Storm Forecast
Valid: Thu 21 Nov 2013 06:00 to Fri 22 Nov 2013 06:00 UTC
Issued: Wed 20 Nov 2013 22:59
Forecaster: TUSCHY
A level 1 was issued for parts of Corsica, Sardinia, parts of Italy, parts of the NE/E-Adriatic Sea, the Ionian and the Aegean Sea mainly for excessive rain, gusty winds and an isolated tornado/waterspout risk.
A level 1 was issued for the Ionian Sea, where in addition to the rain risk, large hail is also forecast.
SYNOPSIS
A major branch of the polar vortex becomes attached to Europe. Various more or less pronounced disturbances circle around that feature and cause unsettled conditions over most of Europe. The influence of the E-Europe high gradually diminishes from west to east.
In the lower troposphere, the main story will be a southbound moving cold front, which exits S France around noon and crosses most of the W/C Mediterranean thereafter. Not much thunderstorm activity is anticipated however.
DISCUSSION
... W/C Mediterranean ...
Widespread shower and thunderstorm activity is anticipated beneath an eastward shifting cold thermal trough and attendant mid/upper trough axis during the daytime hours and again with an approaching cold-core vortex from France during the overnight hours. At lower levels, a broad vortex encompasses most of the W/C Mediterranean.
No BL moisture recovery from the last intrusion of cold air was possible so only modest moisture will be available. Shear and CAPE fields will be separated for most of the time, so the main risk will be marginal hail, gusty winds and heavy rain. The highest rainfall risk exists along westward facing slopes of Corsica, Sardinia and Italy. In addition, 15 m/s LL flow along the SE fringe of the consolidating surface depression over the NW Mediterranean points to NW/W Italy with a gradual shift betimes to the south. This could result in temporal back-building storms with excessive rain. All those areas were upgraded to a level 1. Given recent heavy rainfall amounts in many of these regions, flash flooding will be well possible.
A similar hazard exists with repeatedly onshore moving showers and isolated thunderstorms along the NE and E coasts of the Adriatic Sea. Once again, Montenegro and Albania seem to be the main area of concern regarding excessive rainfall amounts of up to 100 mm/24h.
... Ionian Sea, Aegean Sea and Greece ...
Ample of forcing will exist with a NE-ward sliding upper trough and a gradually structuring surface low, which cross the area of interest during the evening and overnight hours from SW to NE.
Until 18Z, scattered to widespread showers and thunderstorms are forecast mainly over the Ionian Sea. Overlap of 15-20 m/s DLS and 400-800 J/kg MLCAPE cause numerous multicells with an isolated large hail, strong wind gust and heavy to excessive rainfall risk. Convection over the Aegean Sea remains more isolated and less severe.
During the evening hours, thunderstorm probabilities increase over the S-Ionian Sea and a cluster of storms moves rapidly to the NE. This activity will have the best access to rich BL moisture and widespread heavy to isolated excessive rain will be the main hazard ... approaching the Aegean Sea beyond midnight. Parts of Greece were added to the level 1as active convective period (offshore) probably also results in high rainfall amounts over S/C parts of Greece with numerous onshore moving shower/thunderstorm clusters.
For all the Mediterranean: Cool mid-levels and seasonable BL moisture result in enhanced LL CAPE for the complete area. A few waterspout reports are possible
... SW France ...
Marginal prefrontal BL moisture and moderate LL shear exist ahead of the approaching occlusion, which crosses the area of interest during the evening hours. An isolated tornado event can't be ruled out, but degree of destabilization remains too uncertain for upgrading.
