Storm Forecast
Valid: Thu 18 Jul 2013 06:00 to Fri 19 Jul 2013 06:00 UTC
Issued: Wed 17 Jul 2013 23:40
Forecaster: GATZEN

A level 1 was issued for north-eastern Spain mainly for large hail.

A level 1 was issued for north-eastern Algeria and northern Tunisia mainly for large hail.

SYNOPSIS

The mid-level ridge extending over north-western Europe will strengthen and forms a closed high that slowly moves eastward across the British Isles. Progressive troughs will affect northern Europe. Further south, only weak mid-level flow is expected through-out the period except stronger westerly winds over northern Africa.

DISCUSSION

North-eastern Spain

Latest Barcelona sounding indicates an unstable air mass due to steep mid-level lapse rates and rich low-level moisture. On Thursday, not much change is expected as easterly winds will evolve in response to the diurnally driven thermal low across Iberia. CAPE in the order of 1500 J/kg will be possible. Along the sea-breeze convergence and due to upslope flow, another round of storms is forecast. Given the easterly low-level winds and weak westerly flow aloft, vertical wind shear will increase during the day. In the lowest 3 km, 10 to 15 m/s bulk shear and some veering will be possible. Storms may organize into multicells or supercells, capable of producing large hail. Later on, cold pool driven storm clusters may also produce an isolated wind gust. Storms will weaken soon after sunset.

Northern Algeria and Tunisia

Mid-levels will remain relatively cold during the period. This allows for CAPE given rich Mediterranean moisture advected inland combined with strong diurnal heating. Storms are expected to initiate over the mountains where the capping inversion will become weaker in response to the heating. Although the flow and QG forcing will be weaker compared to Wednesday, some intense storms may be possible given the moderate deep layer vertical wind shear that can increase to 20 m/s. Large hail seems to be the main threat, although a severe wind gust is not ruled out especially in the southern portions due to the deep dry boundary layer.

Southern France, north Mediterranean, Italy, northern Balkans

At the southern flank of the north-west European high, a region with relatively weak mid-level geopotential extends from the Bay of Biscay across southern France and northern Italy into the northern Balkans. Main low-level feature is a zone of convergence driven by the sea-breeze and thermal lows that evolves during the daytime. Onshore winds can advect adequate low-level moisture originating from the Mediterranean Sea into southern France, northern Italy and portions of the Balkans. This moisture can overlap with rather steep lapse rates originating from the Iberian Peninsula that have spread across the west Mediterranean. Moderate CAPE up to 1000 J/kg will form in the morning hours and storms will start along outflow boundaries quite early. During the afternoon, convective activity will likely concentrate along the sea-breeze convergence zone, where clusters of storms can result. Additionally, storms are quite likely over the Massif Central, the western and central Alps, and the Apennines. Across western and central France as well as from the eastern Alpine region to the Balkans, weak low-level moisture will limit the chance of thunderstorms.

Due to the weak flow, most storms are forecast to be weakly organized. Pulse storms may be capable of producing marginally large hail and excessive precipitation. Storm clusters may also produce an isolated severe wind gust. During the night hours, convection will weaken again.

Baltic States

Ahead of an approaching mid-level trough, a tongue of moist air spreads into the Baltic States that allows for CAPE during the day. Along a frontal boundary that moves onshore in the late morning hours, thunderstorms will likely form. Main focus will be the northern portions where strongest QG forcing is expected. Additionally, vertical wind shear will be best over the northern parts in the range of a rather strong westerly low-level jet. Therefore, a few multicells may be capable of producing marginally severe hail. The threat seems to be too low to issue a level 1 threat, though.