Storm Forecast
Valid: Sat 06 Jul 2019 06:00 to Sun 07 Jul 2019 06:00 UTC
Issued: Fri 05 Jul 2019 23:27
Forecaster: PISTOTNIK

A level 3 is issued for NE Italy and S Austria mainly for large hail and to a lesser degree for severe convective wind gusts. In addition, tornadoes are possible in NE Italy.

A level 1 and level 2 are issued for a belt from central France to W Hungary and Slovenia mainly for large hail, severe convective wind gusts and excessive convective precipitation.

Level 1 areas are issued for S France and E Spain for large hail and severe convective wind gusts.

A level 1 is issued from the E Ukraine into W Russia mainly for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts.

SYNOPSIS

Low 500 hPa geopotential stretches from Iceland to Scandinavia and NW Russia. An unseasonably strong mid-level jet is present at its southern flank. It gradually connects with a second jet streak over SW Europe ahead of a cut-off low west of Portugal, resulting in a broad zone of enhanced zonal flow across central Europe.
The associated frontal zone near the surface stretches from France to the Ukraine and remains stationary. Hot air is present to its south.

DISCUSSION

... France and greater Alpine region ...

An elevated mixed layer (EML), originally created over NW Africa and the Spanish plateau, is picked up by the increasing westerly flow and advected over the Alpine region, where it is reinforced by strong daytime heating. Low-level moisture has recovered well under the capping inversion on Friday. Since the upstream regions over France and Spain where largely devoid of convective overturning, the robust CAPE predictions that are offered by the forecast models for the warm side of the frontal zone on Saturday appear realistic or even too conservative. Expected CAPE magnitudes by Saturday afternoon are 500-1500 J/kg over central France, Switzerland and S Germany, 1000-2000 J/kg over S France and 2000-3000 J/kg over N Italy, but peak values along convergence zones may still be 1000 J/kg higher. The highly unstable airmass is overspread by steadily increasing vertical wind shear, often reaching or even exceeding 20 m/s across the 0-6 km layer in the afternoon and evening. This impressive CAPE-and-shear overlap sets the stage for the first day of a severe weather outbreak of possibly extraordinary dimensions.
The day may start with elevated and initially weak convection across central France near the forward rim of the EML. It is not ruled out that this convection will already start to root down until noon and bring first severe storms that move from E France into SW Germany and Switzerland. Otherwise, convective initiation becomes increasingly likely over orographic features from early afternoon onward. Model agreement about its timing and placement is limited, but subtle warm air advection, numerous crossing vorticity maxima and persistent upvalley and upslope circulations offer plentiful sources of lift across all scales, and the question is not IF, but rather WHEN convection will form. Towards evening, secondary initiation at outflow boundaries and other convergence zones becomes increasingly likely as well.
Storms will quickly organize into strong multicells, supercells and bowing lines or grow upscale into large clusters. Large hail and severe wind gusts are likely and scattered extreme events are foreseen across the entire level 2 area. Supercells with extremely large hail are more likely over the Alpine forelands, where enough low-level moisture is maintained beneath the capping inversion to allow fat CAPE profiles. in contrast, possible swaths of widespread severe to extreme wind gusts can explixitly include the Alpine terrain as well - deeper mixing and higher cloud bases can compensate a somewhat weaker storm organization, and the expected storm motion from west to east appears favorable for their gust fronts to burst out into major west-east running valleys.
The risk of extreme developments culminates on the southern side of the Alps in the late afternoon and evening. From 14 UTC onwards, storms from the southern Alps will likely start to descend into the Klagenfurt Basin (S Austria) and the plains of Friuli and Veneto (NE Italy), where they encounter the highest CAPE and the best wind profiles on the moist side of a dryline. Fine-meshed models agree on several right-moving supercells which can bring large to giant hail, downbursts and very heavy rain. In addition, a remarkable tornado risk unfolds in the NE Italian plains, where sea breezes enhance low-level shear and may boost 2m dewpoints to 24 or 25C by sunset. A few tornadoes are possible and even a strong tornado is not ruled out in case the track of a supercell can connect with an outflow boundary, the sea breeze front or another convergence zone.

Uncertainties of this forecast grow towards the east (E Austria, Slovenia, N Croatia, Hungary), where the CAPE buildup needs some hours more, the capping is stronger and synoptic lift arrives later. A few "cap breakers" over orographic features can organize well and turn severe, but could fall victim to the cap again soon after they detach from the mountains. There is some consensus that a large storm cluster will cross Austria and Slovenia overnight with an ongoing risk of severe wind gusts, excessive rain and localized large hail before it gradually decays over Hungary and Croatia. Tail-end storms to the south may still turn supercellular and propagate towards the unconsumed air further south, hence N Italy could repeatedly be affected by corridors of all kinds of severe to extreme weather throughout the night.

... SW Europe ...

The stationary cut-off low west of Portugal prolongs the warm air advection regime under a strong SW-erly mid-level flow. Whereas hot and dry conditions are present in the interior of the Iberian Peninsula, robust CAPE is in place - but strongly capped - over coastal and maritime areas as well as in the French Pyrenees foreland.
There is a limited chance for isolated convective initiation in the afternoon at the sea breeze fronts over N and E Spain. The capping inversion and strong 0-6 km shear of 20-25 m/s will make convective initiation a big challenge. However, if persistent updrafts can be maintained, an organziation into supercells with a risk of large hail and (dry) downbursts is easily possible. Likewise, a few well-organized storms could carry some severe weather into SW France in the evening and night, in case they manage to form over the Pyrenees and detach from them.

... E Ukraine into W Russia ...

Ahead of a cold front, some hundred J/kg of CAPE overlap with deep-layer shear around 15 m/s (more CAPE to the south, more shear to the north). In the late morning to afternoon, scattered to widespread storms are expected. The main risk is excessive rain, as lapse rates are weak and the shear vector is largely parallel to the front. However, discrete multicells could also bring a few large hail and severe wind events. Convection will quickly become elevated and weaken in the evening.

... S Scandinavia, Baltic Sea, Baltic States into NW Russia ...

Low-topped, not necessarily electrified convection is expected in the well-mixed, cool maritime air to the north of the frontal zone. Lift of two travelling short-wave troughs makes deeper and thundery convection more likely over S Sweden and from Finland into parts of NW Russia. The southern parts of these areas overlap with moderately enhanced 0-3 km shear (10-15 m/s), hence some multicells are expected to form. However, the CAPE magnitude appears too low to issue a level 1.
Convection will largely decay around sunset over land, but may go on over the Baltic Sea overnight. A few non-supercellular tornadoes are possible near the coasts throughout the forecast period.