Storm Forecast
Valid: Fri 28 Jun 2024 09:00 to Sat 29 Jun 2024 06:00 UTC
Issued: Fri 28 Jun 2024 08:56
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for large parts of Spain and Portugal mainly for excessive convective precipitation and, especially towards the E, also for large hail and severe convective wind gusts.

A level 1 is issued for W France mainly for excessive convective precipitation and large hail.

A level 1 is issued for SE France, Switzerland and N Italy for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 and level 2 issued for W Poland, S and E Germany, Austria, W Hungary, and parts of the Balkans for excessive convective precipitation, large hail and to a lesser degree for severe convective wind gusts.

A level 1 and level 2 are issued for N and central Poland, the Czech Republic and Slovakia mainly for excessive convective precipitation and to a lesser degree for large hail.

SYNOPSIS

A dominant cyclone is centered between Iceland and Scotland. At its south(east)ern flank, a strong mid-level trough and an associated cold front cross the North Sea, the N half of Germany, Poland and S Scandinavia, followed by cool maritime air.
The rest of Europe is under warm, often also humid airmasses and weak pressure gradients. Two small cut-off lows translate from Portugal to E Spain and from Bulgaria to W Turkey, respectively, opposed by a weak mid-level ridge from the W Mediterranean region to Belarus.

DISCUSSION

... S Sweden, Germany, Poland, Czech Republic, Austria, Slovakia, Hungary into the Balkans ...

Plentiful low-level moisture and insolation assist in the buildup of 500-1500 J/kg CAPE. Scattered to widespread afternoon and evening storms are expected, first over orographic features and along and ahead of the cold front pushing from E Germany into the Poland and the Czech Republic, later also at outflow boundaries or other convergence zones over flat terrain. With modest lapse rates and under weak vertical wind shear, the majority of storms will be disorganized, and excessive rain is the main hazard. Strong pulse storms may also produce large hail and a few severe downbursts. These latter hazards are somewhat higher near the western rim of the discussed area over E Germany and W Poland, where deep-layer shear increases to around 15 m/s, and over E Austria and adjacent Hungary and Slovenia, where an advection of an "Alpine plume" of steeper lapse rates over moist upvalley circulations can also push deep-layer shear to 10-15 m/s and enhance CAPE to up to 2000 J/kg. These regions are upgraded to a level 2 for these combined hazards, along with N Poland, where vertical wind shear is weak, but a particularly high storm coverage maximizes the excessive rain threat.
Storms should quickly fade and dissipate after sunset towards the south and east, whereas lift from the short-wave trough and the cold front will push them further eastward into E Poland, Kaliningrad and the Baltics overnight with gradually decreasing severe weather hazards.

... France, Switzerland, NW Italy ...

Increasing warm air advection ahead of the western cut-off low with a "Spanish plume" of steep lapse rates keeps the airmass capped, while daytime heating of the boundary layer and plentiful evaporation may push CAPE well above 2000 J/kg. Under rather anticyclonic flow and strong capping, convective initiation should stay isolated and confined to elevated terrain of the Pyrenees, the Massic Central and the Western Alps. Current thinking is that these storms will not be able to detach from the mountains and tap into the environment of high CAPE and around 15 m/s or more deep-layer shear that would be very conducive for severe weather. An explosive development into a few multi- and supercells moving over the forelands, and perhaps even upscale growth into one or two large clusters, cannot completely be ruled out for the evening, though, like it is simulated by AROME. For now, a level 1 and a low-probability lightning area seem to be the best compromise. A careful monitoring of this volatile setup is recommended!
With ongoing warm air advection and the arrival of a few vorticity maxima, elevated initiation becomes more and more likely over SW and perhaps central France overnight. There is a minor threat of heavy rain and perhaps large hail, but the elevated nature should keep it limited until beyond Saturday 06 UTC.

... Spain, Portugal ...

Moderate CAPE up to 1000 J/kg is in place near the western cut-off low and at its forward flank. Deep-layer shear exceeds 20 m/s near the Spanish east coast but quickly goes down to very low values near the center of the cut-off low, which slowly progresses E- to NE-ward. Some storms are already active now (Fri 09 UTC) near the cold front, and synoptic and orographic lift in the prefrontal southeasterly flow will produce more widespread storms in the next couple of hours. Under wind profiles favorable for side- and backbuilding convection and a long time window with only slow propagation of the cold front, excessive rain is the main hazard, and some dangerous events are not ruled out! Towards the east, more discrete ("tail-end") storms under stronger vertical wind shear can also organize into multi- and supercells and produce large to isolated very large hail and severe downbursts.
The activity will cluster betimes and slowly shift northeastward, but can continue well into the night before it gradually dissipates after midnight.