Storm Forecast
Valid: Mon 11 Oct 2010 06:00 to Tue 12 Oct 2010 06:00 UTC
Issued: Sun 10 Oct 2010 21:32
Forecaster: TUSCHY
A level 2 was issued for Sicily, S-Italy and the N-Ionian Sea mainly for excessive rainfall, large hail, severe wind gusts and tornadoes.
A level 1 surrounds the level 2 with a more limited risk. The primarily hazard will be heavy rainfall.
A level 1 was issued for the Balearic Islands and SE-wards mainly for excessive rainfall amounts. Strong wind gusts, large hail and an isolated tornado event are also possible with more discrete storm structures.
SYNOPSIS
An intense trough over NE Europe builds southeastwards with strong CAA affecting a large area. There are signals for some convection over N-Norway/Sweden, but updraft strength looks too weak for longer-lived thunderstorms which could take profit of intense shear. Another, major upper low over the western Mediterranean remains in place and causes widespread shower/thunderstorm activity. Stable conditions prevail over NW-Europe.
Tropical storm Otto just north of the Azores probably acquires enough post-tropical characteristica for extratropical transition although phase diagrams still indicate a chance for a deeper warm core structure. Nevertheless, no heavy rain risk affects the Azores during the upcoming 24 hours.
DISCUSSION
... Tyrrhenian Sea, Sicily, S-Italy, parts of the S-Adriatic Sea and the N-Ionian Sea ...
As the upper low over the Iberian Peninsula digs southwards, a potent mid-/upper jet ejects out of its base and builds rapidly eastwards. Adjacent geopotential height falls over the area of interest are weak to non-existent, as mid-/upper ridge slides eastwards. However, pressure decreases beneath 3km with a very moist and warm BL air mass in place, so the environment sets up for a continued risk of excessive rainfall and severe.
Current thinking is that a large mass of DMC (north-south aligned) moves from the Tyrrhenian Sea (morning hours) eastwards with a more stratiform part affecting the offshore areas of the Tyrrhenian Sea, the Adriatic Sea and central Italy. Larger scale isentropic ascent above a shallow, mixed continental air mass (central/N-Italy) support an heavy rainfall risk for those regions, probably with embedded thunderstorms as MUCAPE decreases gradually to the north.
The southern part of this area of DMC however reveals the best chance for organized severe, including tornadoes, large hail, severe wind gusts and excessive rainfall. We highlighted that area with a level 2, due to the favorable connection to a very moist air mass south of Sicily, a strong LLJ at 850 hPa pointing northwards and with 20-25 m/s 3 km / 6 km bulk shear in place. A V-shaped MCS is forecast and areas next to its southern end (probably Sicily and extreme S-Italy) may see excessive rainfall due to the slow eastward propagation of this MCS. Otherwise, all kind of severe is possible within this level 2.
... The Balearic Islands ...
Placed near the eastern fringe of the upper low, showers/thunderstorms are forecast already from the beginning onwards. Shear remains weak with meager CAPE, so despite a few strong thunderstorm events, nothing severe is foreseen. However, conditions for organized DMC become more supportive after sunset with another depression consolidating over N-Africa. This causes a quasi-stationary west-east aligned boundary over the Balearic Islands and increasing thunderstorm probabilities just off the coast of Tunisia. It is not yet clear how far to the north thunderstorms spread and what the exact timing of this event will be, but conditions for a large, potentially backward building MCS SE of the Balearic Islands are supportive. Heavy to excessive rainfall may affect the Balearic Islands during the morning hours, but right now, we went with a broad level 1 to reflect the still existing model discrepancies. An upgrade may be needed, if new model data confirm the possibility of a near stationary, back-building MCS with favorable inflow trajectories from a very moist air mass to the east.
