Storm Forecast
Valid: Wed 13 Mar 2013 06:00 to Thu 14 Mar 2013 06:00 UTC
Issued: Wed 13 Mar 2013 03:17
Forecaster: TUSCHY

A level 1 was issued for parts of N-Italy mainly for locally heavy rainfall amounts and an isolated tornado risk.

A level 1 was issued for Montenegro and parts of NW-Albania mainly for excessive rainfall amounts.

SYNOPSIS

The long-lived cold-core vortex over the Bay of Biscay finally starts its long anticipated motion to the SE as the major branch of the polar vortex/ high longitude longwave trough continues to build south. This vortex opens up as it rounds the tip of the trough and remains progressive during the forecast as it enters the C-Mediterranean with a positive tilt. This evolution finally creates a more cyclonically curved flow pattern within the subtropical jet, which also causes an increasingly diffluent flow pattern over SE-Europe.

At lower levels, the frontal boundary, which caused havoc over parts of C-Europe in respect to snow and freezing rain during the past few days now shifts south and will be the nucleus for a gradually organizing depression, which becomes a mature storm system over E-Europe during the following days.

North of that boundary (which encompasses all of N-Europe), cold and dry air continues to keep weather quiet regarding thunderstorm development. A few low-end lightning areas were issued, where some daytime driven convection may produce deep enough updrafts for a few short-lived thunderstorms.

DISCUSSION

... Mediterranean ...

A developing depression with constantly falling pressure slowly shifts east and will be placed over the C-Mediterranean during the end of the forecast. The leftover from an eastward pointing subtropical tongue of moist air advects east and affects areas like the Tyrrhenian, Ionian and Adriatic Sea next to Italy. As the progressive and opening cold-core vortex reveals a negative and later-on a neutral tilt throughout most of the forecast period, warm mid-levels atop that moisture plume keep CAPE magnitude in check and hence only 200-500 J/kg MLCAPE are expected. Despite a nice shear/CAPE overlap over Sicily and S-Italy, CAPE seems to be too marginal there for any long-lived DMC as updrafts either become sheared apart due to intense 45 m/s DLS or remain low-topped due to warm mid-levels.

Widespread but non structured thunderstorm development is expected over most of the W/C Mediterranean. We kept the lightning areas north of the subtropical jet, where cold mid-levels will be found over the mild Mediterranean water. Weak shear keeps organized thunderstorm risk low, but slow storm motion may cause heavy rainfall amounts on the local scale. Also, some LL CAPE with weak background winds may result in an isolated waterspout event over the Balearic Islands to the east, before the cold front ushers in a brisk NW-erly flow regime. None of the aforementioned risks seems to be high enough for a level-1.

With the southward surging cold front being delayed by the Alpine rim, a typical N-Italian scenario sets-up with good BL moisture beneath cooling mid-levels as the main upper trough draws near from the NW. Uncapped MLCAPE around 500 J/kg are forecast with daytime driven scattered thunderstorm development. Weak shear assists in rapid upscale growth of numerous disorganized clusters which may produce some heavy rain. This rainfall risk may be enhanced by moderate easterly storm-relative inflow and PWAT of 15-20 mm and this area was highlighted by a marginal level 1. Beside that, an isolated tornado event can't be ruled out given LCLs below 800 m and modest LLCAPE. After sunset, thunderstorms gradually decay with the longest activity confined to the N-coast of the Adriatic Sea.

... S-Adriatic Sea, Montenegro and W-Albania ...

During the overnight hours, the stage is set for an heavy rainfall event as the surface depression approaches from the west. With the onset of a 25 m/s low-level wind maximum, rich BL moisture advects north beneath increasing upper-level divergence. Despite a temporarily weakening trend of the mid-level lapse rates, some cooler air filters in from the west after midnight, which should help to increase the MUCAPE to 100-300 J/kg. This in fact could support a few embedded thunderstorms within a large cluster of stratiform rain. One fact, which could really help to push rainfall amounts upwards will be the capped inflow fetch, with more serious convection over the Ionian Sea not expected to develop before 03Z. Therefore, 100 mm/12h are likely within the level 1 area as a training cluster of showers and embedded thunderstorms is forecast.