Storm Forecast
Valid: Thu 06 Jun 2024 06:00 to Fri 07 Jun 2024 06:00 UTC
Issued: Wed 05 Jun 2024 20:53
Forecaster: TUSCHY

A level 2 was issued across parts of SE Austria mainly for large hail and severe wind gusts.

A level 2 was issued for parts of E-Czechia into S Poland mainly for large hail and severe wind gusts.

Both level 2 areas get framed by level 1 areas with similar hazards but lower probabilities. In addition, heavy rain and an isolated tornado event are possible (including the level 2 areas).

A level 1 was issued for SE France into far W Italy mainly for large hail and severe wind gusts.

A level 1 was issued for parts of Greece mainly for isolated large hail and severe wind gusts.

A level 1 was issued for far E Europe into W Russia mainly for heavy to excessive rain.

SYNOPSIS

Ongoing negative NAM/NAO pattern with high latitude blocking causes a near stagnant European rossby wave pattern with an extensive area with low geopotential heights over the Norwegian Sea, framed by ridges over/S of Greenland and another one over far NW Russia.
A rather active branch of the subtropical jet over the N-CNTRL Antlantic expands further E. This brings a low-latitude wave with further amplification into the Azores, which is a rather anomalous height pattern for this area for this time of year.

This kind of height configuration assists in deep WAA from the SW and influx of modified polar air from the NW, stretching and intensifying thermal gradient over CNTRL Europe with increasing thermal wind driven shear. Although not exceptional strong, an atmospheric river builds into CNTRL Europe beneath a plume of northward fanning EML which results in a belt with enhanced instability from N Spain to S Poland.

The mentioned CAPE tongue builds along/just S of an extensive cold front, which gets stuck over CNTRL Europe and turns into a wavy boundary. It becomes the focus for a potential multi-day severe event. This cold front remains more progressive over E Europe with an extensive/unstable warm sector from E Europe into W Russia.

DISCUSSION

... A belt from SW France to the Alps ...

Mentioned near zonally aligned region with overlapping better tropospheric moisture, mid-level lapse rates and shear all create a favorable corridor for organized convection over CNTRL Europe. This extensive region will be devided into the following sub-regions:

... France into Switzerland/S-Germany...

Onshore flow from the 1-2 K anomalous warm Mediterranean Sea with ongoing moisture pooling along the wavy boundary creates favorable low-tropospheric moisture beneath weak/modest mid-level lapse rates. This offers capped MUCAPE in the 1.5 kJ/kg range over SW France. Traversing weak waves, the orography to the S and/or weak LL convergence along the frontal boundary may cause a few elevated storms. Most likely CI seems to be along the boundary, where MUCAPE atop the cap is weakest, so expect mainly isolated hail. The area just N of the Pyrenees bears watching for spotty CI along the mountains. In case an updraft manages to move off the orography into SW France, rich MUCAPE may support isolated large hail. Confidence in such an event however seems too low for any upgrade with dominant anticyclonic background flow.

Further E into S-CNTRL and SE France/far W Italy, a gradually eroding cap supports CI along the Massif Central with SEward moving multicells and supercells. Kinematics are supportive for large hail with 20 m/s DLS and 0-3 km shear in excess of 15 m/s. Prognostic hodographs show rich/deep effective streamwise inflow with elongated streamwise vorticity vectors in the 1-3 km layer. Upper ventillation won't be the best, but nevertheless we could see a few large hail/severe wind gust events worth a level 1. This activity weakens as it moves offshore over the Gulf of Genoa with drier background conditions and weaker CAPE.

CI for Switzerland probably awaits peak heating and a passing short wave to its N with best conditions for organized DMC activity over N-Switzerland, where 20-25 m/s DLS and MUCAPE in excess of 1000 J/kg overlap. Stronger subsidence behind a passed low-amplitude wave to the SE should suppress convection before the mid-afternoon hours. A few hours with favorable conditions for a few multicells/supercells exist into the evening with large hail and strong to severe gusts before vanishing CAPE/increasing CIN and eastward departing forcing should lower the severe risk. A diurnal driven lowering of the LCL could spark an isolated tornado with maturing storms.

