Extended Forecast
Valid: Mon 03 Jun 2019 06:00 to Tue 04 Jun 2019 06:00 UTC
Issued: Sun 02 Jun 2019 01:48
Forecaster: TUSCHY

A level 1 was issued across parts of N-Turkey mainly for large hail and severe wind gusts.

A level 1 was issued across parts of Bulgaria, Romania and Moldova mainly for excessive rain and large hail.

A level 1 was issued for parts of the S Ukraine into Russia mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for extreme E France, parts of Germany and S-Sweden mainly for large hail, severe wind gusts and excessive rain.

SYNOPSIS

Locked jet stream wave pattern continues over S/SE Europe with a durable closed upper low over the S-Balkan States drifting leisurely to the NE towards Bulgaria. Daily clustering thunderstorm activity and the approaching vortex caused a slight thickness increase over Belarus/Ukraine which connects tentatively with the strong subtropical ridge over Turkey and surroundings.
The main pattern shift is a constantly falling geopotential height pattern affecting far W/NW Europe as an extensive upper trough approaches Ireland, Scotland and England. Downstream SW-erly flow regime extends from the Bay of Biscay all the way to Scandinavia and features only subtle embedded mid-level waves.

The positive tilt of the upper trough favors an extensive baroclinic zone, which runs from the N-Iberian Peninsula over Germany to Sweden. Despite a general eastward motion over France and Germany, the diggig nature of the main trough result in a constant slow-down of the front and the cold front could transform into a warm front over parts of France and NW Spain during the night.
Embedded in a broad warm sector over CNTRL Europe a well defined surface pressure channel and convergence zone over Germany is anticipated to drift east.

DISCUSSION

... SE-Europe ...

Yet another day with scattered to widespread CI in a weak shear/moderate CAPE environment. Convection from the previous day will play a significant role in the final thunderstorm threat and adds an elevated uncertainty to this extended outlook.

Latest idea is once again a decaying convective cluster over most of Romania, which limits diabatic heating at least until noon, lowering final CAPE magnitude in this area. To the S and E (over Bulgaria, E Romania into Moldova), MLCAPE around 1000 J/kg and DLS less than 10 m/s combined with steadily increasing synoptic-scale forcing from the S support another round of clustering thunderstorms with large hail during the initiation stage and later on with excessive rain and isolated severe wind gusts the dominant threat. A broad level 1 was added for now and some refinements may be needed in the SWODY-1.

Further W towards Serbia, limiting diabatic heating and widespread/early CI preclude a level area. Nevertheless, heavy rain is possible with clustering and slow moving thunderstorms.

Another level 1 was issued for the S-Ukraine with 1-2 kJ/kg prefrontal MLCAPE and 10-15 m/s DLS. Large to isolated very large hail, a few severe downbursts (well mixed subcloud layers) and locally excessive rain are possible.

... NW-Turkey ...

A diffluent/slightly divergent upper flow pattern affects NW Turkey, where point forecast soundings indicate 1500 J/kg MLCAPE and 20 m/s DLS. Elongated and straight hodographs point to a few splitting supercells with large to isolated very large hail and severe wind gusts. A level 1 was issued and the main reason for not upgrading to a level 2 was the weak cap/early CI, which could limit the time span for discrete convection somewhat.

...Far E-France, Germany and S-Sweden ...

Rather smooth SW-erly flow regime affects the forecast area, so synoptic-scale forcing will be an issue. However, the eastbound drifting convergence zone, the following cold front and a passing and deamplifying mid-level short-wave all converge over Germany during peak heating to support at least isolated to scattered CI. A rapid decrease of CIN due to favorable insolation ahead of the front/convergence zone is also beneficial for CI.

Excluding aggressive GFS due to unreasonable BL moisture anomaly, we still expect early CI (probably before noon) along the convergence zone from far E-France to W/NW Germany or re-development of elevated thunderstorms from the previous night (which become more surface based with ongoing diabatic heating).

1-2 kJ/kg MLCAPE evolves along the convergence zone (decreaseing to .5 - 1 kJ/kg over S-Sweden) with DLS decreasing from 15-20 m/s over S-Sweden/N-Germany to less than 10 m/s over CNTRL Germany and further S.

This would favor organized multicells/isolated supercells over N-Germany and far S-Sweden. Enhanced curvature in forecast hodographs (1-4 km layer), shear magnitude and well /deeply mixed subloud-layers point to a few severe wind gust and large hail reports due to organized convection. Weak LL flow and LCLs aoa 1km should keep the tornado threat very low.
Further south, initating convection poses a large hail and isolated severe wind gust threat before growing upscale into numerous slow moving clusters, which bring heavy to locally excessive rain. Although not all models indicate CI all the way to Switzerland, weak CIN and orography with some mesoscale and synoptic-scale support increase confidence adequately to expand the level 1 far S.

The cold front, although rather inactive due to limited diabatic heating and very weak mid-level lapse rates, needs to be monitored closely over the Netherlands into NW Germany for CI. Not much airmass modification is needed for several hundreds J/kg in a strongly sheared environment with lowered LCLs. This would increase the tornado threat somewhat. Right now we did not include this area in a level 1 but an upgrade may become necessary later on.

Lightning and level areas are rather broad to account for ongoing timing issues of convergence zone/cold front in latest model suite.