Storm Forecast
Valid: Sun 27 Aug 2023 06:00 to Mon 28 Aug 2023 06:00 UTC
Issued: Sat 26 Aug 2023 17:06
Forecaster: TUSCHY

A level 3 was issued for the Balearic Islands mainly for multiple rounds of all kind (extreme) severe events, including significant tornadoes, damaging winds and excessive rain.

A level 2 extends from SE Spain to the Alps with varying potential for significant severe. Corse/Sardegna with damaging winds/large hail turning more into an excessive rain/tornado threat over NW Italy.

A level 1 was issued for a belt from Czechia to Poland into the N-Ukraine mainly for excessive rain, wind and hail. An isolated tornado is possible mainly over CNTRL/NE Poland into SW Belarus.

A level 1 was issued for NW Germany and parts of the Netherlands mainly for an isolated tornado event (Germany) and heavy rain (the Netherlands).

A level 1 was issued for SW Turkey mainly for isolated large hail/severe wind gusts.

SYNOPSIS

An anomalous trough over W Europe becomes the main driver for this severe weather outbreak. The main energy drops south into a sharp upper trough configuration, which shifts its tilt from positive to negative during this forecast while crossing the W Mediterranean. Its 500 hPa geopotential height anomaly is rather impressive for this time of year. Even more impressive is the low/mid-tropospheric pressure response with a rapidly deepening low. ECMWF brings this depression to record level over the Ligurian Sea during the end of the forecast with similar results of GFS. This evolving depression pushes a cold front from the W to the CNTRL Mediterranean.

The northern part of this trough evolves into a lower amplitude wave, which crosses N Germany during this forecast period.

East of this trough configuration a wavy frontal bounday extends from the Italian depression NE towards Poland, where IFS-ENS agrees nicely in another evolving/weak depression.

Overall NWP guidance agrees well in this setup with ongoing manageable discrepancies regarding the speed of the Mediterranean cold front and geometry/placement/intensity of the LL depression over the Ligurian Sea. Those discrepancies affect mainly the nowcast and placement of potential most significant severe. Overall agreement in numerical guidance increases confidence in a widespread severe thunderstorm outbreak over parts of Europe.


DISCUSSION

... Far N Austria/ Czechia into Poland and Belarus/Ukraine ...

Overall CAPE-shear space is supportive for organized convection in the brisk SW-erly flow regime along the W fringe of a still anomalous strong/warm ridge but uncertainties regarding CI remain high.

Main focus for CI will be the wavy synoptic-scale front, which evolves into an E-ward pushing cold front over Czechia into S Poland and a N-ward surging warm front further E/NE as the aforementioned surface depression consolidates over E/NE Poland during the day (driven by the ejecting energy over N-Germany and an approaching (but not constructively interferring) right entrance region of a 300 hPa speed max).

Uncertainties exist how much residual convection (decaying MCS with potential PV+ anomaly/MCV activity from yesterday's Austrian level 3 event) will cross the area until noon but growing concern exists for some forenoon activity. Would not be shocked to see a temporal weakening (forenoon) cluster exiting E/SE Poland, before re-intensifying over S/CNTRL Belarus beyond noon in an 1.5 kJ/kg MLCAPE / 15m/s DLS environment. Large hail and severe gusts (inverted-V) with heavy rain will be the main risk. This is not yet explicitly shown in NPW guidance as models like ICON/IFS and GFS seem to track any developing PV+ anomaly to the NE deep into CNTRL Belarus, whereas we expect the convection to turn more to the E into S-Belarus, maybe even SE into NW Ukraine, follwing 1000-500 hPa thickness and growing into weaker capped/CAPE rich air.

Further W (Czechia into Poland), conflicting signals exist regarding synoptic-scale lift either due to dominating background subsidence or traversing very weak IPV maxima. Spotty diurnal CI is possible along orography or where convective temperature will be exceeded, but otherwise the main activity should reside along the cold and warm front. In case of the aggressive ICON/UK solution regarding a stronger LL depression, organized convection with all kind of severe is possible from Czechia into Poland during the afternoon onwards but models remain rather reluctant for now. Hence no level 2.

... W-Mediterranean ...

One focus for organized convection exists along the E-ward pushing cold front, which gains momentum dependant on where the developing depression evolves (far E Gulf of Lion like ICON indicates or the Ligurian Sea as see from UK/GFS with IFS in-between). The across-spread in NPW guidance for Monday 00Z is roughly 100-150 km regarding the cold front placement.

