Storm Forecast
Valid: Sat 07 Jun 2025 06:00 to Sun 08 Jun 2025 06:00 UTC
Issued: Fri 06 Jun 2025 15:45
Forecaster: PISTOTNIK

A level 3 is issued for the bordering region of SE Poland, SW Belarus and the NW Ukraine mainly for large hail and severe convective wind gusts, and to a lesser degree for tornadoes.

A level 2 and level 1 are issued for S and E Poland, parts of Belarus and Russia for large hail, severe convective wind gusts, excessive convective precipitation and to a lesser degree for tornadoes.

A level 2 and level 1 are issued for parts of the Ukraine mainly for large hail and severe convective wind gusts.

A level 1 is issued for the E Czech Republic, parts of Austria and Slovakia for large hail, excessive convective precipitation and severe convective wind gusts.

A level 1 is issued for parts of Germany, NW Poland and the W Czech Republic for severe convective wind gusts and tornadoes.

SYNOPSIS

An unseasonably intense mid-level low remains moored in the Norwegian Sea. Its circulation influences large parts of Europe. The axis of a 25-35 m/s mid-level jet runs from the Bay of Biscay to France, central Europe, Poland, Belarus and NW Russia. It is largely collocated with a broad and diffuse, more or less stationary frontal zone. In response to vorticity maxima aloft, several small wave- or comma-like disturbances run eastward on the cyclonic side of the frontal zone from England to the Baltic States. Their timing and intensity, and hence their interaction on convective developments, are limited but difficult to assess.
South of the frontal zone, very warm to hot and anticyclonic conditions dominate across Iberia, the Mediterranean Region and the Balkans. A weak upper-level low is placed over Greece and W Turkey.

DISCUSSION

... SE Poland, Belarus, Ukraine into Russia ...

An elevated mixed layer of Saharan origin has been advected far NE-ward, and low-level moisture has accumulated beneath its cap and especially along the frontal zone. A reservoir of moderate to high CAPE therefore resides in this large region, whose NW and N fringes are overspread by moderate to strong vertical wind shear and lift impulses of travelling short-wave troughs aloft. This cocktail was/is responsible for an outbreak of severe to extreme thunderstorms on Thursday and Friday, with the climax likely on Friday afternoon to evening (at the time of writing).
On Saturday, remnants of the elevated mixed layer will likely either move further eastward or be dissipated by this massive convective overturning, weakening the mid-level lapse rates to some degree. However, plentiful low-level moisture remains in place along the frontal zone and on its warm side. After some hours of largely unimpeded daytime heating, forecast models suggest again the buildup of 1000 to 2000 J/kg CAPE, with possible peak values up to 3000 J/kg where low-level moisture is concentrated (e.g., along outflow boundaries and other convergence zones). The wind field shifts very little compared to Friday, leaving vertical wind shear around or below 10 m/s over larger parts of the Ukraine, whereas it quickly ramps up under the flank of the jetstream over the NW Ukraine into SE Poland and Belarus, where 0-6 km and even 0-3 km shear can rise to 20 m/s or even slightly beyond. With respect to synoptic-scale lift support, two main short-wave troughs can be identified: the first one moves E-ward over Russia and exits our forecast areas by evening, the second and stronger one crosses Poland and Belarus in the afternoon and evening. In-between, synoptic-scale subsidence seems to prevail over much of the Ukraine, possibly fanning into adjacent regions of Belarus and Russia.

The highest severe weather hazard develops with the arrival of the second trough, which is also predicted to spark a shallow frontal wave travelling somewhere along the Lithuanian/Belorussian border, though its intensity and track cannot fully be assessed yet. The mid- and low-level wind field and the frontal boundary will further tighten in response to its dynamics. The model pool agrees on a corridor between Krakow (Poland) and Salihorsk (Belarus) where CAPE up to 2000 J/kg, deep-layer shear in excess of 20 m/s and 0-3 km storm-relative helicity in the 300-500 m^2/s^2 range shall overlap, providing prime conditions for a few long-tracked and intense supercells. Convective initiation is predicted for the mid-afternoon. Despite the rather narrow corridor with the best conditions - which is bounded by the cold front the the NW and possibly by drier air descending from the Carpathian mountains the SE -, a level 3 seems to be warranted for a remarkably enhanced hazard of large hail and severe wind gusts, including possible swaths of extreme events. Rather low cloud bases and enhanced 0-1 km shear also point to the possibility of a tornado, which might also be long-tracked and strong, especially if moisture and low-level wind shear can further be augmented underneath the capping inversion.
The hail hazard dominates in the earlier stages of these storms. Later on, upscale growth into a bow echo with a shift to a primary wind hazard is not ruled out, which could propagate some distance into the hotter and drier air torwards the E to SE, though conditions for such a scenario seem to be rather marginally supportive. In any case, some well-organized storms and possibly large clusters can persist well into the night and travel across Belarus into Russia. Depending on the storm mode (discrete vs. linear), at least isolated to scattered hail and/or wind events can occur throughout the night, alongside with a possibility of excessive rain especially in case of training storms and/or large storm clusters.

