Storm Forecast
Valid: Mon 24 Jul 2023 06:00 to Tue 25 Jul 2023 06:00 UTC
Issued: Mon 24 Jul 2023 00:42
Forecaster: GROENEMEIJER

A level 3 was issued across Austria, Northwest Hungary, and western Slovakia mainly for damaging winds and very large hail, and to a lesser extent for very heavy rainfall.

A level 3 was issued across northern Italy, northern Slovenia, and extreme southern Austria for very large to giant hail, damaging winds, and tornadoes, and to a lesser extent for very heavy rainfall.

A level 2 was issued across a larger area surrounding the level 3 for all hazards, particularly large hail, and severe wind.

A level 1 was issued across a large part of Europe, for isolated large hail, severe wind gusts, and heavy rainfall.

A level 1 was issued across parts of eastern Turkey, Georgia, Armenia, and Azerbaijan, primarily for large hail.

SYNOPSIS

At the anticyclonically curved northern edge of a deep elevated mixed layer across the Central Mediterranean region, the mid-/upper-level flow is strong. A shortwave trough with an axis from southern Britain to NW Spain slowly translates eastward and tightens the height gradient a little further, creating very strong shear. Subtle impulses for upward motion traveling along the edge will be the focus for a few rounds of storms, that in combination with moderate to very high CAPE will likely produce widespread severe or extremely severe weather including all convective hazards: damaging winds, very large hail, tornadoes, and heavy rainfall.

DISCUSSION

Level 2 region from France to Switzerland, southern Germany, Czechia, Austria, Poland, Slovakia, Hungary, and Romania...

A nocturnal thunderstorm cluster that developed across France in response to frontogenetic forcing should be located approximately across Switzerland by the start of the forecast period. Ahead of it, considerable CAPE will develop across and near the Alps with values of 1500 - 2000 J/kg across the lower terrain of Austria.

A rapid increase in storm coverage and evolution into an eastward racing bow-echo is anticipated. Initially, large to very large hail will a big risk, but this should fairly quickly transition to wind. With embedded supercells, large hail will still remain a possibility, though. This transition is accelerated by strong evaporational cooling in the deep boundary layer to the north of the Alps and the particularly strong 0-3 km bulk shear of 20 - 25 m/s along the central Alps. Most convection-allowing models simulate a long swath of severe winds from western Austria into Slovakia and Hungary, including locally extremely severe winds > 32 m/s. The storm system may well continue eastward across the Pannonian Plain and ultimately reach northwest Romania. A scenario whereby new storm cells develop across Hungary, or several intense bow echoes move from Slovenia and/or Southern Austria across Hungary towards and into Romania is also suggested by some models.

Further north, across Southeast Germany, Czechia and southern Poland, linearly organized convection may form as well, however with a somewhat lower intensity than further south. Nevertheless, severe winds are also possible in this region. In the wake of they first storms, more storms are expected to develop across eastern France, Switzerland and southern Germany. These will develop in an environment with fairly low (~ 1000 J/kg) MLCAPE, but strong linear deep-layer shear of around 25 m/s. These storms may produce some large, but not extremely large, hail and locally severe winds. The storm activity will likely continue overnight.

North Italy, southern Switzerland and Austria, Slovenia, Croatia...

Within this second area of risk, supercells are expected to develop during the afternoon across the Alps. These do so in an environment of 25 m/s shear in the 0-3 km layer only which will boost their organization and vigor. Large to very large hail in combination with severe winds will create a high damage potential with these storms that may transition into small bow echoes over time.

During the evening, the temperature in the 800-500 hPa level will start to drop from the west, facilitating the movement of storms to the lower terrain. This is the point where the strongest storms are simulated by the convection-allowing models, such as ICON-D2, which shows among the most intense updraft helicity tracks seen in this model. This can be understood by the high 2000 - 4000 J/kg of MLCAPE, and the extreme high shear. These storms will be capable of producing very large (5 - 10 cm) to giant (> 10 cm) hail in some places, extreme severe winds gusts, as well as wind-driven hail, the very damaging combination of the two.

The predicted low-level wind profiles are not very favorable for tornadoes per se, as the wind shear in the lowest kilometre, or even lowest 500 metres, is limited, and mostly not streamwise. But as rare past cases have shown, in such an exceptional high-CAPE/high-deep-layer-shear situation, tornadoes or even strong tornadoes are well possible.

A first round of storms is forecast to affect the low land of Northern Italy and subsequently the northern Adriatic and finally Slovenia and/or northern Croatia during the evening and early night. Afterwards, stabilisation will gradually and temporarily reduce the risk of such extreme storms, however, additional elevated storms with primarily a risk or large to very large hail may continue during the night, in particularly across Italy, and affect areas slightly further south, i.e. Liguria, Emilia-Romagna, and northern Tuscany.

Benelux, Northeast France, North Germany, Northwest Poland...

As some 500 - 1000 J/kg of CAPE should develop within a fairly strongly sheared flow (20 - 25 m/s 0-6 km bulk shear), convective storms that evolve in this weakly capped environment should organize into small supercells or lines. Given the high buoyancy close to the lifted condensation level that is predicted and fair low-level shear, a few short-lived tornadoes cannot be ruled out. In addition, some very local severe wind gusts may also evolve as well as isolated marginally severe hail (2-3 cm).

East Spain...

With a short time window ahead of a surge of cooler, drier air from the northwest, the capping inversion atop the warm, moist boundary layer should erode and a window for possible deep convection should exist across the area and even, possibly, along the coast or sea. Given the high CAPE and shear, there is a risk that a potential storm would become severe and produce large hail, very heavy rainfall, and/or strong winds. A level 1 was issued for this low but existing risk.

Belarus, adjacent Russia...

An environment of limited (400-800 J/kg) CAPE and moderate deep-layer shear also exhibits fairly short but curved low-level hodographs. This suggests that some stronger low-level mesocyclones may develop with a risk of a tornado or two, and possibly some marginally severe hail.

Eastern Türkiye and adjacent countries...

Rather steep mid-level lapse rates, i.e. between 700 and 500 hPa in this region enhance the risk of hail by providing additional buoyancy and updraft strength. This elevates the risk of large hail to where a level 1 is clearly warranted.