Storm Forecast
Valid: Fri 02 May 2025 06:00 to Sat 03 May 2025 06:00 UTC
Issued: Thu 01 May 2025 19:45
Forecaster: TUSCHY

A level 2 was issued for parts of Spain mainly for large to very large hail, severe gusts and heavy rain. A tornado risk (including a strong event or two) exists as well.

A broad level 1 surrounds the level 2 with lowered probabilities. The heavy rain and isolated tornado threat dominates towards Portugal.

A level 2 was issued for parts of SE Turkey mainly for large to isolated very large hail and strong to severe gusts as well as for heavy rain.

A level 1 surrounds the level 2 for similar hazards with lowered probabilities.

A level 1 was issued for parts of Germany, Poland into S Lithuania mainly for severe gusts and large hail. A low-end tornado risk can't be ruled out.

A level 1 was issued for parts of E-CNTRL Sweden mainly for severe gusts and isolated hail.

SYNOPSIS

There is not much new input to serve as ongoing blocking event keeps general pattern rather static for most parts of Europe. In fact the stout ridge intensifies over the CNTRL Mediterranean with only a marginal net motion to the E.

Framed by those two long-lived cut-offs, unsettled conditions persist for parts of SW and SE Europe. The main focus convective-wise turns towards Spain, where a wavy front becomes mobile as it transforms into an eastbound surging cold front.

A progressive neutral tilted trough crosses S Norway/Sweden before approaching Finland during the night while acquiring a more negative tilt. This feature gets accompanied by a potent basal jet with 90 kt at 300 hPa and 50 kt at 500 hPa respectively. While crossing the Scandinavian Mountains, a re-forming lee cyclogenesis pushes a cold front E/SE, which aligns more parallel to the background flow as it affects N Germany into Poland. This assists in a broad overlap of augmented shear along the front with modest CAPE. A post frontal well mixed airmass covers most of Sweden with some diurnal driven convection forecast during the day.

EFI CAPE shear peaks over N-CNTRL into NE Germany including positive SOT values and is mostly affected by strong shear values. The same but with even more impressive values and areal extent is valid for most of Spain. EFI 2m temperature highlights the vast areal extent of temperatures well above average.

One word to a northward advancing dust plume ahead of the western cut-off. Yesterday's impact over Spain was marginal despite modest dust load values. For Spain, the main dust should stay E/N for the main convective period. Current expectation is that favorable timing for dust induced cloudiness will stay over France with some advection towards CNTRL/S Germany as the frontal intrusion into N-Germany should keep highest dust load to the south of the area of interest. Nevertheless, wet deposition is possible into NE Germany but with no serious diabatic impact. This has to be monitored closely however.


DISCUSSION

... N-CNTRL to NE/E Germany, N/CNTRL Poland to the Baltic States ...

A S/SE ward moving cold front interacts with a CAPE plume, which peaks to the W (e.g. Germany) and weakens to the E. Scattered to widespread thunderstorms occur during the forecast in a rather broad sector.

The prefrontal airmass features low to modest moisture content. Despite some moisture advection from the W, strong diabatic heating should support a deep/well mixed BL. This airmass turns unstable during the day in response to some prefontal steepening of the mid-level lapse rates and offers 500 to locally 1000 J/kg MUCAPE over N Germany with a constant decreasing trend towards Poland/Baltic States.

Strongest synoptic-scale ascent passes well N of the cold front, so background forcing becomes a bit diffuse and is probably focused along the front itself.
Prefrontal capping is weak and should erode during the afternoon, which could als support a few prefrontal thunderstorms with initiation along the orography. Modest forcing should initially support semi-discrete cells before clustering occurs.

Shear is rather strong with readings in the 15-20 m/s range for DLS, while 0-3 km shear resides mostly below 10 m/s, although an increase is seen next to the cold front itself. Forecast hodographs are elongated and curved with SR hodographs showing good conditions for precipitation venting away from the updraft's path. Numerous multicells and supercells are forecast beyond noon with hail and locally heavy rain from N-CNTRL Germany to NE Poland. Inverted V shaped BL profiles support swaths of strong to severe gusts and there is a distinct chance for numerous bowing segments. EPS data indicates enhanced probabilities for numerous 25 m/s gust events. A more robust event could have been expected in case of better CAPE build-up, but even with limited chances for entrainment, forecast CAPE is modest at best. A few long-tracked supercells with confined swaths of severe are still forecast but no focus exists in NWP data for any upgrade. The large hail threat should peak towards N Germany with better CAPE and weak/modest LL shear.

