Storm Forecast
Valid: Thu 06 Jul 2023 06:00 to Fri 07 Jul 2023 06:00 UTC
Issued: Wed 05 Jul 2023 23:10
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for NE Spain and S France for large hail, severe convective wind gusts and excessive convective precipitation.

A level 2 and level 2 are issued for N Italy for large hail, excessive convective precipitation and to a lesser degree for severe convective wind gusts.

A level 1 and level 2 are issued for SE and E Europe, as well as for W Turkey, mainly for excessive convective precipitation and large hail.

SYNOPSIS

A complex weather pattern is in place in the European sector. A cyclone (responsible for an extraordinary summertime windstorm in the Netherlands and NW Germany on Wednesday) fills up over S Sweden. Two other cyclones are present west of the British Isles and Iberia. All in all, this configuration results in a broad, meandering westerly to southwesterly flow over most of the continent. At 500 hPa, the flow is split up into a polar jet from the British Isles towards Finland and a subtropic jet over Iberia and the Mediterranean Sea. Near the surface, the accompanying diffuse frontal zone acts as a cold front from the Baltic States to the Alpine region and turns into a warm front towards France and Spain.
Last but not least, an extensive but weak mid-level low is present over SE Europe with seasonable temperatures and unsettled weather.

DISCUSSION

... NE Spain, S France ...

High CAPE, probably on the order of 1000-2000 J/kg, builds in the sea breeze and upvalley flow regime of the Spanish east coast and the Ebro Valley, which feeds plenty of low-level moisture with 2m dewpoints in the upper tens beneath an elevated mixed layer (EML) that detaches from the high and dry inland areas. Convective initiation is hampered and delayed by a capping inversion beneath the EML, but synoptic lift (both from warm air advection and from subtle short-wave troughs) appears strong enough to allow upvalley/upslope circulations to break this cap. Courtesy to the subtropic jetstream aloft, deep-layer shear rises from 20 m/s in the Basque region to 30 m/s near the Spanish east coast, providing excellent conditions for storm organization.
One or two clusters of elevated and probably non-severe storms may already be active in the morning, but should decay and/or move away towards the NE soon enough to allow plentiful sunshine and strong daytime heating. By early afternoon, a pronounced sea breeze front / dryline will likely establish downstream (NE) of the Iberian System mountains, and scattered convective initiation becomes more and more likely over those mountains that can still be reached by the moisture tongue from the Ebro valley. A quick organization into strong multicells, supercells and later perhaps a larger linear system, driven by strong downbursts in the hot airmass, is expected. The main hazard is large hail with discrete storms (in particular supercells), before it turns to severe wind gusts and heavy rain in case convection grows upscale. The possibility of both a high coverage of severe weather and of a handful of extreme hail and/or wind events justifies a level 2.
Similar CAPE and shear conditions are in place on the French side of the Pyrenees, which will likely be reached by the storms by sunset. There can still be a time window for surface-based and severe storms, before they finally turn elevated and weaken while moving further NE overnight.

... N Italy ...

A volatile and interesting setup for organized storms is also in place on the warm side of the almost stationary frontal zone in N Italy. Cooler low-level air infiltrates the region from the NE, but is exposed to sufficient insolation and picks up enough moisture from the Adriatic Sea to obliterate the airmass exchange. By afternoon, CAPE on the order of 500 J/kg over the southern Alps and up to 2000 J/kg near the Appennines should build again, collocated with 15-20 m/s deep-layer shear thanks to the westerly mid-level flow. Upvalley and upslope circulations, outflow boundaries from overnight's convection and several travelling short-wave troughs provide a multitude of possible lift/initiation mechanisms. Consensus of the forecast models on the timing and placement of convection is almost non-existent - some realizations show a strong surge of convection as early as Thursday morning, some as late as Thursday night, again others in-between in response to peak daytime heating. While it is therefore almost impossible to give any details about the expected development, the possibility of a severe weather outbreak at some time within the forecast period is high enough to upgrade the southern part of the highlighted region to a level 2, also backed up by a pinch of climatology.
The dominant convective mode is expected to be multicellular with main hazards of heavy rain and isolated large hail in the southern Alps and the adjacent forelands (level 1 area). Towards the south, higher CAPE and more discrete initiation increase the chances for supercellular organization. Especially if a right-moving storm can synchronize well with the sea breeze front, also a persistent supercell with a long track of large to very large hail and perhaps damaging winds may materialize. A tornado cannot be ruled out if a supercell picks up enough vorticity at the sea breeze front and/or deviates strongly from the steering mid-level winds, though rather weak 0-1 km shear below 10 m/s keeps this hazard subordinate.

... SE and E Europe, W Turkey ...

CAPE between 500 and 1500 J/kg, locally more, is present across wide areas, and another round of numerous daytime-driven thunderstorms is expected. CAPE might be lower towards the north, also depending on the amount of residual clouds in the morning, but vertical wind shear is somewhat stronger there (10-15 m/s across the 0-6 km layer versus mostly 5-10 m/s further south). Expected convective modes are single cells, multicells and probably various larger clusters. The dominant hazard is excessive rain, while strong pulse storms can also produce marginally to moderately large hail and perhaps a few downbursts. The areas where the highest storm coverage is foreseen are upgraded to a level 2, the northern one owing to substantial lift both from the advancing cold front and a supporting mid-level trough, the southern one based on the fairly impressive CAPE magnitude.
Moderate to high CAPE also extends well northeastward into larger parts of the Ukraine, but convective initiation during peak daytime hours is less likely there due to a lack of synoptic lift, while the advancing storms in the evening and overnight gradually lose their severe weather potential as they detach from the surface and become elevated.