Storm Forecast
Valid: Sun 23 Jul 2023 06:00 to Mon 24 Jul 2023 06:00 UTC
Issued: Sun 23 Jul 2023 00:03
Forecaster: PISTOTNIK

A level 2 is issued for parts of Bosnia-Herzegovina, Montenegro, Serbia, North Macedonia and Bulgaria as well as for S Austria for large hail, excessive convective precipitation and severe convective wind gusts, surrounded by a larger level 1 mainly for large hail.

Level 1 areas are issued for the south Carpathian foothills in Romania and for E Spain for large hail.

A level 2 is issued for parts of S France for large hail, severe convective wind gusts and excessive convective precipitation, surrounded by a larger level 1 mainly for excessive convective precipitation.

A level 1 is issued for parts of SW Russia for excessive convective precipitation, large hail and severe convective wind gusts.

A level 1 is issued for parts of NW Russia for excessive convective precipitation and large hail.

SYNOPSIS

A "high index" pattern remains in place with low 500 hPa geopotential from NW Russia through Scandinavia to the British Isles. An unseasonably strong jet stream runs along its southern flank. As a new mid-level trough starts amplifying over the British Isles, the jet gradually tilts into a SW-NE direction and further increases in strength. Late in the forecast period, even 25-30 m/s wind speeds at 500 hPa are expected in some regions along the jet axis from Spain to Belarus. In response, warm air advection increases again over central Europe in the warm sector of a weak surface cyclone over the North Sea.
Further downstream, weak cold air avection overspreads the Balkans, easing both the heatwave and the potential for organized, severe storms to some degree. Scorching heat meanwhile continues over the Mediterranean region under anticyclonic conditions.


DISCUSSION

... belt from the Italian and Austrian Alps across the W Balkans to Bulgaria and Romania ...

Though all "ingredients" for severe convective storms relax a bit compared to the previous days, a combination of 500-1500 J/kg CAPE and 15-20 m/s deep-layer shear remains in place in the range of the diffuse frontal zone, which slowly transitions from a weak cold front over the Balkans into a weak warm front in the Alpine region. The most plentiful moisture, and therefore the highest CAPE, can be maintained in the S Alpine forelands and in a belt just NE of the main Dinaric crest on the W Balkans, where the westerly mid-level flow is undercut by moist upslope and upvalley circulations.
In the absence of noteworthy synoptic lift, convective initiation is mostly tied to orographic features and expected to happen rather late and scattered at best. However, storms that form can turn multi- or supercellular, and large to isolated very large hail is quite likely once persistent updrafts form.
The level 2 areas may see a higher storm coverage due to hints of synoptic lift from travelling vorticity maxima (over the W Balkans rather early, in Austria towards evening). The majority of convection-resolving models supports this scenario, in which also excessive convective precipitation and severe downbursts become additional hazards. Convective activity could continue into the first half of the night, or even grow upscale into a larger MCS around the bordering regions of Serbia, North Macedonia and Bulgaria.

... level 1 areas in Romania and Spain ...

In these two regions, isolated afternoon storms are possible at dryline boundaries downwind of the mountains. The overlap of robust CAPE (on the order of 1000 J/kg south of the Carpathians in Romania and probably up to 3000 J/kg at the Spanish east coast) and 15-20 m/s allows well-organized convection in form of strong multi- and supercells. Limiting factor is synoptic subsidence, which makes convective initiation difficult, if not impossible.

... France, BeNeLux, W and S Germany, Switzerland ...

Some elevated and embedded thunderstorms may be active along the almost stationary frontal boundary from W and N France to Belgium and NW Germany, but weak CAPE and the elevated, embedded nature keeps the severe weather hazards low (if anything, some heavy rain is possible due to the almost flow-parallel orientation of the rain band). Towrds the south and later in the forecast period, CAPE and deep-layer wind shear are predicted to rise towards 1500 J/kg and 20-25 m/s, respectively, in the warm air advection regime. While some storms become increasingly likely in the course of the day,
indications are that they will mostly be elevated, which does not warrant more than a level 1 for possible excessive rain events despite the robust CAPE-and-shear overlap.
Only late in the forecast period, stronger synoptic ift weakens the cap and increases the chances for surface-based initiation in S France. In these case, large hail and severe wind gusts become possible as well, reflected by a level 2 area. Upscale growth into a late MCS is possible towards Monday morning. In the meantime, remnants of elevated convection will further spread into Switzerland and SW Germany.

... Russian level 1 areas ...

Scattered, mostly daytime-driven thunderstorms are expected in an environment of moderate CAPE up to 1000 J/kg and synoptic lift ahead of mid-level vorticity lobes. In the northern area, vertical wind shear is weak to moderate, hence excessive rain and marginally large hail are the main hazards. In the southern area, a higher magnitude of both CAPE and deep-layer shear allows better-organized storms with a higher hail and wind threat.
Agreement of global forecast models (ECMWF, GFS, ICON-EU) on the placement and timing of convective initiation is low, which is why only broad thunder and level 1 areas were issued to compromise between these differing guidances.