Further downstream into Russia and the Ukraine, outflow boundaries and other mesoscale convergence zones are the main focus for convective initiation in the afternoon to evening. Storm coverage is expected to be higher in Russia, where the departing first short-wave trough provides some lift support, but more isolated in the Ukraine due to the subsidence between the two troughs. The CAPE-and-shear space is similar to the western rub-region discussed above, with high CAPE under weak vertical wind shear towards the south and moderate CAPE under increasing vertical wind shear towards the north, and so are the expected severe weather hazards. Well-organized storms, including supercells and large clusters, are likely and upscale growth into one or two bowing lines is nor ruled out. Confidence of this forecast is limited due to the lack of high-resolution models, surface observations and radiosondes in this area, therefore a broad level 2 seems to be the best compromise for large hail, severe wind gusts and excessive rain, and one or two tornadoes in case mesoscale details work together to enhance moisture and shear at low levels. However, it is worth noting that one or two corridors may emerge with a coverage of extreme hail or wind events high enough for a level 3, while other parts of the level 2 could stay largely or completely devoid of severe weather.
The northward extension of (severe) convection is somewhat unclear, as CAPE fans out towards the north. However, rather low-topped storms there may still organize well under 15-20 m/s deep-layer shear. Therefore the level 1 is extended well northward for isolated to scattered large hail, severe wind and excessive rain events. In addition, low cloud bases and 0-1 km shear around 10 m/s point to the possibility of a tornado. With mostly unidirectional wind profiles, storms would, however, have to deviate off the hodograph for this scenario to unfold.

... Alpine region, E Czech Republic, Slovakia ...

The tail of the cold front makes a slow SE-ward progress, temporarily halted by the Alps. While some models still show up to 1000 J/kg CAPE from central Austria eastward, the solutions with lower CAPE due to poor lapse rates have taken overhand in recent model runs. A particular concern is a lack of synoptic lift, as the short-wave troughs only graze this area to the north, or even net synoptic subsidence towards the west, where cold air advection starts to dominate the balance of vertical motions.
Scattered storms are expected mainly in the afternoon and evening along the cold front. Deep-layer shear in the 20 to 25 m/s range would be more than adequate for good organization, but in the meantime most high-resolution models agree on only embedded convection near the front line. The level 1 is extended westward into Austria for a few heavy to excessive rain events, as long as storms "swim" along the cold front, along with a more conditional hail and wind hazard in case one or two supercells yet manage to detach from the frontal boundary and propagate SE-ward into the warmer air.

A mesoscale CAPE maximum up to 1000 J/kg also remains in place in NW Italy, where an "Alpine plume" of steeper lapse rates overspreads rich low-level moisture in the Po Valley. Synoptic subsidence will likely suppress convective initiation, but a close monitoring is recommended, as any storm the forms could easily turn supercellular and produce large hail.

... England, France, BeNeLux, Germany, W Czech Republic, W and N Poland, Baltic States ...

The maritime polar airmass on the cool side of the frontal boundary features a neutral to marginally unstable stratification under intense vertical wind shear (more than 25 m/s across the 0-6 km layer, 10-15 m/s across the lowest km). Scattered and partly thundery showers are expected in response to daytime heating. Though they will not be able to consume the entire vertical wind shear, an organization into low-topped multicells, supercells and short bowing lines is expected. Isolated severe wind gusts and tornadoes are possible. Two regions in parts of Poland, Germany and the Czech Republic, where synoptic lift ahead of vorticity maxima should enhance the number of showers, are covered by a level 1.

... SW Turkey ...

Beneath the diffuse upper-level low, scattered, daytime-driven pulse storms under weak to moderate CAPE can produce a few marginally severe hail and rain events.