Storm Forecast
Valid: Sun 17 Jun 2018 06:00 to Mon 18 Jun 2018 06:00 UTC
Issued: Sat 16 Jun 2018 18:51
Forecaster: TUSCHY

A level 1 was issued across SE-Sweden mainly for isolated large hail and an isolated tornado event.

Two level 1s were issued across parts of Romania, Moldova and the Ukraine mainly for large hail and excessive rain.

SYNOPSIS

Strong zonal flow over the NE-Atlantic opens out into an ongoing split flow regime over SE-Europe. The northernmost frontal zone affects N-Europe and embedded short-waves bring unsettled conditions. The most prominent feature is an intense mid-level impulse, which races from Denmark to SW-Finland during the forecast period. Another but much weaker impulse lags behind the first wave but gradually consolidates while moving from England towards N-Germany.
The southernmost branch of strong mid/upper level winds curves south along the eastern fringe of an anticyclone over the Bay of Biscay and spreads over the Mediterranean towards the Middle East. In-between, weak geopotential height gradients prevail.
The passage of an occlusion / cold front over S-Sweden to Germany marks the most active synoptic-scale front convective-wise. Elsewhere, mesoscale features and the orography dominate the picture regarding CI.

...SE-Sweden to Germany...

The sweet spot for the best overlap of ingredients evolves over SE-Sweden. Strong QG forcing affects the highlighted area during the afternoon/evening as diurnal heating peaks. A plume of better BL mixing ratios surges northward ahead of the approaching lift and beneath steeper mid-level lapse rates as mid-levels cool down. Resulting 400-800 J/kg MLCAPE are enough fuel for a few better organized updrafts. An accompanying 35 m/s mid-level speed max circles that impulse and affects far S-Sweden during the late afternoon/evening. This pushes DLS in excess of 20 m/s with forecast hodographs showing elongated and straight structures ... supportive for splitting storms. In addition, abundant lift and a weakening cap should support conglomerating convection with numerous line segments. Still, initiating and book-end thunderstorms could become organized multicells and temporal supercells with isolated large hail and strong to severe wind gusts the main hazard. The wind gust risk increases betimes with upscale growing convection and swaths of augmented strong to isolated severe wind gusts are possible (although modest LL flow limits the overall wind gust risk). Very low LCLs and marginal SRH-1 (especially in the eastern part of the level 1) point to a low-end tornado risk with deviating storms. The thunderstorm risk diminishes after sunset with lowering CAPE and increasing CIN.

Further to the NW, towards S-Norway, diminishing wind shear limits the risk of organized convection but cooling mid-levels/lowering WBZs still indicate a chance for numerous graupel/hail events, although probably not severe.

Further south towards Poland and Germany, forcing and shear constantly decrease to the south. An isolated multicell is still anticipated over NW-Poland with 15 m/s DLS (non severe hail and gusty wind) but the general convective mode transforms more into pulsating convection towards Germany. Gusty winds and graupel remain the main hazard.

... S-/SE-Europe...

Another day with diurnal driven convection is forecast over a broad area. The orography and mesoscale boundaries dictate the degree of convective activity on a regional scale. Most of the times, MLCAPE resides in the 400-800 J/kg range but locally exceeds 1kJ/kg in a very weak shear environment. This amount of CAPE assists in the development of robust updrafts with large hail and heavy rain on a local scale. A focus for more concentrated severe exists over S-/E-Romania, Moldova and parts of the Ukraine where a weak mid-level wave interacts with the unstable air mass, resulting in upscale growing convection and slow moving clusters. Heavy rain and isolated large hail remains the main risk. Enhanced chaces for large storm clusters were the reason for upgrading.