Storm Forecast
Valid: Sun 26 Aug 2018 06:00 to Mon 27 Aug 2018 06:00 UTC
Issued: Sat 25 Aug 2018 09:26
Forecaster: TUSCHY

A level 1 was issued across parts of the Adriatic Sea and surroundings to W-Romania, Ukraine and Belarus mainly for locally excessive rain, isolated large hail and severe wind gusts. A waterspout risk exists over the N/CNTRL Adriatic Sea.

SYNOPSIS

A change in the weather pattern occurred with low geopotential heights now situated over N Europe. Mid-level waves circle that pool of low thickness. Embedded in a strong polar front jet they remain progressive during the forecast and bring unsettled conditions to many parts of CNTRL/N Europe. A piece of energy over CNTRL Europe drops off of the westerlies over the Adriatic Sea during the night and evolves into a southbound moving cut-off. Another cut-off southwest of Portugal approaches the Iberian Peninsula during the day but with limited impact on today's thunderstorm forecast.

Numerous synoptic-scale fronts affect NW Europe but meager mid-level lapse rates keep CI quite unlikely. A wavy and quasi-stationary front runs from the Adriatic Sea to Belarus to the White Sea. This front separates a warm and unstable air mass to its east from a cooler and drier one to its west and hence it will serve as focus for augmented thunderstorm activity.

DISCUSSION

... Italy, Adriatic Sea, most of the Balkan States to W-Romania and W-Belarus ...

The air mass over the Adriatic Sea features high moisture content especially in the lower troposphere. Ongoing thunderstorms from the previous night still pose a flash flood threat over NE Italy and Croatia with enhanced LL convergence but drier air from the NE filters in rapidly before noon and hence this risk should diminish during that time.
During the day, numerous thunderstorms evolve over the Adriatic Sea and surrounding coasts with MLCAPE in excess of 1000 J/kg and adequate DLS (15 m/s) for organized thunderstorms. Rapid clustering and slow storm motion indicates an excessive rainfall risk but in general the amount of CAPE and shear also highlights a risk of large hail, especially with more discrete thunderstorms. Offshore and along the coasts a few waterspouts can occur - especially during the night with a cooling low troposphere and a temporal weakening of the LL flow.

Another severe risk evolves from W Romania to Belarus along and ahead of the wavy front. Deep layer flow is rather parallel to that front, so initiating storms tend to cluster fast. This lowers the overall severe risk, but nevertheless isolated large hail and a few swevere wind gust events (15-20 m/s 0-3 km shear) are possible. Heavy rain becomes the main risk betimes and could produce flash flooding especially over W Romania/far W Ukraine, where favorable upper divergence and highest amount of CAPE/moisture overlap.

Futher south, towards Greece, CAPE and shear both weaken rapidly and hence the main hazard becomes an isolated heavy rainfall threat with slow moving thunderstorms.

... NW Turkey ...

Very moist marine air from the Black Sea undercuts steep mid-level lapse rates over NW Turkey. Subtle waves, embedded in a strong subtropical jet and diurnal heating may spark a few orographic storms despite warm mid-levels. 15 m/s DLS and 800-1000 J/kg MLCAPE may support an isolated and temporal hail risk. Limited coverage precludes a level 1.

...Baltic Sea, Sweden and Finland ...

A sharp upper trough (with an embedded distinct LL/mid-level vortex) crosses the area of interest from the SW. Attendant forcing and weak CIN with widespread 400-800 J/kg SBCAPE support scattered CI of pulsating thunderstorms during the daytime hours. Graupel and gusty winds will be the main threat. However, a few waterspouts over the warm sea are also well possible.
Clustering of thunderstorms is possible over the N Baltic Sea ahead of the strongest forcing and locally heavy rain accompanies that activit (affecting NE Sweden and W Finland during the evening and night).