Storm Forecast
Valid: Mon 06 Mar 2017 06:00 to Tue 07 Mar 2017 06:00 UTC
Issued: Mon 06 Mar 2017 03:01
Forecaster: VAN DER VELDE

A level 2 was issued for SW to SE France and Corsica mainly for severe convective wind gusts and tornadoes.
A level 2 was issued for the west coast of Italy mainly for tornadoes, large hail and convective wind gusts.
A level 1 was issued for the Adriatic coast of the Balkan mainly for excessive convective precipitation and isolated tornado chances.

SYNOPSIS

A large upper trough dominates weather across Europe, except Scandinavia, Baltic region and Iberian Peninsula. At 06Z, a shortwave mid level trough with a surface low lies near Brittany. It lies in a well-defined left exit region of a 500 hPa jet streak of 65 m/s, with strong associated lifting. A warm sector over southwestern France lags behind a mid-level cold front, such that the warm sector is destabilized, and the GFS model computes up to a few hundred J/kg CAPE. 850 hPa wind speeds increase to 30 m/s, and 45 m/s at 700 hPa. The system moves rapidly to the east-southeast.
By 15Z the system arrives to the Mediterranean Sea, and forms a deepening low over Italy in the lee side of the Alps, with steep pressure gradients. The northwestern flow will reach over 35 m/s at 850 hPa between the Provence and Corsica. The mid-level cold front proceeds along the Tyrrhenian coast of Italy while triggering a line of storms, reaching Sicily in the next morning (03-06Z).
Mainly from 00-06Z, the north coast of the Adriatic Sea will see a surge in CAPE when the trough (PV gradient) comes in.

DISCUSSION

...southwestern France...

The mid-level cold front is aligned with the low-level warm front at 06Z, at the southwest coast of France. Convection is likely in the coastal region, and occur under very strong flow (>25 m/s storm motion) and shear conditions (300-400 m2/s2 SREH over 0-3 km). Tornadoes are possible as these values support mesocyclone formation and 0-1 km shear is near 20 m/s. CAPE grows a bit after 09Z over SW to SE France, and the surface cold front may induce a convective line, although convergence signals are only weak. So, isolated cells may be favored, with tornado chances (even marginal large hail) together with severe wind gusts. If a linear system forms, widespread severe gusts should be the main threat. The chance of this increases toward/after 12Z when GFS shows convergence line and solid PV intrusion through central France.

...southeastern France and Corsica...

By 12-15Z the activity should reach the southeast, and near Switzerland the convective precipitation can be locally excessive by orographic enhancement. Further south the model predicts rather dry air which may reduce this. The shear is still extreme, with continued tornado and severe gust threat from storms. As the PV intrusion shifts into the Mediterranean, very strong mid level lapse rates arise, although the models predict little convection apparently due to low level capping (see GFS ML LFC-LCL difference). As pressure gradients and object flow increase around the west side of the Alps, 850 hPa wind speed increases over 35 m/s, so if any convection appears near Corsica, it can enhances the already extreme background 10m wind to produce gusts of such speed.

...Italy...

The mid-level cold front will likely trigger a storm or line of storms moving down the coastline with chances of severe gusts, tornadoes and large hail. MLCAPE increases over the Tyrrhenian Sea over 500 J/kg. By 18Z the line is predicted to be halfway Italy. While the wind speed in the environment and low-level shear will be lower than over France, the 0-3 km SREH of 250-450 m2/s2, supporting mesocyclones that may produce large hail and tornadoes. Further to the south, the front may slow down more, which increases the risk of local excessive convective rainfall.

...Croatia to Albania...

Starting 00Z, CAPE develops in GFS over the northern Adriatic Sea as cold air and lifting arrives to the area. The main threat will be isolated excessive convective rainfall as storms move slower, are helped by orographic lifting of moist air, and can be triggered over a longer period as it remains near the fringes of the PV feature. There is also decent vertical shear values supporting large hail and tornadoes.