Storm Forecast
Valid: Sat 05 Apr 2014 06:00 to Sun 06 Apr 2014 06:00 UTC
Issued: Fri 04 Apr 2014 22:37
Forecaster: TUSCHY

A level 1 was issued for Croatia and parts of Slovenia/SW Hungary mainly for excessive rain.

A level 1 was issued for parts of Greece, Macedonia and W-Bulgaria mainly for large hail and excessive rain. Strong to isolated severe wind gusts will be also possible on an isolated scale.

A level 1 was issued for parts of S-Italy mainly for excessive rain.

SYNOPSIS

A split-flow pattern continues over Europe as ridge-over-low forces jets to swerve to N-Europe and N-Africa. Upstream of a progressive and pronounced trough over Russia, a gradual increase in mid-level heights forces a cut-off over the CNTRL Mediterranean to stay put. Brisk westerlies start to affect far W-Europe.

At the surface, a moist SE-erly flow affects the Balkan States along a decaying occlusion, while latest MIMIC data confirms a plume of modified subtropical air (TPW aoa 30 mm) to spread east, affecting the Iberian Peninsula, W-France and UK/Ireland during the forecast.


DISCUSSION

... Germany to Greece ...

A plume of warm and moist air extends from CNTRL all the way to SE Europe. Mid-tropospheric air mass remains trapped in large-scale blocking pattern, so steep lapse rates remain atop that moisture plume. Increasing low/mid tropospheric convergence and diurnal driven evapotranspiration increase the BL moisture content, which counteracts slowly weakening mid-layer lapse rates regarding CAPE build-up. Storm motion around 10 kt with effective PWs in the range of 15-20 mm and 0-6 km bulk shear of 10 m/s or less all indicate a risk of slow moving and rapidly clustering storms with heavy rain and isolated large hail the main risk....the hazard will be maximized in the 50-% lightning areas. Although driven by diurnal heating, ongoing storms will last well into the night as complex outflow-outflow and outflow-orography interaction may trigger new storms in a weakly capped/unstable air mass. A level-1 was added for Croatia, parts of Slovenia and Hungary, where depth and thickness of the forecast CAPE profiles might support isolated large hail or large amounts of small hail but also locally very heavy rainfall amounts.

Greece, Macedonia and parts of Bulgaria still reside beneath a 20 m/s mid-layer jet , which keeps DLS in the 15-20 m/s range ... although relaxing betimes. This favors better organized multicells with large hail and strong wind gusts. During the overnight hours, the risk for organized storms shifts to the NW-Aegean Sea, as fetch of moist SE-erly winds beneath moderate upper divergence increases. Temporary training storms are possible along the NW coast of the Aegean Sea.

An isolated waterspout risk exists over the CNTRL Mediterranean with most promising conditions being forecast over Sicily and along the W-coast of CNTRL Italy. As the center of the vortex crosses far S-Italy from WNW to ESE, scattered slow moving storms could bring heavy to excessive rain....especially if persistent W-E aligned convergence zone in the eastern part of the level-1 verifies.

... NW-Spain ...

Despite a ridge sitting over Spain, weak disturbances graze far NW Spain while moving from the Atlantic to the Bay of Biscay. Onshore flow of modified subtropical air with meager mid-layer lapse rates covers the area of interest. Either diabatic heating, persistent SW-erly upslope flow or the weak mid-layer impulses help for isolated to scattered CI. Convection remains non-severe.

Sporadic and non-severe thunderstorm initiation is possible in the frontal zone over UK/ Ireland, as numerous mid-layer impulses cross that baroclinic zone.