Storm Forecast
Valid: Sun 02 Jul 2023 06:00 to Mon 03 Jul 2023 06:00 UTC
Issued: Sat 01 Jul 2023 13:15
Forecaster: PISTOTNIK

A level 1 is issued for North Macedonia, N Greece and parts of Bulgaria for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for parts of Albania, Serbia, Romania and the Ukraine mainly for excessive convective precipitation.

A level 1 is issued for Denmark, S Sweden, the S Baltic Sea, Lithuania and Latvia for severe convective wind gusts.

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

A level 1 is issued for parts of Spain for severe convective wind gusts.

SYNOPSIS

An extensive area of low 500 hPa geopotential stretches between Karelia, Denmark and Iceland. A strengthening mid-level jet covers its southern flank from the British Isles towards Belarus with an axis between 50N and 55N . Several pronounced short-wave troughs are embedded in this flow, but their lift interacts with stable to only marginally unstable maritime air.
Near the surface, a diffuse and largely stationary frontal zone runs from Belarus to France. Its placement on the anticyclonic side of the jet stream awards it an "ana front" character with limited weather activity.
Quiescent synoptic conditions previal over the southern half of the continent. Slightly lower 500 hPa geopotential with somewhat cooler air and unsettled weather gradually shifts eastward from the central Mediterranean region and the Balkans towards the Black Sea, followed by a gentle mid-level ridge with hotter air building over SW Europe.

DISCUSSION

... Balkans into Ukraine and Belarus ...

A diffuse mid-level low provides an environment of rich low-level moisture, rather poor lapse rates (yielding CAPE on the order of a few hundred to more than 1000 J/kg) and mostly weak vertical wind shear. Scattered to widespread, disorganized and mostly daytime-driven thunderstorms are expected, and upscale growth into some larger clusters is possible. The main hazard is excessive convective precipitation, which may lead to some flash floods in mountainous terrain. Towards the south (North Macedonia, the Greek mainland and Bulgaria), somewhat steeper lapse rates and 10-15 m/s deep-layer shear may also promote multicellular storm organization with additional hazards of large hail and perhaps a few localized downbursts.

... Denmark and S Sweden into Baltic states ...

Lively shower activity is expected in the well-mixed maritime airmass on the cool side of the main frontal zone, daytime-driven over land and throughout the forecast period over water. The deepest shower cloouds may produce limited amounts of lightning. The southern part of the level 1 area overlaps with increasing vertical wind shear beneath the jet stream (10-15 m/s across the 0-3 km layer, 15-20 m/s across the 0-6 km layer), therefore some multicells and/or bowing line segments can produce marginally severe wind gusts. Vertical wind shear is weaker further north, but strong 850 hPa winds (20-25 m/s) along with some evaporative cooling may also promote marginally severe wind gusts even with poorly organized convection.

... N Italy, Slovenia ...

A confined area with noteworthy CAPE, possibly up to 1500 J/kg, is predicted in the moist sea breeze regime that is overspread by steepening lapse rates from the Alps with a WNW-erly flow aloft. A few afternoon and evening storms are possible along the Alpine rim, and if robust updrafts manage to form, they could turn multi- to supercellular under 15-20 m/s deep-layer shear and bear a hazard of large hail.
A possible failure scenario is the lack of synoptic lift during daytime, which could make it difficult for upvalley/upslope circulations to break the cap. The majority of convection-resolving models simulates only a handful of storms which shall decay soon again after detaching from the mountains. However, some runs simulate continuing or even intensifying activity into the night, when warm air advection finally adds some synoptic lift. Convection would be elevated overnight, hence the hail hazard is much lower then, but heavy rain is not ruled out if this more aggressive scenario verifies and one or two larger storm clusters form.

... Spain ...

Sea breezes and upvalley circulations feed enough moisture inland to allow patchy CAPE on the order of some hundred J/kg. Scattered storms are possible over the orography in the afternoon and evening, but will likely struggle against strong entrainment in the hot and dry airmass. A few severe downbursts may occur, driven by strong evaporation and possibly even in the absence of precipitation reaching the ground. In addition, lightning strikes could spark wildfires.