Storm Forecast
Valid: Fri 15 Apr 2016 06:00 to Sat 16 Apr 2016 06:00 UTC
Issued: Thu 14 Apr 2016 20:09
Forecaster: TUSCHY

A level 1 was issued for most parts of Portugal and NW Spain mainly for an isolated large hail and tornado event.

A level 1 was issued for parts of France mainly for an isolated large hail event. Also, an isolated tornado event can't be ruled out.

A level 1 was issued for parts of the Ukraine mainly for large hail and excessive rain.

A level 1 was issued for far NW Turkey, mainly for large hail and severe wind gusts.

SYNOPSIS

Tenaciously rex blocking continues during the forecast and keeps thundery pattern in place for parts of Europe. Another upper trough, filled with frosty air (500 hPa temperature of -33 °C or less), urges in from the Norwegian Sea to the south and causes a constant increase of the geopotential height gradient over CNTRL Europe.
A weakening trough over SE Europe lifts to the NE but shears apart / elongates to the SE during the forecast. Except those major troughs, smaller scale short-waves impact many parts of W/CNTRL Europe. A somewhat stronger wave exits N-France to the NE during the start of the forecast, crosses Germany around noon and enters Poland after sunset. More subtle waves, embedded in the brisk SW-erly flow regime from Spain to Poland, insert a constant risk of thunderstorm development over a vast region.

The broad channel of low pressure, running from Norway to the Bay of Biscay and further west/offshore, starts to weaken/break apart. This scenario adds enhanced uncertainties to the final development of subtle low-tropopsheric waves or depressions. However, a weak disturbance over the W-English Channel will drift east ahead of the approaching trough from the north. A temporal weakening trend is expected, as it resides beneath the unfavorable jet region.
Another LL vortex is present beneath the deep vortex over SE-Europe. Not much change in strength is anticipated as it crosses E-Ukraine during the forecast.
Aforementioned mid-level waves will partially be accompanied by weak surface waves, like the one affecting Germany during the daytime hours. Hence, a wavy front probably extends from the S-Bay of Biscay to Poland.

DISCUSSION

... Portugal and SW France ...

A weak depression approaches Portugal from the west and opens during the course of the day. This scenario would allow a warm front to be pushed well onshore and to the north - probably residing over the Cantabrian and Basques Mountains. Models agree with nice offshore fetch of moist subtropical air, which covers most of Portugal and W/NW Spain. Even though mid-level lapse rates remain weak to modest, stout BL moisture should ensure MLCAPE of up to 500 J/kg. Airmass will be weakly capped but concerns exist regarding little diabatic heating due to lots of cloud cover. However, temporal subsidence between NE-ward traveling short-waves should support at least temporal heating. Expect an increase of thunderstorms during the noon/afternoon with scattered storms mainly in the level 1 area. DLS of 20 m/s and 15 m/s in the lowest 3 km point to organized multicells/isolated supercells with strong wind gusts and isolated large hail. Neither LL lapse rates nor mid-level CAPE profiles indicate a substantial risk of both hazards, but isolated events can't be ruled out. In addition, enhanced LL shear (SRH-1 in excess of 150 m^2/s^2 and LCLs below 500 m) indicate a risk for a few tornado events.
Further south (S-Portugal) and east (W-Spain), CAPE signals remain too marginal for a severe risk.
Isolated thunderstorms continue offshore during the night, as the vertical temperature gradient remains supportive for a few stronger updrafts.

Uncertainties exist in how far north the warm front surge will occur, but there are some hints of different models, that some of the moisture could reach far SW France. There, weak N/NE-erly BL flow also results in an increase of the BL depth just north of the Pyrenees. A divergent scenario exist, if an EML plume can advect that far to the NW to overlap with the aforementioned moisture. GFS is very bullish with this scenario. In addition to ongoing SW-erly mid-level flow and potential foehny effects, uncertainties remain high regarding how much CI and CAPE will occur and evolve. Any longer-lived thunderstorm, which grows deeper into the troposphere could produce large hail and strong to isolated severe wind gusts as kinematics support organized multicell structures.
During the night, a decrease of thunderstorms is anticipated and no severe risk is forecast. Some MUCAPE and strong DLS may support marginal hail with strongest updrafts.

... S-CNTRL France ...

Frontal boundary will serve as focus for isolated CI. Overall cloud cover should loosen during the afternoon with some diabatic heating possible. Moisture pooling and modest mid-level lapse rates may support a confined area with 400-800 J/kg MLCAPE. Combined with 25 m/s DLS, a few organized storms with large hail and strong wind gusts will be possible. LL shear increases around sunset, so a small window of opportunity exists for a tornado event. During the night, more elevated nature of isolated storms should keep the hazard confined to an isolated large hail event.

... Benelux and Germany ...

Kinematics will be good (DLS increasing from 15 to 25 m/s from Benelux to E-France/S-Germany) but BL moisture will be modest at best. Combined with only marginal mid-level lapse-rates, it will be hard to see more than 500 J/kg MLCAPE during this forecast. In addition, a short-wave affects Germany until noon, which should support thick clouds and more stratiform rain. Dry slot moves in from the west around noon with clearing skies expected mainly over CNTRL Germany. Subsidence behind that wave in addition to slow BL recovery should help to keep the thunderstorm risk low for a couple of hours after the wave's passage. Nevertheless, forecast soundings show not much modification (e.g. heating) is needed for elongated/thin CAPE profiles with 300-500 J/kg MLCAPE. Main chance for a few thunderstorms exists over N/NE Germany around noon. Elevated CAPE profiles indicate a marginal to isolated large hail risk with strongest updrafts.
During the afternoon hours, W/SW Germany will see borderline conditions for thunderstorm development. As mentioned, not much modification is needed for deeper updrafts with 25 m/s DLS. For now, we kept probabilities on the lower end side.
During the night a few elevated storms - non severe - may occur over far SW Germany.

... CNTRL-/E-Ukraine ...

NE ward moving depression is filled with moist air from the W-Black Sea. Beneath cold mid-levels, 400-800 J/kg MLCAPE is forecast. DLS remains weak and convection should grow upscale into numerous clusters. Magnitude of CAPE could support a few large hail reports and slow moving clusters may add a few flash flood incidents, so a level 1 was issued. Thunderstorms weaken during the night as they leave the Ukraine to the NE.

... NW Turkey ...

A E/SE-ward racing cold front sparks numerous thunderstorms, which grow in an environment of 30 m/s DLS and 400-800 J/kg MLCAPE. Large hail will be the main hazard and even an isolated very large hail event is possible. Besides, severe wind gusts are also possible as 0-3 km shear increases to 20 m/s.The main reason for not issuing a level 2 is the short time-frame, thunderstorms will have before leaving the forecast area to the E. The risk ends from W to E during the afternoon.