Storm Forecast
Valid: Wed 21 Jun 2023 06:00 to Thu 22 Jun 2023 06:00 UTC
Issued: Tue 20 Jun 2023 22:02
Forecaster: GROENEMEIJER

A level 2 was issued across an area stretching across Spain, France, the Alpine countries, southern Germany, Czechia, Slovakia, Hungary, and Serbia for severe wind gusts and large hail, and to a lesser extent for heavy rain.

A level 1 was issued across Spain for hail and to a lesser extent for wind and rain.

A level 1 was issued across Northeast Germany and Poland for hail and heavy rain and to a lesser extent for severe winds.

A level 1 was issued across central France, northern France, southern Belgium and Luxembourg for heavy rainfall and to a lesser extent for hail and wind.

A level 1 was issued across central France, northern France for heavy rainfall and to a lesser extent for hail and wind.

A level 1 was issued across parts of Türkiye and the Caucasus region for hail and wind and heavy rainfall.

SYNOPSIS

A ridge with an axis extending from Italy to the Baltic Sea is filled with warm air and a pronounced dry adiabatically stratified elevated mixed layer (EML) at its core between 850 and 600 mb. The mid-level flow curves anticyclonically around the fringes of the EML, with a pronounced south-southwesterly speed maximum developing on the western flank across eastern Spain and Southeast France as a shortwave trough approaches the ridge from Portugal to western France. Within the entire periphery of the hot air, stretching from Spain across the Alps to Serbia, the strong mid-level flow creates strong vertical wind shear to sustain organized convection, and – similar to the last two days – storms are expected to develop scattered within this area as CAPE should be adequate for their formation across this zone. Pinpointing the locations of initiation is next to impossible perhaps with the exception perhaps of eastern Spain, where the approaching shortwave trough will force synoptic-scale upward vertical motion and in initiation should be relatively widespread. In response to this trough, cyclogenesis and widespread convective initiation are expected in the evening across Southwest to Central France, resulting in a mesoscale convective system producing possibly very high rainfall accumulations. NWP model, however, do not agree on the track of this activity which may move anywhere between almost due north to western Belgium and northeastward to Saarland by Thursday 06 UTC.

DISCUSSION

Iberia...
Ahead of the approaching shortwave trough storms are expected to initiate across large parts of the peninsula. The strongest storms will likely form across the eastern mountain ranges where CAPE is highest and shear is strongest as well. Hence a high probability of rapid organization is forecast, with a tendency of supercells quickly transforming into one or more bowing systems as a result of fairly strong evaporational cooling of downdrafts, resulting in a rather high severe wind risk. Besides severe winds, large hail will be especially likely with young and early mature convective systems. Very heavy rainfall will be an important hazard as well.

France, Southern Germany...
As during the previous days, a number of storms will likely form in a broad zone from southern to northeastern France into Switzerland and southern Germany. As a result of the moderate 1000-2500 J/kg MLCAPE and strong wind shear of 20 m/s in the 0-3 km layer, rapid organization into supercells and bowing systems is likely. Large hail and severe wind gusts are the primary risks. Some hail in excess of 5 cm diameter will be possible. During the evening the risk of very heavy rainfall will increase as upward motion ahead of the shortwave trough will boost convective coverage as discussed above.

Switzerland, Austria, Slovenia...
Being situated in the same peripheral zone with adequate deep-layer shear and abundant MLCAPE of 1500-3000 J/kg storms managing to initiate within this region will have the potential to rapidly produce severe weather. Convective initiation may be somewhat hindered by some CIN, but a number of convection-allowing models do produce severe storms. Among those, the ICON-D2 ensemble produces a strong system developing across the Eastern the Alps. Given the sizable shear and instability, the primary convective mode here supercells transitioning into bow echoes will be as well.
 
Czechia, Northeast Germany, and Poland...
Shear weakens going northward from the Alps, but an axis of relatively high MLCAPE of 1000-2000 J/kg should extend all the way to the Baltic Sea coast of northern Poland. This suggests that storms may well become severe with very heavy rain, large hail and isolated severe wind gusts possible. NWP guidance suggests that storm coverage across Poland and northeast Germany will be most widespread, but the higher shear further south will create a higher risk of severe weather with storms developing across Czechia.
NE Austria, Slovakia, Hungary, Serbia, Romania…
Yet another location of likely convective initiation are the Tatra mountains or a zone of convergence in the Vienna Basin. These storms would move southeast- and later southward across the Pannonian Plain and may affect western Romania and northern Serbia. Again, large hail and with more mature bowing systems, the wind risk would be the main hazards.

Turkey and southern Caucasus region…
Scattered storms are expected to once again initiate across both higher and low terrain within the region. In spite of the low CAPE available, the strong deep-layer shear may render these storms well-organized with a modest risk of large hail and severe wind gusts.