A level 2 was added for Corse/Sardegna for the afternoon/evening as the cold front crosses E with capped prefrontal CAPE/shear space offering MUCAPE in excess of 3 kJ/kg and 3/6km shear around 25 m/s. It remains hard to pinpoint where/when the surface cold front runs into too much cap (right now we expect this S of Sardegna) with DMC affecting mainly N/CNTRL Sardegna and Corse. Any discrete cell poses an all kind (sig.) severe threat, whereas bowing/lining-up convection poses a damaging/hurricane-force wind gust threat with ICON-EPS even showing weak signals of 140 km/h (+) gust potential. A level 3 was considered but uncertainty in any corridor with most damaging events remains too high right now.

Over the Balearic Islands, numerous rounds of (extremely) severe convection is anticipated.
As the main upper trough with a distinct IPV maximum ejects out of E Spain, it acquires a more negative tilt during the day with maxed out Q vector convergence indicating deep/strong background lift, enhanced by the E-ward surging cold front.
First round of DMC activity probably evolves over far SE Spain during the start of this forecast, before growing upscale into a progressive MCS event with embedded supercells. All kind of (extremely) severe hazards are forecast including a tornado risk.

This risk continues beyond noon as the upper trough approaches. Final low-mid tropospheric wind field depends on where the surface depresssion to the NE evolves, but scanning through NWP guidance, forecast hodographs/soundings support an ongoing risk of supercells with severe wind, excessive/flash flood producing rain, large hail and (significant) tornadoes. Overall expected high coverage of (extremely) severe events supports a level 3 upgrade. This risk diminishes during the night as drier air filters in from the NW.

... Ligurian Sea into S Switzerland ...

Another area of concern regarding flash flood producing rain exists within the deep/moist warm conveyor belt (WCB) regime (EFI in the 0.9 - 1 range with positive SOT).

First concern exists for a forenoon band of training convection around Genoa with a strong pre-cold frontal convergence zone in mentioned deep/very moist WCB environment. Could see a confined band with 100-200 l/qm rain in 6-12h like inidcated by ICON-EPS and COSMO 2.2. Like so often, a few tornadoes along this convergence zone area also possible.

A second round of excessive rain with training storms is possible during the overnight hours along the N coast of the Ligurian Sea. Pinpointing where any training convection sets-up is not possible with uncertainties regarding placement/strength of the LL depression, but in general a level 2 worth risk of excessive rain exists all night long with local extreme rainfall amounts possible (would not be surprised about a few 24h amounts in excess of 300 l/qm). Once again, a few tornados along any convergence zone are possible. This overnight event could evolve into a serious flash flood event!

Finally all of NW Italy into far SE Switzerland will see a prolonged period of favorable conditions for training thunderstorm activity with flash flooding. Strong WAA flow regime with 25-40 kt at 850 hPa feeds storms with copious amounts of moisture. Troposphere is more or less saturated with PPWs in excess of 40 mm with a deep warm cloud layer and elongated CAPE profiles. In addition, SRH-rich environment supports a few intense supercells (e.g. tail-end storms) with an augmented tornado risk next to hail and strong to severe gusts.
This risk continues into the night and expands E all the way to the Austrian/Italian border. Not sure if this activity also justifies a level 2, but given the dynamic and moist environment we decided to go E with the level 2. Despite decreasing CAPE values, a few rotating storms with severe gusts and an isolated tornadoes are still forecast.

During the end of the forecast organized thunderstorm also erupt S along the W coast of Italy with all kind of severe possible.

... NW Germany and parts of the Netherlands ...

Placed in a moist environment with low T-Td spread, moderate CAPE up to 7 km AGL and top weighted shear profiles indicate a chance for scattered showers/thunderstorms in a weakly capped environment. Any more organized cell with deviant motion will have adequate SRH for some rotation and an isolated tornado threat/hail risk exists. Activity weakens after sunset.

Slow moving convection over the W-Netherlands poses a local heavy rainfall risk.

... SW Turkey ...

A few storms in vicinity of an upper trough may bring large hail and severe wind gusts along the coast, where ingredients overlap. A level 1 was added.

... Lightning areas ...

Parts of CNTRL Sweden may see a few slow moving storms with heavy rain but not enough for a level area.