Storm Forecast
Valid: Sun 25 Aug 2019 06:00 to Mon 26 Aug 2019 06:00 UTC
Issued: Sun 25 Aug 2019 00:50
Forecaster: PISTOTNIK

Level 1 and level 2 areas are issued for parts of NE Algeria, NW Tunisia, Corsica, Italy, Austria, Germany, the Czech Republic, Slovakia, Romania, Hungary, Slovenia, Croatia, Bosnia-Herzegovina, Montenegro, Albania, Georgia and SW Russia mainly for excessive convective precipitation and large hail. In addition, non-mesocyclonic tornadoes (waterspouts) are possible in coastal areas.

A level 1 is issued for parts of Portugal and W Spain for large hail, severe convective wind gusts and excessive convective precipitation.

SYNOPSIS

A broad, subtropic ridge filled with very warm to hot air covers most of Europe. It is flanked by small cut-off lows near Ireland and Portugal and by a long-wave trough over Russia. The main frontal zone is deflected to the far north and runs across the Norwegian Sea and Scandinavia.

DISCUSSION

... Portugal, W Spain ...

Diurnal heating and synoptic lift create some hundred J/kg CAPE under 10-15 m/s deep-layer shear. Scattered storms will form in the afternoon and evening. An organization into multicells is likely, and isolated large hail and severe wind events are possible, especially in the interior of the Iberian Peninsula, where lapse rates are steepest. Convection will drift northward and may survie into the night, possibly growing into one or two clusters. Additional storms can also initiate further south at outflow boundaries or zones of enhanced mesoscale ascent ahead of the cut-off low. The severe weather risk, while overall decreasing, shifts to isolated heavy rain overnight.

... other level 1 and level 2 areas ...

Rich low-level moisture in the warm airmass and strong insolation under quiescent synoptic conditions allow the build-up of 500-2000 J/kg CAPE. Even more CAPE around 3000 J/kg (see the Sat 12 UTC Brindisi and Trapani soundings), albeit strongly capped, will be present over the Tyrrhenian, Adriatic and Ionian Sea.
Another round of scattered to widespread thunderstorms is expected mainly in the afternoon and evening and over orographic features. Under weak vertical wind shear, storms will be single- to multicellular and will likely grow upscale into large clusters with erratic, outflow-driven motion. Activiy will peak in the afternoon and evening and over orographic features, but a few storms at outflow boundaries or other convergence zones may also be active over flat terrain and at any time of the day. Flash floods and moderately large (2-4 cm sized) hail are the main risks. The level 2 areas face the highest flash flood risk due to most plentiful low-level moisture and preconditioned soils due to a history of heavy downpours over the past few days.
Storms propagating to coastal and offshore areas along nocturnal land breeze fronts may spawn some waterspouts.

... central Sweden ...

A short-wave trough and the cold front of a cyclone over the Norwegian Sea move east. Scattered showers or weakly electrified thunderstorms will form in the afternoon and evening in an environment of strong synoptic lift, marginal CAPE and strong 0-3 km shear around 15 m/s. Multicells or transient supercells might bring a few marginally severe wind or hail events and an isolated tornado. Limiting factor against a level 1 is the low depth of the convective clouds and a possible lack of CAPE.