Storm Forecast
Valid: Sun 08 Jul 2018 06:00 to Mon 09 Jul 2018 06:00 UTC
Issued: Sat 07 Jul 2018 06:42
Forecaster: TUSCHY

A level 1 was issued across N Spain for large hail and isolated severe wind gusts.

A level 1 was issued across NE Algeria and NW Tunisia for large hail and isolated severe wind gusts.

A level 1 was issued across NE Italy, Slovenia and far NW Croatia for large hail and severe wind gusts.

A level 1 was issued for parts of Greece for large hail and excessive rain.

A level 1 was issued for NE Bulgaria and E Romania mainly for large hail and excessive rain.


SYNOPSIS

An anticyclone over NW Europe steers one trough towards Norway and another one from the Czech Republic towards the Adriatic Sea. Excluding the Mediterranean and surroundings a rather dry and stable air mass occupies most of N/E and CNTRL Europe. As a result only few areas see conducive conditions for DMC activity.

No synoptic-scale fronts impact today's thunderstorm forecast.

DISCUSSION

... N-Spain...

A confined upper low over the far S-Bay of Biscay and extreme N-Spain weakens during the forecast and opens up into a zonally aligned channel of low geopotential heights. Attendant cool mid-levels start to warm a bit during the day but still assist in adequate mid-level lapse rates atop a moist BL for weakly capped 500-1000 J/kg MLCAPE. Initially poor DLS strengthens during the day as the dissolving upper low drifts north and as a subtropical ridge pushes towards S-Portugal/Spain. DLS of 15-20 m/s during the afternoon and evening indicates a good chance for organized multicells and isolated supercells with large hail and isolated severe downburst events. An isolated tornado event can't be ruled out with 150 J/kg LLCAPE and LCLs around 800-1000 m agl. The activity weakens during the night.

...Slovenia and NE-Italy...

A passing upper trough and attendant brisk N-erly mid/upper level flow push an alpine EML south towards the area of interest. There, a moist BL awaits the steeper mid-level lapse rates and resulting overlap supports 500-1000 J/kg MLCAPE. With the passage of a mid-level speed maximum DLS increases to 20-25 m/s and 0-3 km shear to 10-15 m/s. Initiating thunderstorms along the southern rim of the Alps spread rapidly south and organize into multicells and supercells as they encounter the better CAPE plume. Long and straight hodographs point to a chance for splitting supercells with large to isolated very large hail and severe wind gusts. The activity becomes elevated over the Adriatic Sea and in general tends to weaken beyond sunset due to lowering CAPE. Still a lingering severe risk is possible along the coast of NE Italy into far NW Croatia during the first half of the night.

...Greece to Romania...

Low geopotential heights atop moist LLs cause widespread 400-1000 J/kg with CAPE values increasing towards Greece. Weak capping and low DLS favor early CI of pulsating thunderstorms, which grow upscale during the day into numerous clusters. Initiating thunderstorms bring a hail and wind gust risk (locally severe) but the main hazard will be excessive rain with slow moving and clustering convection.

With increasing CAPE and shear towards Greece the large hail risk seems to warrant the issuance of a level 1.

Another level 1 was issued for areas next to the Black Sea, as LL mixing ratios in excess of 13 g/kg assist in SBCAPE of more than 1000 J/kg despite meager mid-level lapse rates. Resulting robust updrafts may contain large hail and produce flash-flood producing rain due to slow storm motions.

...W-Russia ...

A low shear and weak to moderate CAPE setup is forecast with diurnal driven thunderstorm activity. Pulsating thunderstorms bring graupel/isolated hail and gusty winds. The activity weakens during the late evening hours and overnight.

...Tell Atlas ...

Onshore flow advects a marine air mass ashore north of the mountains while an EML plume with mid-level lapse rates in excess of 8 K/km spreads north over that air mass. With 850 hPa temperatures shy below 25 degree Celsius a stout cap is expected during most of the day.
Probabilities for isolated CI exist during the night, as an upper-level impulse circles the subtropical ridge and grazes N-Algeria while moving to the SE. Effective 15 m/s mid to upper-level shear and 500-1000 J/kg elevated CAPE could support an isolated but organized elevated thunderstorm activity over NE Algeria/NW Tunisia with large hail the dominant risk. Not sure if any downdraft may break the thick cap but at least an isolated event can't be excluded due to the elevated terrain.