Storm Forecast
Valid: Wed 26 Jun 2024 06:00 to Thu 27 Jun 2024 06:00 UTC
Issued: Tue 25 Jun 2024 14:57
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of France, Germany, Switzerland, N Italy, Austria and the Czech Republic mainly for excessive convective precipitation and to a lesser degree for large hail.

A level 1 aand level 2 are issued for Slovakia, Hungary, much of the Balkans, and central Italy for excessive convective precipitaion, large hail and severe convective wind gusts.

A level 1 is issued for the coastlines of the N Adriatic Sea mainly for (non-supercellular) tornadoes.

A level 1 and level 2 are issued for parts of Spain for severe convective wind gusts, large hail and excessive convective precipitation.

A level 1 is issued for far-east Turkey and Armenia for severe convective wind gusts, large hail and excessive convective precipitation.

SYNOPSIS

An ill-defined blocking anticyclone centered over the Baltic region and two cut-off lows over the Adriatic Sea and west of Portugal create a blocking pattern. The main frontal zone stretches from the British Isles to N Scandinavia, before it turns sharply southward into Russia. Much of Europe southwest of a line from Denmark to Romania is covered by warm and moist airmasses, with particularly warm (to hot) conditions at the forward flank of the two cut-off lows over Spain and the Balkans. Cooler and drier air is present over NE Europe.

DISCUSSION

... France, Germany, Switzerland, N Italy, Austria, Slovakia, Hungary, Romania ...

The warm and moist air features moderate to high CAPE often in the 500-1500 J/kg range, regionally even higher. Much of the area is also exposed to at least weak synoptic lift throughout the forecast period due to warm air advection, positive vorticity advection, or a combination thereof ahead of the Adriatic cut-off low. Scattered to often widespread thunderstorms will form as soon as daytime heating and/or mesoscale zones of more concentrated lift erode a weak capping inversion.
In central Europe, rich low-level moisture (with 2m dewpoints often in the upper tens or even scratching 20C) and weak vertical wind shear direct at prime conditions for excessive rain. Their impact may be further aggravated by the recent history of repeated abundant rainfalls. Strong pulse storms can also produce marginally large hail or large amounts of small hail in their early stages, whereas moist air and weak vertical wind shear should keep wind gusts mostly below the severe level. The hail and wind hazards increase in Slovakia, Hungary and Romania, fostered by gradually steepening lapse rates and 10-15 m/s vertical wind shear across the 0-3 and 0-6 km layers, which can promote a better storm organization into multicells and perhaps temporary cold-pool-driven bowing storm clusters.
Rather low cloud bases in conjunction with numerous outflow boundaries and other convergence zones over flat terrain also indicate a possibility of several weak and short-lived tornadoes across the entire discussed area. Best chances for such spin-ups are over the N Adriatic Sea in the morning hours, when some storms are expected to form in a cyclonic background wind field near the center of the low.
While the majority of convection will follow a strong diurnal cycle, a few storm clusters may be active already in the morning or well into the night where enough synoptic and/or orographic lift is present. A combination of synoptic and orographic lift can also create a period of prolonged rain with further embedded storms on the back side of the cyclone in SW Germany, W Austria and parts of Switzerlnd well into the night.

... Balkans, central Italy ...

CAPE shows a similar magnitude (500-1500 J/kg and regionally more), but is more based on steeper lapse rates - the fringes of an elevated mixed layer from the Sahara. An enhanced SW flow also creates moderate vertical wind shear between 10 and 20 m/s. The primary storm mode will be multicells, forming from the late morning onwards first over the mountains, along a dryline boundary NE of the Dinaric Alps, or along a similar sea breeze front along the Italian east coast. Later a somewhat erratic propagation into the Pannonian Basin is expected, following the course of outflow boundaries. Next to excessive rain, the better storm organization along with steeper lapse rates also enhances the hail and wind hazards. Isolated extreme hail and wind events are possible towards the southeast, in particular in the characteristic CAPE and shear maximum on the moist side of the Dinaric dryline, where CAPE up to 2000 J/kg is collocated with 0-3 km shear up to 20 m/s. The hail hazard is maximized in case of supercells, the wind hazard in case the subtle synoptic lift can make convection grow upscale into a cold-pool-driven bowing line.
Storms will gradually weaken after sunset, but a few of them can continue all night long.

... Spain ...

A plume of very steep lapse rates also overspreads Iberia ahead of the western cut-off low, and is further reinvigorated by strong daytime heating over the elevated terrain. CAPE strongly depends on the availability of low-level moisture, which can easily be mixed out over inland areas. However, CAPE up to 1000 J/kg appears possible in the larger valleys, and up to 2000 J/kg in N Spain, where a sea breeze front pushes inland from the Basque Country, Cantabria and Asturia. A fairly strong cap and synoptic subsidence will likely delay or regionally inhibit convective initiation, but daytime heating in conjunction with a short-wave trough should spark scattered storms towards evening. 0-6 km and 0-3 km shear are mainly between 10 and 15 m/s, though the complex terrain can render them lower or higher on a more local scale. All in all, multicells and cold-pool-driven clusters are the main expected convective modes. Especially the latter can produce some swaths of severe to isolated extreme wind gusts, aided by steep lapse rates and plentiful evaporation in the rather dry airmass. Apart from that, large hail and localized excessive rain may occur as well. Storms will weaken overnight, but synoptic lift can cause occasional flare-ups until morning (also further south).

... S-central Norway and Sweden, central Finland into adjacent Russia ...

Patches of CAPE are present just south of the main frontal zone, and isolated to scattered thundery showers are expected mainly in the afternoon and evening. It appears unlikely that they can grow deep enough to benefit from 15-20 m/s deep-layer shear and produce severe weather.

... E Turkey into Georgia, Armenia and Azerbaijan ...

Ahead of a short-wave trough travelling southeastward, scattered and mostly daytime-driven thunderstorms are possible in an environment of low to moderate CAPE and moderate vertical wind shear. A few large hail, severe wind and excessive rain events may occur.