Storm Forecast
Valid: Tue 08 Oct 2013 06:00 to Wed 09 Oct 2013 06:00 UTC
Issued: Mon 07 Oct 2013 21:07
Forecaster: PISTOTNIK

A level 2 was issued for the Ionian and S Adriatic Sea for excessive precipitation, tornadoes, severe wind gusts and large hail.

A level 1 was issued for the Tyrrhenian Sea and the N Adriatic Sea mainly for waterspouts and excessive precipitation.

A level 1 was issued for much of Italy mainly for excessive precipitation.

SYNOPSIS

A temporarily zonal flow stretches across the Northern third of Europe. An embedded short-wave trough moves from the North Sea to Finland during the forecast period. A second, more pronounced trough approaches the British Isles overnight and chokes off the zonal flow again. Cold air advection starts to increase behind the first trough and escalates to an outbreak of polar air behind the second trough. In response to QG lift ahead of the troughs, lapse rates steepen enough to allow patches of marginal CAPE over Southern Scandinavia and Scotland, but equilibrium levels are thought to be mostly too low for cloud electrification.
Meanwhile, the Southern two thirds of Europe see quiescent synoptic conditions with weak geopotential gradients everywhere. An old cut-off low still sits on top of Italy and prolongates the current severe storm episode over the Central Mediterranean region.

DISCUSSION

... Level 2 area: Ionian and S Adriatic Sea, W Balkans ...

A tongue of particularly moist air with surface dewpoints between 18 and 20°C is advected northward, overspread by at least moderately steep lapse rates from the Southwest with a mid-tropospheric wind speed maximum. Resulting CAPE and deep-layer shear increase from roughly 1000 to up to 2000 J/kg and from 15 to 20 m/s, respectively, within the level 2 belt. The weak warm air advection is also associated with slightly enhanced storm-relative helicity.
While upper levels show only patchy lift support, the low-level fields reveal a striking convergence zone which stays stationary throughout the forecast over the Ionian Sea and moves slowly eastward over the Adriatic Sea. Numerous rounds of large thunderstorm clusters are expected along this convergence zone, and with the training activity excessive precipitation is certainly the main risk again. Most of the storms will occur over water, but they may well graze Apulia, possibly also Calabria and Sicily. Embedded supercells are also possible, especially with tail-end storms, and they pose an additional risk of large hail, severe wind gusts and tornadoes (low-level shear is not particularly enhanced, but cloud bases are very low and buoyancy starts close to the surface).
As the forecast period proceeds, storms will also get more and more widespread over the Southern and Central Adriatic Sea, moving onshore along the coast of the West Balkans overnight with similar severe weather risks.

... Level 1 area: Corsica, Tyrrhenian Sea, Italy, N Adriatic Sea ...

Uncapped CAPE in the 500 to 1000 J/kg range will be available again over the warm water, and numerous vorticity maxima pivot around the center of the upper-level low and enhance convective initiation. Scattered to widespread, unorganized thunderstorms are forecast. With plenty of low-level buoancy and a very cyclonic wind field, a fairly large number of waterspouts is expected again, especially in the morning hours between Corsica and Central Italy and over the Northern Adriatic Sea. Afterwards the parent vortex starts to fill up and to move eastward, but the spout risk remains enhanced till the end of the forecast period.
Much of mainland Italy was included into the level 1 as well: storms may move onshore or, at least in the noon to afternoon hours, may also form over land. Localized excessive precipitation is possible with this activity.