Storm Forecast
Valid: Tue 09 Sep 2025 06:00 to Wed 10 Sep 2025 06:00 UTC
Issued: Mon 08 Sep 2025 18:26
Forecaster: TUSCHY

A level 2 was issued over the Balearic Islands and far E Spain with a repeated risk for severe to isolated extremely severe hazards like excessive rain, large to very large hail, severe/damaging wind gusts and/or a few tornadoes with a significant event (over the Balearic Islands) not ruled out.

Two level 2 areas were issued for W-CNTRL Italy and far NE Italy with an excessive rain and coastal tornado threat. Sardegna will also see a large hail risk.

A level 1 surrounds all other level areas for similar hazards with a lowered severe risk.

A level 1 was issued over parts of W Germany with an heavy rain risk.

A level 1 was issued for parts of E-CNTRL Europe mainly for isolated heavy rain and a tornado or two.

SYNOPSIS

A still amplifying upper trough over W Europe faces a blocking anticyclone over NE Europe into Russia. This trough taps into a very unstable airmass over the W/CNTRL Mediterranean and accompanying impressive shear assists in a large-scale DMC risk.
Numerous cut-offs along the S fringe of the blocking anticyclone bring unsettled conditions to parts of E Europe.

The main synoptic-scale front for thunderstorm activity is a wavy and slow moving boundary over W/SW Germany, whereas otherwise the activity will be driven more from the mid/upper levels and from mesoscale activity in the BL.

DISCUSSION

... W-/CNTRL- Mediterranean ...

A rather classic severe thunderstorm outbreak is forecast for parts of the W/CNTRL Mediterranean. A broad overlap of strong shear and modest/high CAPE creates a favorable setup for organized DMC activity.

A prolonged period of cyclonic SW-erly flow with numerous embedded short waves supports repeated CI for the highlighted area. A pronounced lead short wave exits the base of the longwave trough and crosses the W Mediterranean to the NE until 00Z. Thereafter, the main upper trough axis approaches from Spain and supports further CI.

Ongoing from the previous night, expect DMC E of Spain/atop and near the Balearic Islands in 20-25 m/s DLS, 10-15 m/s 0-3 km shear and 1500 to 2500 J/kg MUCAPE. More subtle background forcing by the lead impulse should induce semi-discrete CI and long-lived/severe supercells are forecast. Storm-intern dynamics like RIJ formation could induce temporal line-ups with focused swaths of severe gusts. Otherwise all kind of severe is possible with DMC activity including excessive rain, very large hail (in excess of 5 cm not ruled out), severe/damaging gusts and a few tornadoes. A significant tornado event cannot be ruled out.
This cluster of supercells or bowing segments approaches Sardegna during the afternoon hours. While outrunning the best shear, kinematics still support organized multicells and a few supercells with all kind of hazards. The main uncertainty is how well mesocyclones fight against increasing CIN and weakening background forcing as the lead impulse turns more to the NE (towards the Ligurian Sea).

In the meantime, ongoing SW-erly flow regime still affects the W Mediterranean basin and as the main upper trough approaches from the W, persistent onshore flow advects the marine airmass well ashore. Despite weakening LL/0-3km shear, impressive DLS and upper divergence affect E Spain all the way to the Balearic Islands and another round of DMC activity is forecast - once again with all kind of hazards. Dependant on where mesoscale convergence zones and a structuring diffuse LL depression set up between E-CNTRL Spain and the Balearic Islands, we could see confined swaths of extreme rainfall amounts also onshore. In addition, more discrete thunderstorms produce large to very large hail, severe gusts and a few tornadoes with a few significant hail/wind events not ruled out.
A level 3 upgrade was considered for the Balearic Islands but it remains uncertain, if those islands indeed will be hit by any supercell or line segment, which get influenced also by hard to forecast marine boundaries.

Over parts of CNTRL/N Italy, background conditions improve for organized DMC activity during the overnight hours as the lead impulse approaches from the SW and impignes on a very moist and unstable airmass with LL mixing ratios offshore in excess of 14-16 g/kg and MUCAPE in the 1000-2500 J/kg range. The northerly path of the lead impulse results in low-tropospheric mass response atop the CNTRL Mediterranean and a persistent and strengthening S-erly flow regime is anticipated. This points to hard to forecast localized V-shaped and training MCS events along the coasts, which evolve along mesoscale convergence zones, partially shaped by the orography. Right now, we highlighted the areas of most ensemble/deterministic support, where to expect an augmented risk for excessive rain. A few tornado events along the coast are also forecast. This risk extends beyond 06Z.

In addition, we could see an overnight risk of and organized MCS with swaths of severe gust affecting parts of Sardegna into the Tyrrhenian Sea. This risk extends also beyond 06Z.

... W-Germany ...

A plume of unseasonable moist air resides in the area of interest with TPW background values of 200% or more compared to the climatology.
A complex interaction of a temporarily structuring LL depression over W-CNTRL Germany and a N-ward lifting low-amplitude wave beneath strong upper divergence in a coupled jet configuration seem to be prime for clustering and slow moving convection. In addition a residual plume of 0-4 km CAPE exists, so embedded thunderstorms accompany the more stratiform rain event, increasing rainfall amounts on a local scale. With TPWs in the 30-35 mm range and slow storm motions, expect an ongoing and N-ward lifting cluster, which weakens betimes as the diurnal minimum arrives. However, dependant on any SR flow enhancement by (partially convectively initated and enhanced) positive PV anomalies, regional excessive rain is possible until noon/the early afternoon. Despite rapid decline of thunderstorm activity, we kept this event in our level scheme mainly due to the ongoing convective dominant nature for the most organized rain bands. This event ends from S to N during the afternoon into the evening hours.

... NE Germany ...

During the afternoon into the evening hours, a more focused band of mass fluss convergence evolves over parts of N/NE Germany, which is mainly highlighted by ID2. If this verifies, we could see a band of repeated thunderstorm activity with a confined risk for heavy rain. No level area was added for now due to lingering uncertainties in NWP guidance. However this area needs to be monitored for higher probabilities.

... Far E Poland to far W Ukraine ...

Two upper cut-offs undergo a Fujiwhara effect over E-CNTRL Europe. The initial dominant but constantly weakening cut-off exits Czechia E into SW Ukraine, while another one moves from S Belarus into NE Poland and intensifies. Both mid/upper level vortices tap into an unstable airmass of 400-900 J/kg and offer weak shear. Clustering and slow moving convection creates an heavy rain and isolated tornado risk. This threat ends until sunset with a lingering risk towards far NE Poland (mainly rain risk).