the following hours.

The disturbances, embedded in the subtropical jet, are hard to detect on WV imagery with one stronger impulse now sliding eastwards along the north coast of Algeria. Scattered thunderstorms evolved during the past few hours. Some doubts exist that this activity will be long-lived due to the diffuse nature of this disturbance next to a forecast weakening during the following hours.

The attention then turns to the afternoon and night hours primarily over the level-2 area. Surface data reveal a very moist boundary layer with dewpoint readings in the lower to mid-twenties and elevated instability release of 1-2 kJ/kg. DLS gradually increases throughout the next few hours and peaks out at 20-30 m/s with some veering present. Given deep and dry subcloud layer (daytime mixing of the boundary layer airmass assumed), severe downbursts and large hail are forecast with those storms, and isolated very large hail possible, too. Although thunderstorm coverage may be not as widespread, a few extreme hail events can't be ruled out and hence a level 2 was issued.

Low thunderstorm probabilities were included for Sardinia.


... Slovenia and Italy ...

The level 1 was downgraded as latest model data revealed weaker shear compared to what was forecast yesterday. Still isolated large hail is possible but overall risk is too marginal for a level 1.

The general thunderstorm area over NE-Spain was downgraded to lower probabilities as instability release and forcing will be too marginal for more than a few, isolated thunderstorms.

No significant changes were performed over NE-Europe.