Storm Forecast
Valid: Sun 21 Aug 2022 06:00 to Mon 22 Aug 2022 06:00 UTC
Issued: Sat 20 Aug 2022 23:34
Forecaster: PISTOTNIK

A level 1 and level 2 are issued from Poland to Greece and for S Italy mainly for excessive convective precipitation, towards the east (Bulgaria, Romania and W Ukraine) also for severe convective wind gusts and large hail.

A level 1 is issued for NE Spain for large hail.

SYNOPSIS

A deep, steering cyclone is located near the Faroe Islands and surrounded by a fairly strong mid-level jet. An embedded, negatively tilted short-wave trough and an attendant cold front swing northeastward across Norway and Sweden. In their wake, the flow turns more zonal and a pronounced Atlantic warm front approaches the British Isles.
Further south and east, quiescent and warm, towards the SW and SE even hot conditions continue. Slightly lower 500 hPa geopotential stretches from central Europe to the Balkans, opposed by mid-level ridges and surface anticyclones over Russia and west of Iberia.

DISCUSSION

... from Poland to the Balkans, Greece and S Italy ...

Another round of scattered to widespread, disorganized, mostly daytime-driven thunderstorms is expected beneath the shallow mid-level low and at its forward flank. Plentiful low-level moisture allows CAPE up to 1000 J/kg despite rather poor lapse rates, worked over by Saturday's widespread convection. While synoptic lift continues to decrease, the long and wavy, weakening cold front still acts as a convincing mesoscale trigger mechanism.
Weak vertical wind shear around or below 10 m/s keeps storms poorly organized. Excessive rain is the main hazard. Leading storms in the previously "untouched" airmass, most notably in Bulgaria, Romania and the W Ukraine, may still benefit from somewhat steeper mid-level lapse rates, and a few hail and wind events are possible in addition. The highest storm coverage is expected in the level 2 area, where quite numerous large clusters may evolve towards evening.
Towards the south (Italy, Greece and surrounding sea waters), CAPE is higher thanks to abundant maritime moisture, but the capping inversion will also keep storm coverage lower. Nonetheless, most models show scattered and weak precipitation signals also in coastal and offshore areas overnight, which hints at a possibility that the cap might locally be broken by nocturnal land breeze fronts. In this setup, some waterspouts are well possible toward Monday morning (not necessarily requiring electrified convection).

... NE Spain ...

A WNW-erly background flow advects an elevated mixed layer from the mountains on top of abundant low-level moisture, which results in CAPE up to 2000 J/kg (already sampled by Saturday's 12 UTC Barcelona sounding). With a stout capping inversion in place and in the absence of synoptic lift, convective initiation will be isolated and tied to high mountains, but forecast models show remarkable agreement on a few afternoon storms over the eastern parts of the Pyrenees. As soon as they tap into the sea breeze regime with the highest CAPE and up to 20 m/s deep-layer shear, an organization into one or two supercells with a prominent hazard of large hail is expected. Isolated very large hail is not ruled out. Limiting factor that precludes a level 2 is the narrow window (both in space and time) for a successful penetration of the capping inversion.

... Norway, Sweden ...

The fast-moving trough exerts strong lift upon a neutral to slightly unstable airmass. Scattered, low-topped and weakly electrified showers are expected to cross much of Norway and Sweden during the forecast period. Moderate 0-3 km shear between 10 and 15 m/s allows multicellular organization. A few marginally severe wind gusts are not ruled out.