Storm Forecast
Valid: Sat 31 Jul 2021 06:00 to Sun 01 Aug 2021 06:00 UTC
Issued: Fri 30 Jul 2021 14:13
Forecaster: TUSCHY

A level 3 was issued for N Italy into extreme S Austria mainly for very large to giant hail and severe to extreme wind gusts.

A level 2 surrounds the level 3 mainly for (very) large hail, severe wind gusts and heavy rain.

A level 2 was issued across far E Spain to the Balearic Islands mainly for (very) large hail, severe wind gusts and heavy rain.

A level 1 surrounds both level 2 areas with similar hazards but with less coverage. An isolated (nocturnal) tornado threat exists over NW and NE Italy into Slovenia/far NW Croatia. Also along the E-CNTRL coast of Spain and over SE Austria into extreme W Hungary.

A level 1 was issued for the SE Ukraine into SW Russia mainly for hail and excessive rain.

A level 1 was issued for parts of the Baltic States/Sea and SE Sweden mainly for severe wind gusts. An isolated tornado event is possible.

A level 1 was issued for N Algeria mainly for large hail and strong to severe downbursts.

A level 1 was issued for far N Germany mainly for a few severe wind gusts and an isolated tornado threat.

SYNOPSIS

One of the four dominant north hemispheric Rossby waves is forecast to cover most of Scandinavia down to the Iberian Peninsula. This wave has numerous smaller scale embedded vorticity maxima - one crossing Denmark E/NE with another one impacting most of Spain/SW France during this forecast.

The eastern fringe of this Rossby wave is framed by a wavy and gradually eastward shifting frontal boundary, which separates a warm/hot and humid airmass over S-Euope from a drier/more stable one to its north.
Downstream ridge covers most of CNTRL/E Mediterranean into E Europe with a broad/ill defined upper wave over the Near East.

Overall model performance is good, with models showing a nice consistency during the past few runs - valid for GFS/IFS and ICON. Some discrepancies regarding timing exist with the Spanish wave as ICON f.ex. speeds up in the past 2 runs. However it is still unclear if this trend persists. Overall this won't lower the expectations in a severe risk as general geometry of the hight field agrees well in NWP.

DISCUSSION (UPDATE)

A widespread outbreak of severe thunderstorms is forecast for parts of SW into S-CNTRL Europe.

For a more detailed discussion please check yesterday's extended outlook which is attached. We will just update any changes for this SOWDY 1.

... E Spain to the Balearic Islands to N Italy into Hungary/Austria and Switzerland...

Still a complex forecast over E/NE Spain to the Balearic Islands with less aggressive NWP guidance compared to this pugnacious outlook. However there is not much in the way for developing organized convection over E/NE Spain which grows upscale while racing towards the Balearic Islands. Intense QG forcing for this time of year should overcome capping issues, although offshore moving convection might become more elevated betimes. Very large to giant hail with damaging wind gusts are possible in this level 2. An isolated tornado is possible along the coastal areas of E-Spain, where maturing supercells interact with the sea breeze. Locally excessive rain is an issue with merging and initiating cells along the orography.

The level 2 over N Sardegna/Corse was re-issued despite meager QPF signals in NWP guidance. I weightened the EPS output more as a few members seem to break the cap. Passing short-wave should add enough input for elevated convection with large hail and heavy rain the main risk. This part of the level 2 remains quite uncertain and is a nocturnal event.

Not much change to the level 2 over N Italy into parts of Austria/Hungary and Slovenia despite a broad upgrade to a level 3. Some LAMs keep convection attached to the Alps, but this seems unlikely. Apart from the mountains CI will be delayed until the afternoon. Thereafter, long-tracked severe supercells with very large/giant hail, damaging winds (locally in excess of 65kt) and heavy rain are forecast. The tornado risk ramps up in the eastern part of the level 3 with enhanced LL directional/speed shear and rather low LCLs - only the upscale growth into a severe bow echo may lower the general risk a bit (isolated discrete convection still possible next to the N-Adriatic Sea).

There is not much reasoning against this MCS to race trough E-Austria during the night, attached to the progressive short-wave. A general decrease in severe is forecast with weakening CAPE but still impressive shear.

The level 3 was expanded into SE Austria, where a cluster of supercells are forecast during the afternoon/evening (prior to the nocturnal MCS event). (Very) large hail/excessive rain and severe wind gusts occur next to an isolated tornado threat.
We expanded the level 2 more into W Hungary as maturing multicells/supercells from SE Austria grow upscale during the afternoon/evening hours. Large hail/severe wind gusts will be the main issue but an isolated tornado event is possible with deviating cell motion.

The level 3 was expanded into the Lombardy for numerous waves with (extremely) severe events - first supercells during the afternoon/evening and clustering convection during the night with a serious flash flood threat (some ID2 members approach 180 l/qm/6h during parts of the overnight hours).


... N-Algeria ...

Same uncertainty exists for this area, but latest EPS guidance increased confidence for scattered elevated CI to add a level 1 area mainly for hail and strong to severe wind gusts.

... N Germany ...

A shallow warm-core low is still forecast to cross this level 1 during the daytime hours. Expect a more organized line of convection to accompany this depression, which would increase the local severe wind gust threat with strong background flow.
In addition, an isolated tornado event is possible along the S fringe of this low with somewhat augmented LL shear within a low LCL environment. The mesoscale/nowcast dictates this threat.

