Storm Forecast
Valid: Fri 09 Oct 2015 06:00 to Sat 10 Oct 2015 06:00 UTC
Issued: Thu 08 Oct 2015 20:44
Forecaster: TUSCHY

A level 2 was issued for N-Algeria/Tunisia, Sicily, parts of Italy and the Tyrrhenian Sea mainly for damaging wind gusts, excessive rain, large to very large hail and tornadoes. A significant tornado event is possible over Sicily.

A level 1 surrounds the level 2 with similar hazards but less coverage and less intense.

SYNOPSIS

Most parts of Europe feel the influence of an extensive high pressure area, centered over Norway and Sweden. A strong mid/upper-level cyclonic vortex remains anchored over W-Russia and ensures ongoing influx of cold air to the south. Along the leading edge of that continental air mass, a strong moisture gradient has established from NE Germany to the N-Black Sea, which tends to shift a bit more to the W/SW during the forecast.
Further west, from Spain to the UK, ingredients like moisture and mid-level lapse rates remain poorly defined and with westward building surface high from Scandinavia, no thunderstorm activity is forecast.
Maritime convection beneath a strong cold-core low northwest of Scotland could produce a few lightning strikes, but activity should stay north of our forecast area.
Ex-Joaquin also approaches the western part of our forecast domain, but attendant moisture plume remains offshore until Saturday 06 UTC. Interesting to see ongoing shallow warm-core structure with tight low-tropospheric gradient and ongoing gale-force winds. Overall weakening trend will continue.

Pattern recognition features a classic event for the S-CNTRL Mediterranean regarding an outbreak of organized thunderstorms. Positive tilted trough over the W-Mediterranean acquires a neutral and during the end of the forecast even a slightly negative tilt while moving east. Substantial strengthening of the trough is forecast during the overnight hours, especially beyond 00Z. A sharp baroclinic zone, which runs from E-Algeria to N-Tunisia to Sicily, serves as focus for LL cyclogenesis. Not a big surprise to see ongoing model discrepancies about the final structure of the LL vortex. Local models even show two unique vortices evolving along that boundary and affecting the Tyrrhenian Sea during the forecast.

DISCUSSION

... Sicily, Sardegna and the Tyrrhenian Sea ...

Latest IR images indicate the front over NE Algeria/NW Tunisia (as of 20 Z) with strong BL convergence and impressive DMC activity. Not much eastbound progression is forecast.

During the daytime hours, the evolving low south of Sardegna steers a plume of well mixed air at mid-levels (EML) towards Sicily. EML spreads offshore over a very moist low-tropospheric air mass and results in strong CAPE values. MLCAPE exceeds 2 kJ/kg.

06-18 UTC:

Latest idea is that bulk of daytime activity occurs along the cold side of the front and north/west of the developing cyclone, which is placed south of Sardegna. That's the region with strongest BL convergence. This air mass is less capped and abundant forcing shoud assist in widespread CI from Algeria to Sardegna. 20 m/s DLS and 1-2 kJ/kg MLCAPE result in organized DMC with large hail, strong to severe wind gusts and excessive rain. Would not be surprised to see training convection over S/E Sardegna with 25-30 kt S/SE-erly inflow.
Enhanced LL shear along the coastal areas is already enough for a tornado threat.
Further to the SE (e.g. Sicily), elevated convection along NE-ward spreading warm front may already become organized with 1-2 kJ/kg MLCAPE and 20 m/s DLS. Strengthening cap with strong low-tropospheric WAA should limit the wind gust and tornado risk. Large or very large hail and excessive rain will be the main hazard.
Over NE-Algeria, slow moving clusters with all kind of severe are forecast. A significant flash flood risk exists!

18-06 UTC:

Structuring vortex moves towards Sardegna/Tyrrhenian Sea with a developing broad warm sector covering an area from Tunisia to S-Italy. Warm sector features capped extreme CAPE values (MLCAPE of 3-4 kJ/kg) with steep lapse rates atop 16 g/kg mixed-layer mixing ratios. Around sunset, elevated thunderstorms continue to spread NE towards the Ionian Sea with an ongoing hail risk. Thereafter a temporal lull in activity is forecast (until 00Z).
Strong synoptic lift approaches Sicily beyond 00Z which should support a rapidly eroding cap and increasing chances for CI. MLCAPE in excess of 3 kJ/kg, 20 m/s 0-3 km shear and SRH-1 in excess of 300 m^2/s^2 will overlap and supercells with very large hail, excessive rain, damaging wind gusts and tornadoes are anticipated. A significant tornado event can't be excluded as forecast hodographs show a looped signature with 50 kt at 800 hPa around 03Z over W-Sicily (GFS).
This activity spreads E/NE until 06Z and affects S-Italy, too. A level 3 was strongly considered, but timing issues of DMC activity and ongoing model discrepancies in coverage of storms kept this event in an high-end level 2.

Further north over the Tyrrhenian Sea and Sardegna, widespread organized DMC activity, most likely in form of numerous intense clusters, continues all night long. 2 kJ/kg MLCAPE and 20 m/s DLS/3 km shear favor well organized convection with large/very large hail, excessive rain, severe wind gusts and an isolated tornado risk. CNTRL Italy will be affected during the end of the forecast and also over the S-Adriatic Sea.