Storm Forecast
Valid: Tue 22 Aug 2023 06:00 to Wed 23 Aug 2023 06:00 UTC
Issued: Mon 21 Aug 2023 15:20
Forecaster: PISTOTNIK

Level 1 areas are issued for the Czech Republic, S Poland, Slovakia, N Spain and S Italy for large hail, excessive convective precipitation and severe convective

SYNOPSIS

A zonal flow with a 500 hPa jet axis from England to the Ukraine divides Europe into a rather cool northern half and a very hot southern half. Several embedded short-wave troughs cross S Scandinavia, the Baltic Sea, the Baltic States, Belarus and adjacent Russia and create partly thundery but non-severe shower activity in the only slightly unstable, cool, maritime airmasses.
Near the surface, a diffuse, inactive and stationary frontal boundary runs from France to the Ukraine, i.e., on the anticyclonic side of the jetstream. South of it, the heat wave continues, especially under a mid-level ridge from Iberia to the Alpine region, whereas weak NE-erly flow slightly dampens the heat over the Balkans and in the Mediterranean region.

DISCUSSION

... belt from France to Poland ...

In the range of the inactive frontal zone, somewhat cooler but moist low-level air slowly slips southward near the surface, while at the same time warm air advection from the SW increases at mid-levels. This configuration results in a belt with augmented low-level moisture and CAPE mostly in the 500-1200 J/kg range (also confirmed by the Mon 12 UTC soundings), but also a capping inversion around the 850 hPa level. Deep-layer shear is mostly moderate and rises from 10-15 m/s towards the south to 20 m/s or slightly more towards the north in vicinity of the mid-level jet, most notably near the Czech/Polish border.
The chances for convective initiation are best in and around the Czech Republic in the late afternoon and evening, when the fringes of a short-wave trough along with increasing warm air advection create some synoptic lift. Orographic lift over the Giant Mountains (despite their improper naming) along the Czech/Polish border and over the High Tatras in Slovakia can also assist in breaking the cap. Scattered storms are expected then, mostly multicells but also some supercells, some of which could bring large hail, localized excessive rain and downbursts (the latter supported by rather large temperature/dewpoint spreads in the hot airmass). Some storms can go on well into the night and continue to travel E- to SE-ward with decreasing severe weather hazards as they decouple from the surface.
Further upstream, i.e., over France, Switzerland, SE Germany and Austria, surface-based convective initiation in the afternoon is less likely, as the capping inversion is stronger and synoptic lift comparably weak. However, a few elevated and probably non-severe storms are not ruled out in the warm air advection regime throughout the forecast perios.

... N Spain, Italy ...

Sea breezes should push enough moisture inland to allow scattered afternoon convection over orographic features in an otherwise calm synoptic setting. Stationary or backbuilding storms can bring localized excessive rain. Apart from that, isolated large hail events are possible especially at the south- and west-facing coasts in S Italy (thanks to 15-20 m/s deep-layer shear where the sea breezes undercut the NE-erly background flow) and severe downbursts especially over N Spain (thanks to strong evaporational cooling in the hot and dry airmass).