Storm Forecast
Valid: Fri 31 Aug 2018 06:00 to Sat 01 Sep 2018 06:00 UTC
Issued: Thu 30 Aug 2018 14:51
Forecaster: TUSCHY

A level 1 was issued across parts of N Italy and NW Croatia mainly for isolated large hail, excessive rain and a low-end tornado threat.

Two level 1s was issued across W-Georgia mainly for excessive rain.

SYNOPSIS

Rather quiescent conditions (convective-wise) are forecast during this forecast as an upper longwave trough over N/CNTRL Europe amplifies/elongates and weakens. This structural change is due to an approaching short-wave which circles the main trough. The wave crosses E France and Switzerlad during the day while evolving into a closed upper low over NW Italy during the night. The LL response will be a consolidating LL depression over NW Italy. Models still show differences in where and when this response occurs but overall falling pressure south of the Alps can be expected. This area will be a focus for organized convection (south of the Alps).

Numerous areas with isolated thunderstorm activity exist over many European regions and were highlighted accordingly with lightning and even level areas.

DISCUSSION

...N-Italy...

As the mid-level wave approaches during the daytime hours winds at 500 hPa increase to 15-20 m/s with similar DLS values expected during the afternoon and evening hours. Forecast soundings reveal a layer with steepened lapse rates between 800-500 hPa and weaker lape rates aloft, so the main CAPE build-up is forecast to occur in the mid-troposphere.

Early morning convection along the S-Alps (over far N Italy)
and attendant outflows probably lay out and sharpen a zonal convergence zone over the Po-Valley, which acquires chracteristica of a warm front with ongoing advection of a warm and unstable air mass to its south from the N-Adriatic Sea to the NW. Diabatic heating and the moist BL cause MLCAPE of 800-1200 J/kg between Rimini-Parma and Venice. This area will see the highest threat for organized multicells and a few supercells as forecast hodographs indicate curving profiles with a gradually strengthening deep layer flow. Amount of CAPE/shear indicate a large hail and severe wind gust threat, especially as convection grows upscale into a line over the far N-Adriatic Sea. An isolated tornado risk can't be excluded with this activity.
The severe risk diminishes over NE Italy and far W Slovenia with lower CAPE, but elevated thunderstorms could still produce hail. Coastal regions of the NE Adriatic Sea could see more surface based activity with lingering BL moisture and hence isolated severe (hail and wind gusts) are still possible during the night.

A similar CAPE/shear setup exists around Genoa and organized multicells can produce isolated large hail and severe wind gusts during the evening.

Despite the progressive nature of the thunderstorms, clustering could support a few flash flood events.


... South of the White Sea ...

Along and just south of a slow moving warm front, 10 g/kg LL mixing ratios beneath slightly enhanced mid-level lapse rates cause unstable conditions with MLCAPE aoa 1000 J/kg. Shear remains very weak and only increases in GFS to the south. Convection is forecast to grow upscale into slow moving clusters with isolated large hail during the more discrete stage, transitioning more into an heavy rainfall threat betimes. The overall risk seems to be too low for a level 1 upgrade and the activity weakens beyond sunset.

... E of the Black Sea ...

High low-tropopsheric moisture from the E-Black Sea advects ashore beneath weak mid-level lapse rates. Hence a tight CAPE gradient exists along the coast with 1000-1500 J/kg MLCAPE expected. Further inland CAPE decreases rapidly to less than 800 J/kg. Wind shear is very weak, so the main risk will be slow moving and clustering thunderstorms with excessive rain. The areas with the highest risk were upgraded to a level 1.

...Other lightning areas ...

Meager overlap of ingredients or subtantial model discrepancies preclude any further level areas. Nevertheless a few severe events can't be ruled out (an isolated severe downburst event over N-Algeria with inverted-V profiles but meager CAPE and isolated large hail/severe wind gust events along the coasts and offshore of NE Spain with a temporal better CAPE/shear overlap during the afternoon and evening).