Storm Forecast
Valid: Tue 03 Nov 2009 06:00 to Wed 04 Nov 2009 06:00 UTC
Issued: Tue 03 Nov 2009 05:40
Forecaster: VAN DER VELDE

A level 2 was issued for W Turkey mainly for tornadoes and severe wind gusts.

A level 1 was issued for S Italy mainly for severe wind gusts.

A level 1 was issued for N France and Belgium mainly for the chance of a tornado and severe wind gusts.

SYNOPSIS

Depressions dominates the map, with a <968 hPa low pressure center stationary north of Scotland and a <996 hPa center moving from Italy to western Turkey during the period. Blocking high pressure holds its position over Russia, with a cold stable airmass over N Balkan, Poland and Scandinavia.
The cold front and occlusion pass early in the period over the British Isles, followed by an increasingly unstable airmass. The system of Italy, Greece and Turkey is dynamically the most active.

DISCUSSION

...Greece and Turkey...

Two rounds of storms should pass during this period. The coldest convective cloud tops and largest humidity and precipitable water are associated with the first round, with cold front characteristics. Likely organized into a line, it will affect Greece mostly between 06Z-15Z and western Turkey from 18Z on. 0-3 km SREH of at least 150 m²/s² overlaps the instability and it is not ruled out that storms will occur with very high values of 500-1000 m²/s² around 18Z at the Turkish coast. Storm rotation is likely. Over land, low level vertical shear will be enhanced to 10-15 m/s, enhancing chances for tornadogenesis. Strong low level winds support gusts of 20-30 m/s.
The second round arrives in southern Greece during the late evening and is backed by a strong mid level potential vorticity maximum. A small area of enhanced SREH and bulk shear is present in GFS. Corfidi MCS vectors could reach 25 m/s (Bunkers vectors 20 m/s, also for Turkey) and severe gust chances are elevated, in particular since also upper forcing becomes strong.
Low-level buoyancy may additionally produce tornadoes / waterspouts in the entire level area, and continuous onshore lifting of moist air and embedded storms may produce local flash floods in particular in SW Turkey later in the period.


...southern Italy...

During the first half of the forecast period, a small area of unstable air and large deep convergence signal in GFS moves southeastward along the Italian west coast. Since storm motion and wind speed in the lower levels are large (>25 m/s) severe convective wind gusts may occur, even though the cloud tops are not predicted to be very cold yet (improves later near Greece). It will likely not move onshore until 15Z-18Z.

...France, Belgium...

GFS shows signals of more than 50 J/kg CAPE with little CIN from some lower levels during the passage of the cold front after 15Z. The signals keep existing till the end of the night all the way over Germany, and significant upper support in the form of intense mid level PV values are present. At the front, large 0-2 km convergence is present in GFS. Several models currently show enhanced precipitation for extreme northern France and Belgium. All of this suggests reasonable chance for a strongly forced convective line in the level 1 area which may persist into Germany. The forecast is kept conservative but may be updated. Should convection develop along the front, it will profit from an environment with 25-35 m/s DLS, >15 m/s LLS, and 175-400 m²/s² SREH and very low LCL heights, which is sufficient for bow echos with severe gusts, and tornadoes.

...west coasts of France, Belgium and Netherlands...

There is some potential for thundery showers to cluster during early Wednesday morning (06Z) in the coastal convergence zones if storm motion becomes small and more SW-erly. The convergence should be stronger now that the temperature difference between land and sea is larger.