Over S-Germany, the diffuse frontal boundary serves as focus for isolated CI with favorable conditions for a few longer tracked supercells during the afternoon into the evening hours. A few hail and strong to severe gust reports are forecast. In addition, a diurnal driven lowering of the LCL could spark an isolated tornado with maturing storms.
During the overnight hours the Swiss storms should also cross S-Germany eastward with upscale growing clusters possible. This activity seems to be more elevated with heavy rain and isolated hail a local threat. Nowcast BL modifications needs to be monitored as not much change is needed for near surface based activity, which would increase the nocturnal severe risk.


... Austria ...

Multiple rounds of convection occur in this area. During the early afternoon, CI is forecast along the Alpine crest with the passage of a low-amplitude wave from the SW. Rapid storm organization is forecast due to 1-1.5 kJ/kg MUCAPE and 20 m/s DLS. Forecast hodographs show nice structures for large hail although the statistic approach remains rather reluctant. Long/straight hodographs indicate a chance for splitting supercells with large hail and severe wind gusts, whereas LCLs aoa 800-1000 m AGL should limit any tornado risk. Splitting activity causes colliding storms which betimes grow upscale into a cluster with an additional heavy rain risk spreading into far E/SE Austria next to the mentioned hail/gust threat. Activity shifts into NE Slovenia/W Hungary before weakening into the overnight hours.
Although global models keep DLS rather modest, higher resolution models show peaks up to 18 m/s DLS (including regional upslope flow regime during the day), so added a confined level 2 for this region (also due to the high coverage of organized storms).

During the night, a cluster of storms from Switzerland moves E with ongoing model spread regarding its track. Either motion into SE Germany or into S Austria is possible. Weakly to uncapped conditions would support a hail/severe wind gust risk next to heavy rain with this nocturnal multicell/isolated supercell activity as it tracks E (clustered with some tail-end/semi discrete storms).

... Czechia into Slovakia/S-Poland ...

A favorable CAPE-shear space exists for long-tracked supercells with large hail and severe wind gusts the main hazard. 500-1000 J/kg MUCAPE with 20 m/s DLS and curved/elongated hodographs support well organized convection, which evolves during the afternoon hours and keeps going into the overnight hours. There could be some increase in tornado probabilities from E-Czechia into S-Poland as maturing storms enter better LL-CAPE environment with a decrease in LCL heights. A few supercells may survive well into the night over far S-Poland. A regional upgrade to a level 2 was added for this activity.

EZ and GFS both agree in the development of a weak LL depression, which crosses Poland to the NE. This may support clustering convection during the night moving into E/NE Poland. The severe risk should be low as this activity leaves the CAPE plume to the NE. Isolated hail and heavy rain may cause some issues and expanded the level 1 well N.

The low-amplitude wave from Austria should arrive during the evening hours and may spark a large cluster of storms over Slovakia, which shifts E. This activity faces modest MUCAPE and some capping issues, so heavy rain and isolated large hail will be the main risk. EZ driven models are the most optimistic ones with this activity.

... Far E Europe into W-Russia ...

A NE-ward sliding upper trough keeps mid-levels rather cool atop a moist low-tropospheric airmass. Weakly capped 500-1000 J/kg MUCAPE evolves during the day and widespread/early CI is forecast. Slow moving convection grows upscale into numerous clusters. Heavy rain with flash flooding but also copious amounts of small hail are forecast next to isolated strong downbursts and a low-end tornado risk (mesoscale accident). This activity weakens during the night .

... Greece ...

A few high-based afternoon storms evolve along the mountains with hail and severe downbursts the main risk. Some better SRH in the lowest 3km may cause temporal updraft rotation and large may approach severe limits on a local/short scale. This activity weakens until the evening hours.