Storm Forecast
Valid: Wed 28 Jul 2021 07:00 to Thu 29 Jul 2021 06:00 UTC
Issued: Wed 28 Jul 2021 07:26
Forecaster: PUCIK

A level 2 was issued across NW Italy mainly for excessive rainfall and large hail.

A level 2 was issued across the Apennines foothills mainly for very large hail, damaging wind gusts, and to the lesser degree for tornadoes.

A level 2 was issued across the N Alps and adjacent lowlands mainly for large hail, severe wind gusts, and to a lesser degree excessive rainfall.

A level 1 was issued across E Czechia, Slovakia, Poland, Ukraine, Romania, and W Russia mainly for large hail, excessive rainfall, and severe wind gusts.

A level 1 was issued across the southern U.K. mainly for severe wind gusts.

A level 1 was issued across S Scandinavia mainly for excessive rainfall.

A level 1 was issued for the Caucasus area mainly for excessive rainfall.


SYNOPSIS

An extensive area of thunderstorms and at least a marginal severe weather risk is forecast across Europe in conjunction with a deep cyclonic vortex across the U.K. and broad W to SW-ly flow across W and Central Europe. Storms are forecast along and ahead of a wavy frontal boundary across the Alpine range, Czechia, and from Poland into S-Scandinavia, where abundant lower-tropospheric moisture, locally between 12 and 14 g/kg is forecast. Towards the S, an EML has been advected from Sahara across much of Italy and Balkans, but either dry air or stout CIN will prevent initiation across these regions. Across the U.K. and the Northern Sea, storms are forecast in a maritime polar airmass with marginal lower-tropospheric moisture and lapse rates enhanced by cold mid-tropospheric temperatures.

DISCUSSION

... Southern U.K. ...

A low-end Lvl 1 is introduced across the area, where marginal CAPE will overlap with moderate to strong 0-6 km bulk shear (15 to 20 m/s), supportive of some splitting supercells. Due to the strong background flow, stronger cells may be capable of severe wind gusts.

... S Scandinavia ...

Widespread initiation is forecast along the diffuse frontal boundary and ahead of the mid-tropospheric trough. Forecast profiles reveal marginal CAPE, slow storm motion, low LCLs, and high RH in the mid-troposphere, which is suggestive of efficient warm rain processes resulting in enhanced risk of heavy rainfall events.

... N Apennines, Alps ...

Stronger mid-tropospheric flow across this area will yield moderate to high values of 0-6 km bulk shear, with the highest values just to the N of the Alpine range and to the E of the Apennines. Across the latter region, the 0-6 km bulk shear will even exceed 25 m/s with increasing hodograph curvature towards the evening hours. The whole region may encounter severe storms as a diffuse mid-tropospheric trough approaches the area, but there are 3 areas with enhanced severe weather threat:

1/ NW Italy: The strongest forcing is forecast across this area with a potential for repetitive storm development across the S Alpine foothills. Thus, excessive rainfall will be the highest severe weather risk, followed by large hail from some right-moving supercells.

2/ N Apennines and adjacent lowland: Currently, there is a disagreement in the NWP concerning initiation, but a combination of a synoptic-scale lift at the upper troposphere combined with simulated uncapped profiles adds more confidence that at least one or two long-lived supercells will form in a kinematically very favorable environment with 0-3 km shear btw. 15 and 20 m/s and increasing hodograph curvature towards the evening hours. Developing supercells will pose threats of very large hail and damaging wind gusts. The tornado threat will remain rather marginal compared to the latter two given rather weak low-level shear. While any storm that develops across this region will likely become very severe, the probability of initiation is also the lowest here out of all highlighted Lvl 2 areas.

3/ N Alps and forelands: Models seem to be in rather good agreement concerning the development of a cluster of storms in the afternoon hours and its movement towards the E/NE in an environment favorable for well-organised storms with 0-6 km bulk shear around 20 m/s. The cluster of storms may consist of some transient supercells and line segments with upscale growth toward the evening hours. Both large hail and severe wind gusts are likely with the storms. Upscale growth may continue in the late evening and early night hours as the system reaches SE Czechia and SW / W Slovakia, possibly traveling to Poland in the prevailing SW-ly flow.

... Slovakia, Poland, and further E ...

Lack of clear forcing signals prevents us from drawing more confined swaths of severe weather. Nevertheless, an extensive area will be under severe weather threat given abundant lower tropospheric moisture, moderate CAPE values, and also moderate 0-6 km bulk shear, reaching 10 to 15 m/s. A lvl 2 was considered for S Poland, but some of the models only show isolated initiation across the area. In the early afternoon hours, the highest coverage can be expected across the Carpathians and along a weak frontal boundary across W Russia. During the evening hours and night hours, coverage will start increasing also across Poland, perhaps in conjunction with an arrival of a storm system that has crossed the N Alpine range and E Czechia / W Slovakia and with an approach of the diffuse mid-tropospheric trough. Storms will spread towards the E and will likely persist to the morning hours as abundant CAPE will remain atop stabilizing boundary layer.