Storm Forecast
Valid: Sat 14 Jun 2014 06:00 to Sun 15 Jun 2014 06:00 UTC
Issued: Sat 14 Jun 2014 00:14
Forecaster: TASZAREKT/GATZEN

A level 1 was issued for NW Italy and surroundings mainly for excessive precipitation, large hail and severe wind gusts.

A level 1 was issued for north-eastern Spain mainly for large hail and severe wind gusts.

A level 1 was issued for the central Mediterranean mainly for excessive precipitation and large hail.

A level 1 was issued for Ukraine/Russia border mainly for large hail, severe wind gusts and tornadoes.

A level 1 was issued for Greece, Albania, Macedonia, Bosnia and Herzegovina, Montenegro and parts of Bulgaria mainly for excessive precipitation and large hail.

SYNOPSIS

Atlantic ridging supports a northerly flow over most of Europe. A polar trough that has moved into western Russia is associated with an intense short-wave trough axis extending from Belarus over Germany into France that will slowly dig southward. It will be located over southern France and the Alps on Saturday.

At lower levels, relatively dry air masses continue to spread south over Europe. Rich moisture remains to the south of the Alps and over the Mediterranean and Black Sea region. This moisture will partly overlap with steep lapse rates over the Mediterranean. Steep lapse rates will also evolve over the Alps in response to diurnal heating.

DISCUSSION

Western Po Valley

Main synoptic-scale feature will be a sharp positively tilted trough axis moving south into southern France and the Alps. It will provide mid-level vorticity advection and cooling that is expected to spread into the area at mid-levels. Diurnal heating will provide steep lapse rates over the Alps that spread southward into northern Italy in the afternoon.

Cyclogenesis is forecast over the Gulf of Genoa that supports an easterly flow over northern Italy, leading to moisture flux convergence that increases the low-level humidity to more than 11 g/kg. The overlap of this moisture with the Alpine lapse rates will allow for CAPE in the order of 1500 J/kg.

Current thinking is that initiation will be in the late morning over the Alpine mountains. Outflow-boundaries are expected to spread into the Po Valley and initiation becomes likely in the afternoon and evening hours. Weak vertical wind shear and small hodographs are indicated by latest GFS model run, given weak northerly winds atop of easterly surface winds. Nevertheless, supercells are not ruled out, capable of producing large hail and severe wind gusts. Due to the slow storm motion, excessive precipitation is expected to be a threat. Storms are forecast to merge and clusters of storms may go on well into the night. Severe wind gusts and excessive rain seem to be the main threat.

North-eastern Spain

The short-wave trough axis will approach over north-eastern Spain in the afternoon, providing QG lift due to mid-level vorticity advection. Daytime heating over Iberia and resulting thermal low will provide a favourable setup for deep moist convection, with easterly moist surface flow, steep mid-level lapse rates, and 15 m/d deep layer vertical wind shear. Upslope flow will support initiation, and a few supercells are forecast, capable of producing large hail and severe wind gusts. Storms may spread into the Mediterranean Sea during the night hours, when severe threat will decrease.

Central Mediterranean

Rich low-level moisture below and elevated mixed layer will support CAPE in the order of 1000 J/kg. Initiation is expected along sea-breeze convergences over Italy. Weak flow will pose a threat of excessive precipitation, whereas large hail seems to be a limited factor. Storms are forecast to cluster and weaken after sunset.

Initiation is somewhat questionable aver Corse, Sardinia, and Sicily due to a stronger capping inversion. Isolated initiation is forecast, though. Large hail and excessive rain will be the main threat.

Northern Ukraine

This area is associated with position of jet streak and high values of vertical wind shear. Daytime heating will likely create CAPE in the rather moist low-level air mass near the Russian border. Given DLS > 20m/s supercells are expected to occur. Large hail and severe wind gusts are not ruled out. Due to the strong low-level vertical wind shear, an isolated tornado event cannot be ruled out.

Greece, Albania, Macedonia, Bosnia and Herzegovina, Montenegro and parts of Bulgaria

Similar as in central and southern Italy, rich boundary-layer moisture which is pushed to the south, will overlap with dry mid-levels from Africa and create CAPE up to 1000 J/kg. Although storms will form in low sheared environment, large hail is expected to be the main threat in this region. Inverted-v profiles will also increase the probability of severe wind gusts. High CIN values in S Greece will be the main limiting factor for convection in this region. NE flow and convergence over the Dinaric mountains may also result in excessive precipitation.