... SE Sweden to the Baltic States ...

An occluding cold front shifts E/NE with a prefrontal unstable airmass offering 500-1000 J/kg MLCAPE. Don't want to exclude an isolated tornado over SE Sweden with better shear in the lowest 1 to 3 km AGL but otherwise this is more a wind gust driven level 1.

Otherwise not much was changed from yesterday's philosophy.




PREVIOUS DISCUSSION from the extended outlook

... E Spain to the Balearic Islands to NW Italy...

A rounding vort lobe adds substantial background (QG) forcing which exits Spain to the E during the daytime hours. Degree of E-coastal onshore flow remains questionable with a general MSLP decrease over the W Mediterranean, but there is a time window until the afternoon for onshore advection of a very moist marine airmass with BL mixing ratios in excess of 14 g/kg, pushing MLCAPE in the 1-2 k/kg range. 25 m/s DLS and 15 m/s 0-3km shear with straight hodographs support organized updrafts with (very) large hail and damaging downbursts (augmented by drying at mid-levels). The level areas were expanded onshore where current EPS guidance gives the best signal.

Further E towards the Balearic Islands, CAPE-shear space improves substantially to 2000-4000 J/kg MLCAPE and 25-35 m/s DLS with up to 25 m/s shear in the lowest 3 km AGL. Capping will be an issue, but a more zonally aligned thermal gradient and a strengthening Tramontane wind event setting up a BL convergence zone in the similar area with an orientation like the thermal gradient - increasing confidece in CI (combined with the intensifying background forcing).
All kind of severe is possible with extreme events likely in such a setup. Past events however showed that NWP guidance often had problems with the cap. But most models indicate either QPF signals or traces of precipitation, which is already a sign for (elevated) convection, so we issued a level 2 for this event.

Further E, a splitting UVV field, strongest surface pressure fall along the French/NW Italian coast and the SE ward spreading Tramontane may cause some weakening between the Balearic Islands and Sardegna/Corse, but this is of low confidence with often understimated QPF of organized convection. We therefore lowered the risk level over Sardgena (elevated convection with heavy rain and (very) large hail) but we kept a level 2 from Corse to NW Italy, where cap weakens. Curved LL to mid-level hodographs along the coastal regons indicate an augmented nocturnal tornado risk - combined with a regional excessive rainfall risk around Genoa with enhanced LL flow ahead of the structuring French depression.

... N Italy into S Austria and Slovenia ...

Another day with a favorable CAPE shear space (20-25 m/s DLS with 0-3 km shear increasing to 20 m/s during the evening hours) is in store for this region. A progressive/flat ridge dampens CI until the early afternoon hours.
CI occurs first along the mountains but already then, multicells/transient supercells pose a large hail/severe wind gust and heavy rainfall threat. Flash flooding is possible along the Alpine rim until convection detaches from the orography.
Later on, this convection enters the lowland of N Italy and SE Austria into Slovenia/Hungary and grows into long-lived multicells/supercells with (very) large (wind driven) hail, damaging wind gusts and locally heavy rain and betimes into an eastward propagating MCS into Hungary, which weakens during the night with a restrengthening cap.

Of concern is the increase of 0-3 km shear to 20 m/s during the evening hours over N Italy, which favors a gradual upscale growth into a severe and long-lived MCS over NE Italy, which moves into Slovenia/SE Austria. Hurricane-force wind gusts are possible including large hail. Expect this MCS to spread far east and probably way more than current NWP guidance indicates. A nocturnal tornado threat exists over NE Italy into W Slovenia/NW Croatia.

... S Poland into W Russia ...

Substantial uncertainties exist with an ongoing MCS from the previous night - with timing and placing issues. A progressive mid-level short wave races E together with a well structured thetae ridge, so there exists no obvious reason why one should not expect re-intensification after the morning depression. CAPE/shear space supports multicellular convection with large hail and strong/severe wind gusts - beside heavy rain.

... N Germany to Denmark to the Baltic States ...

Coupled to a progressive upper trough is a anomalous deep surface depression (-2 to -3 sigma to 79-09 climate), which crosses the German Bight east with a surface pressure of around 994-998 hPa. Enhanced LL convergence along its fringes will be the foci for bands of augmented convection - probably capped around 600 hPa. DLS in the 10-15 m/s range is enough for a localized hail/strong to severe wind gust threat. An isolated tornado risk exists but depends on the final track/strength of the LL vortex and has to be evaluted in further model data.

Further E over the Baltic Sea to the Baltic States, an ongoing severe risk remains attached to the E ward racing wave. Strong to severe wind gusts with local hail remain the main hazard. Again, the LL wind field could also support a local tornado event, especially along the coastal regions of the N Baltic States.

No level area was added for now but a local upgrade may become necessary later on. One exception is the already upgraded area to a level 1 mainly due to severe wind gust issues with nocturnal convection.

...SE Ukraine to SW Russia ...

Along an elongated confluence zone, rich BL moisture pushes MLCAPE in the 1-2 kJ/kg range. Shear at all levels is weak, so pulsating convection with initially a large hail threat but later on an excessive rainfall risk.