Storm Forecast
Valid: Wed 23 Sep 2009 06:00 to Thu 24 Sep 2009 06:00 UTC
Issued: Tue 22 Sep 2009 22:32
Forecaster: PUCIK

A level 1 was issued for Tunisia, Southern Tyrrhenian Sea, Eastern Algeria and Southern Corsica mainly for excessive precipitation and to lesser extent for tornadoes/waterspouts.

SYNOPSIS

A prevailing strong zonal flow is anticipated over most of the northern half of Europe at mid and upper levels of troposphere. Embedded in this flow, a disturbance is expected to reach Scandinavia by Wednesday 15Z with attendant frontal system. To the south a large ridge has overspread Western and Central Europe with stable conditions in these regions. A cut off low will remain over the southwestern Mediterranean, centered over Algeria with slight movement to the East. At the surface a widespread system of lows will affect the weather conditions of Northern Europe with arctic maritime airmass spreading east behind the advancing cold front which will enter Russia by Wednesday 15Z. Otherwise, most of Western, Central and Southeastern Europe will remain under high pressure system. Under the forward flank of the cut off low over Algeria , a shallow low has evolved and it is expected to deepen.

DISCUSSION

... Tunisia, Southern Tyrrhenian Sea, Eastern Algeria, Southern Corsica ...

Widespread and well organised thunderstorm activity has evolved to the East of Tunisia per satellite observations during Tuesday. This trend will keep up also during this forecast period. A belt of high DLS value, forming under the mid-level jet has overspread a belt of high Theta-E airmass which is moderately unstable with MLCAPEs over 1000 J/kg. Towards the Wednesday evening, MLCAPE values should even increase, as shown by numerous NWP outputs. The best potential for the well organised and strong deep moist convection will exist over the southeastern coasts of Tunisia with the highest instability and shear values simulated there. However, this region is out of the forecast area.

As the surface low will deepen, low level circulation should strenghten, yielding 15-20 m/s flow at 850 hPa level and especially over Tunisia LLS values are expected to increase, up to 10 m/s. A band of high SREH values will form, where the low level flow will back as the surface low deepens. spreading northwards during the forecast period. With DLS values of 15 - 20 m/s for southern half of Level and above mentioned conditions, well organised storms might form, including supercells. The primary storm mode should be a large MCS however, as suggested by strong convergence fields in GFS and also, very high precipitation blob in ECMWF runs. High storm coverage will be supported by large scale forcing from the cut off low. The primary threat should be excessive precipitation (thanks to the very high moisture content, relatively slow storm movement and high chance of MCS formation) especially across eastern Tunisia and southern Corsica, where both GFS and ECMWF show the highest amounts of precipitation. Moreover, tornadoes are not ruled out over Tunisia, with high SREH and LLS, low LCLs values simulated over the region. The main limit for the tornadoes should be the fact, that storms will probably not stay as isolated supercells, but instead form a large complex. The threat should be highest between 06-12Z over Tunisia and towards Thursday morning over Corsica. Elsewhere in the marine regions of Level 1 and also over western Mediterranean, waterspouts might form as high low level CAPE release is expected as well as low LCLs over the sea.