High LCLs should lower the tornado risk but nevertheless a low risk exists with cells riding the warm side of the cold front with regionally lowered LCLs or during cell interactions.

Further NE towards the Baltic States meager CAPE should lower the general severe risk but we nudged the level 1 at least a bit into Lithuania for hail/gusts.

... Parts of Spain ...

A severe event unfolds for parts of S/E Spain. The environemntal conditions support long-lived supercells with all kind of hazards despite some issues regarding the final thunderstorm coverage in latest NWP data.

Persistent onshore flow from the Mediterranean advects a moist airmass beneath steep mid-level lapse rates especially along the complex orography of CNTRL Spain. Initial more elevated CAPE turns more surface based during the day with adequate diabatic heating forcast.

Thunderstorms will already be in progress along the front itself with numerous northward racing elevated supercells posing a large to isolated very large hail threat roughly along a line from Seville/Cordoba to Madrid (ongoing from the pervious night probably until noon). This activity is forced by a N-ward lifting upper wave.

Behind this lifting wave, a less disturbed S/SW-erly mid/upper flow regime affects Spain with initial subsidence turning more into weak lift during the afternoon hours (caused by embedded weak low-amplitude waves). A N/NE ward surging dry slot also suppresses CI behind the front but could induce CI over SE Spain during the afternoon as it interacts with the surface cold front. In general models diverge in showing certain spots with more focused convection and hence broad level areas were added.

From noon onwards, the NE ward shifting activity of elevated supercells becomes more surface based but outruns best CAPE plume, so despite a gradual decrease in strength, storms still pose a risk of strong to severe gusts, hail and heavy rain. This is not true for convection along the Pyrenees, which moves N into SW France, where high CAPE and strong shear support an ongoing severe risk with hail, gusts and heavy rain.

In the wake of that activity models diverge where and when thunderstorms erupt but all show at least a few regions with enhanced supercell proabilities. The most volatile environment seems to evolve over SE into E Spain, where 1 to 1.5 kJ/kg MUCAPE, 20-25 m/s DLS and up to 20 m/s 0-3 km shear overlap. Any growing cell is capable of maturing into long-tracked supercells with a risk of large to isolated very large hail, severe to damaging gusts and heavy rain. Forecast hodographs show long and rather straight signatures with varying curvature in the lower troposphere, so expect a mix of splitting and dominant right moving supercells. The rather well mixed BL mitigates the tornado risk somewhat but nevertheless there exist some concerning signals for even strong and longer lived tornadoes as maturing storms see strong streamwise helical inflow into updrafts and regionally lowered LCLs both due to the orogrpahy itself but also due to enhanced BL moisture on a regional scale.

Towards CNTRL and N-CNTRL Spain, CAPE lowers a bit but impressive shear and still adequate CAPE support numerous well organized multicells and supercells with all kind of hazard.

Further W towards W Spain, shear weakens but forecast hodographs still support a mix of multicells and supercells with a growing tornado risk as LCLs continue to lower. However, bad timing during the dirunal cycle and some overturning of morning convection adds uncertainties in how unstable this airmass gets next to constant drying with the approaching dry slot. Hence a level 1 was issued for this area.

During the night, the activity once again turns more elevated with strengthening CIN, so the large hail risk becomes the more dominant threat betimes with an all kind hazard until midnight. In addition, convection ends from W to E as th cold front advances E.

A broad level 2 was issued mainly due to issues with areal coverage of storms as various possibilities for CI along the orography exist next to lingering questions regarding the final impact of nocturnal convection.

... Parts of CNTRL/E Turkey ...

As a jet maximum within the northward bending subtropical jet crosses the area of interest, diurnal heating in combination with onshore advecting moisture create an overlap of modest (and mostly elevated) MUCAPE with strong shear. Widespread thunderstorms are forecast with hail and strong to severe wind gusts. As thunderstorms cluster betimes, heavy rain becomes another issue. Due to the high coverage of severe in an area with minimal CIN, a small level 2 was added. A broad level 1 was expanded well E/NE as cap should strengthen with lowering elevated CAPE values. Still numerous severe event are forecast (hail/gusts and rain).

... E-CNTRL Sweden ...

Numerous showers and a few high-based thunderstorms erupt during the afternoon behind an eastward shifting cold front in modest CAPE but strong shear. A deep/well mixed BL airmass supports severe to damaging gusts with any well established updraft/downdraft interface. We could see some lack of lightning activity but CAPE should expand into the graupel zone at least for some hours so a low prob. lightning area was added.