Storm Forecast
Valid: Tue 11 Nov 2014 06:00 to Wed 12 Nov 2014 06:00 UTC
Issued: Mon 10 Nov 2014 21:51
Forecaster: TUSCHY

A level 2 was issued for S-France and NW Italy mainly for excessive rain.

A level 1 surrounds the level 2 mainly for excessive rain and an isolated tornado risk.

A level 2 was issued for parts of S-Italy mainly for excessive rain, tornadoes and large hail.

A level 1 surrounds the level 2 for similar risks but also for an isolated severe wind gust risk.

A level 1 was issued for SW Spain mainly for excessive rain and an isolated tornado event.

SYNOPSIS

A sharp and negative tilted trough rotates from NE Algeria/N Tunisia towards the Tyrrhenian Sea. This feature weakens during the evening/overnight hours as environmental geopotential heights start to increase.
Another trough over the Bay of Biscay deepens and amplifies to the south. It acquires a positive tilt, too and approaches the far W-Mediterranean during the night.
Further east, over parts of CNTRL and E-Europe, numerous ridges and a stout surface high cause quiet conditions regarding organized DMC.

DISCUSSION

... Malta, S-Italy and the Adriatic Sea ...

As the sharp trough rotates to the NE, a plume of steep mid-layer lapse rates (up to 8K/km) spreads north and affects the area of interest. Trajectories of the low/mid layers emerge off N-Africa and hence quality of air mass (regarding moisture content) will be of concern. Near surface mixing ratios of 10 g/kg will be enough for modest to strong CAPE build-up as EML spreads atop. 1-2 kJ/kg MLCAPE is forecast.
The trough's passage is accompanied by strong winds at low/mid levels with winds in general aoa 25 m/s for the lowest 2-3 km. DLS oscillates around 25 m/s, before it weakens from W to E during the forecast.

Forcing affects Sicily during the morning hours from SW to NE before it subsides around sunset. Conditions for organized DMC are pretty favorable with strongly backed LL flow and decent shear. Especially conditions for tornados look favorable with LL CAPE in excess of 200 J/kg, SRH-1 of 150-200 m^2/s^2 and LCLs in general less than 1000 m. A mixed storm mode is possible. 20 m/s SE-erly LL flow may feed a MCS between NE Tunisia and W-Sicily during the start of the forecast, which gradually lifts to the NE. Tail-end storms could bring excessive rain to parts of W-Sicily. A tornado, large hail and severe wind gust threat also exists.
For the rest of Sicily, models still diverge regarding initiation (mainly due to capping issues, the influx of drier air from the south and the uncertainty of how serious any MCS development could affect Sicily until noon). However, forcing should be enough for isolated to scattered (discrete) thunderstorms, which rapidly gain organization. All kind of severe is possible, including an isolated strong tornado event. Large hail and severe wind gusts are also possible. Strong BL flow may partially offset storm motion and hence back-building storms with excessive rain are forecast.

This risk spreads east towards parts of S-Italy with an ongoing tornado/hail and severe wind gust risk.

Malta was kept in a level 1 due to the chance for storms to build back to the south. Limited time and ongoing advection of dry African air keep the chance for CI on the lower end side.

During the night, thunderstorm activity also increases over the Adriatic Sea. Widespread thunderstorms with excessive rain, large hail and an isolated tornado event are forecast. Weakening DLS/forcing keep this part of the forecast in a level 1.

... NW Italy and S-France ...

During all of the forecast period, NW Italy resides ahead of a gradually approaching trough from the SW. A prolonged period with strong onshore flow is forecast and winds in the lowest 2 km keep their strength of 15 m/s. Offshore air mass is not very moist with mixed-layer mixing ratios of less than 10 g/kg, but due to cool mid-levels up to 1000 J/kg SBCAPE is forecast. Forecast soundings show a chance for surface based convection along the coasts. Up to 100-200 l/qm in 24 h are possible.

Further north, excessive rain is likely due to persistent upslope flow along the SW Alps, but hostile conditions for thunderstorms keep this event out of our level areas.

During the night, widespread thunderstorms are forecast from NE Spain to NW Italy as the trough from the Bay of Biscay approaches. Heavy and locally excessive rain affects S-France as coastal convection starts to train. This risk and also an isolated waterspout risk extend from S-France all the way to the Balearic Islands.

... Spain, Bay of Biscay, the English Channel and surrounding regions ...

Until noon, a prolonged period with heavy rain affects far SW Spain as warm/cold front race east. Impressive dynamics and favorable geometric placement of the mid/upper level jet induce an augmented tornado risk for far SW-Spain, which resides in a strongly sheared warm sector until noon. Severe wind gusts are also likely due to the very strong winds in the lower troposphere. This risk lessens around noon.

Thereafter, a well mixed marine air mass affects Portugal and most of Spain. At peak heating, stronger storms could pose an isolated large hail threat. Complex orography may also induce an isolated funnel/tornado threat. This risk rapidly diminishes around sunset.

Scattered thunderstorms also affect the Bay of Biscay all the way to SW-UK and Ireland. Despite strong wind gusts and graupel no severe risk is forecast.