Storm Forecast
Valid: Fri 26 May 2017 06:00 to Sat 27 May 2017 06:00 UTC
Issued: Thu 25 May 2017 15:38
Forecaster: TUSCHY

A level 2 was issued for NW/N-CNTRL Spain mainly for large to very large hail and severe wind gusts.

A level 1 surrounds the level 2 for a similar risk with less probabilities. In addition, excessive rain is possible.

A level 1 was issued for NW-France and SW-UK mainly for large hail and to a lesser extent for strong to severe wind gusts.

A level 1 was issued for NW-/N-Ireland mainly for an isolated large hail risk.

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

A level 1 was issued for parts of the SW-Ukraine, Romania, Bulgaria, NE-/S- Greece, W-Turkey and the Aegean/E-Ionian Sea mainly for an isolated large hail and severe wind gust risk. The main hazard will be excessive rain. Offshore regions may see a few waterspouts.

SYNOPSIS

A strong ridge persists over W-/CNTRL Europe with calm conditions (convective-wise). To the west, a progressive negative tilted trough interacts with a wavy N-S aligned frontal boundary. This will be the main focus for organized severe convection.

A broad trough over NE-/E-Europe remains more or less in place with no serious eastwad motion forecast. Numerous frontal boundaries remain embedded beneath that trough and shift to the SE. Attendant cool mid-levels atop a moist BL result in a broad area with modest CAPE build-up.

DISCUSSION

...SW-Ukraine, Romania, Bulgaria to W-Turkey and the Aegean Sea ...

A warm and moist air mass with positive anomalies in TPW forecasts resides over SE-Europe. Diurnal heating and a weakly capped air mass result in scattered to widespread CI during the day - in some places probably all day long. 800-1500 J/kg MLCAPE evolves but mainly in areas where diabatic heating materializes. Else, rapid shower/thunderstorm development may limit heating and attendant CAPE build-up. Dependant on the final magnitude of CAPE, strong to severe thunderstorms with a few organized multicells are forecast. In general, weak shear precludes storm organization and supports rapid upscale growth into a mess of various clusters. Heavy rain with slow moving storms will be the main risk with flash flooding possible ... especially over Bulgaria/S-Romania, where rich BL moisture from the NE-Aegean Sea may sneak north. Forecast soundings show elongated CAPE profiles with moderate thickness, so it seems that large hail may be an isolated threat with a higher risk of large amounts of small hail. Maturing storms and cold pool driven clusters my produce confined swaths with strong to isolated severe wind gusts.

The level 1 was not expanded all the way to far W-Russia, where a SE-ward moving cold front sparks numerous storms during the afternoon hours. MLCAPE of 400-800 J/kg and weak shear may result in a few hail and gusty wind reports. Isolated large hail can't be ruled out but the overall setup and expected ingredients don't support a level 1 right now.

The E-Ionian Sea was also added to the level 1 mainly for a few waterspout events.

During the night, onshore convection weakens betimes with a reduced severe risk. However, over the Aegean Sea beneath an eastward rotating pool of 500 hPa temperatures of less than -20 °C, ongoing scattered thunderstorms grow upscale into a large cluster of storms, which approaches Cyprus and SW-Turkey during the second part of the night. DLS increases to 15-20 m/s over Cyprus, where organized multicells with large hail, severe wind gusts and heavy rain are forecast. Further north over the S-Aegean Sea, robust SBCAPE may offset weak DLS and an organized cluster of storms with a well structured cold pool may push east during the night. Severe wind gusts, large hail and heavy rain will be the main hazard. In general an augmented waterspout risk exists in the mentioned areas.

... Spain, Portugal, the Bay of Biscay, NW-France and SW-UK ...

An EML plume spreads north from NW Africa all the way to far S-Ireland. Mid-level lapse rates approach or exceed 8K/km over Spain with less magnitude further north. Mid-level lapse rates regionally offset meager BL moisture, which improves somewhat over SW-France and NW Spain during the forecast. Overlap of both ingredients assists in widespread 800 to 1500 J/kg MLCAPE over NW Spain and similar values over far W-France.
The grazing trough brings stronger winds to NW-Spain and N-Portugal, where DLS exceeds 20 m/s during the day. Further south over Spain, DLS remains aoa 15 m/s with lower values over W-France.

Initiation will be bound to the approaching upper trough, which pushes a cold front east. Ongoing storms over N-Portugal and NW Spain from the previous night continue during the day. A few organized storms are well possible before noon over far NW-Spain and N-Portugal with 800 J/kg MLCAPE and 20 m/s DLS. Large hail will be the main hazard.
As the cold front approaches and pushes east around noon and during the afternoon, widespread CI occurs in the highlighted region (level 2). CAPE/shear overlap favors well organized multicells/a few supercells. Large hail can be expected and an isolated very large hail event (hail diameter in excess of 5 cm) can't be excluded. The severe wind gust risk restricts itself to maturing storms with cold pool driven gusts. Betimes, as storms enter N-CNTRL Spain, a deeper mixed subcloud layer may enhance the overall wind gust threat and an organized MCS may push east over N-CTRNL Spain. We expanded the level-1 area further north to include extreme SW France, which could be affected by that MCS, probably weakening due to increasing CIN. After night, departing forcing to the north and lowering CAPE/increasing CIN lowers the thunderstorm and severe risk over N-Spain, too.

The level 1 was expanded far south to CNTRL-Spain, where similar CAPE and 15 m/s DLS support organized multicells/isolated supercells. Large hail and severe wind gusts will be the main hazard. CI decreases from N to S due to weaker forcing.

Over W-France/SW-UK, the air mass stays capped until sunset. Thereafter, initiation is expected over the NE-Bay of Biscay and far NW-France as the trough and front both approach from the SW. Forecast soundings show elevated MUCAPE in excess of 1 kJ/kg with 15-20 m/s DLS. Thunderstorms grow upscale into an organized cluster of storms quite fast. This cluster crosses the W-English Channel and affects SW-UK during the second part of the night, where large hail will be well possible. In addition, low to mid-tropospheric winds strengthen over that region during the night to 15 m/s in the lowest 3 km. Elevated nature of storms should limit the overall severe wind gust threat, but we can't exclude the chance for stronger downdrafts to penetrate through the stable layer. Therefore strong to isolated severe wind gusts are also forecast. Heavy rain is possible over NW-France but confidence decreases for SW-UK due to the progressive nature of the cluster and its late arrival.

... NW/N-Ireland ...

Eastbound moving cold front interacts with an air mass, which features MLCAPE of 400-800 J/kg and 15 m/s DLS. A few organized thunderstorms with isolated large hail are possible, which move off to the NE. This risk peaks during the late afternoon/evening and vanishes thereafter with decreasing CAPE. During the end of the forecast a decaying cluster from the south approaches S-Ireland but northing severe is forecast.

... N-Algeria ...

CI is less certain with diffuse forcing. However, a weakening mid-level thermal trough atop rich BL moisture along the Atlas Mountain range causes MLCAPE of 1000-2000 J/kg. If storms initiate over the mountains, organized multicells (15m/s DLS)with large hail and severe downbursts (deep/dry subcloud layer) occur. The severe risk decreases beyond sunset.