Storm Forecast
Valid: Fri 24 Jul 2015 06:00 to Sat 25 Jul 2015 06:00 UTC
Issued: Thu 23 Jul 2015 18:17
Forecaster: TUSCHY

A level 2 was issued for NE Spain mainly for large to very large hail, severe downbursts, excessive rain and for an isolated tornado event.

A level 1 surrounds the level 2 mainly for similar risks but with less coverage.

A level 1 was issued for parts of Germany and the Alps mainly for large hail, strong to severe wind gusts and excessive rain. An isolated tornado event is possible.

A level 1 was issued for parts of Italy and Slovenia/Slovakia mainly for isolated large hail, severe downbursts and excessive rain.

A level 1 was issued for N-France mainly for an isolated large hail and a tornado event.

SYNOPSIS

An amplifying trough over the Bay of Biscay and later on over France attains a progressively more negative tilt trough the forecast. It is accompanied by a LL vortex, which temporarily weakens over NW France but re-intensifies thereafter over NE-France and Benelux during the night with a coupled upper-level jet configuration/strong upper divergence atop. Numerical guidance converges with a substantially intensifying vortex especially for that time of year. Of interest is GFS, which constantly shows signs of a shallow warm core structure and even offers a development similar to a Shapiro-Keyser cyclogenesis (onshore!). Back-bent occlusion and attendant wind field could become a major concern for the Netherlands and NW Germany for this time of year (in respect of vegetaion) although the main development occurs later than 06Z. Please keep in touch with your local weather agencies!

With limited time for moisture recovery, it seems like dephasing of approaching lift and slowly recoverying BL moisture keeps the ingredients partly separated trough most of the forecast from France to N-Germany. However, a constant approach/overlap of moisture/lapse rates and lift is noted during the evening and overnight hours over S/CNTRL-Germany. Meager CAPE could be partly offset by impressive lift over NW Germany and Benelux with organized shallow convection possible.

Most of the Mediterranean and SE Europe is placed beneath a broad upper-level trough with cool mid-levels. Combined with diurnal driven diabatic heating and regionally augmented BL moisture (especially next to the Mediterranean), widespread moderate CAPE build-up is forecast with an attendant risk of strong to severe thunderstorms.

Backward (westwards) bending occlusion over Finland and CNTRL Sweden resides beneath an upper trough with cold mid-levels. An eastward pushing surface low and temporal diurnal heating results in modest SBCAPE with scattered thunderstorm development.

DISCUSSION

... NE Spain ...

As the low over the N-Bay of Biscay pushes east during the day, a mixture of diurnal driven pressure drop due to intense diabatic heating over CNTRL Spain and some high-level divergence (e.g. GFS) result in a prolonged period of onshore flow and a very moist air mass advects towards NE Spain. As this air mass creeps in a well developed EML with mid-level lapse rates of 8 K/km, strong MLCAPE of 1500-2000 J/kg evolves during the afternoon and evening hours. Winds at mid-levels constantly increase during the day with 50 kt at 500 hPa around sunset. This causes DLS to increase to 15-20 m/s.

Initiation most likely occurs over the Pyrenées around noon/during the early afternoon hours and then starts to shift to the SE, as overall wind field veers ahead of the approaching trough. Thunderstorms then enter a very unstable air mass and intense multicells/supercells are forecast. Very large hail and severe downbursts are likely beside an excessive rainfall risk. Despite meager LLCAPE and LCL heights in excess of 1 km, an isolated tornado event mainly along the coast can't be ruled out, both due to increasing BL moisture and lowering LCLs during the evening hours but also due to thunderstorm interaction with a potential inland moving sea breeze front.
During the night, storms move offshore. We expanded the level areas more to the east due to 20-25 m/s 0-3 km shear and high CAPE despite northward lifting forcing. Would not be surprised to see a slowly decaying MCS affecting the Balearic Islands during the night with strong to severe winds, large hail and excessive rain.

... Germany ...

During the daytime hours, the focus for isolated organized DMC lies over S/S-CNTRL Germany. All models show a tongue of enhanced BL moisture to be situated just north of the Alps, where a grazing EML plume supports 1000-1500 J/kg MLCAPE, which gradually weakens to the north with abating mid-level lapse rates. However, DLS increases from less than 10 m/s along the Alps to 15 m/s over S-CNTRL Germany.
An eastbound moving short-wave and orographically induced thunderstorm development should result in an overall increase of thunderstorm activity during the afternoon and evening hours. However, numerical guidance remains reluctant with CI, most likley due to a strong cap and overall weak lift until sunset. Given amount of CAPE and strengthening shear to the north, strong to severe thunderstorms with large hail and severe wind gusts area forecast with any deep/longer lived updraft. An isolated very large hail event can't be ruled out over S-Bavaria. Slow storm motion boost the rainfall risk.

During the night, the risk for strong to isolated severe thunderstorms also shifts to CNTRL-/E-Germany as plume of unstable air lifts north/northeast. 20 m/s DLS and 500-800 J/kg MLCAPE are enough for an isolated large hail and strong to severe wind gust risk. Despite increasing LL shear, uncertainty of the LL depressions's strength and attendant amount of moisture advection to the north but also weak mid-level lapse rates and more elevated profiles in forecast soundings over CNTRL Germany keep the tornado risk low for the moment.
As MUCAPE plume advects far north, we had to adjust the lightning areas the same way ... most of the lightning areas will see CI first during the night.

Of interest is an impressive dry intrusion (seen in IPV fields), which could overrun fracturing cold front somewhere over NE France/Belgium/Luxembourg and W-Germany during the end of the forecast. This could induce an upswing of convction along that front in a strongly sheared environment. An isolated tornado risk and severe wind gusts would be the main hazard.

...N-France ...

Moderate SBAPE of 800 J/kg and enhanced LL shear along the depression's track may support an isolated large hail and tornado risk over NW and far N-France. Limited DLS should lessen overall severe risk however due to short-lived convection.

... Alps ...

Numerical models don't show any signals of dispelled BL moisture with the last frontal intrusion. Therefore, with EML atop, another day with strong MLCAPE build-up is forecast, as models show anywhere from 1-2 kJ/kg. DLS on the order of 10 m/s or less does not favor organized DMC, but expect scattered to widespread CI during the day either by intense diabatic heating or by an eastbound travelling short-wave. Slow moving clusters of thunderstorms are forecast with large hail especially during the initiation stage and later on also with excessive rain. In case of temporarily cold-pool driven activity, confined swathes of strong or isolated severe wind gusts can occur.
Switzerland is a borderline level1/2 nominee due to DLS of 15 m/s during the afternoon hours. Would not be surprised to receive a very large hail report with intense convection. Confidence however remains too low for upgrading, mainly due to the uncertainty about how long storms remain discrete before clustering and how significant substantial mountain convection and N-ward fanning cirrus shield could impact the forecast. Thunderstorms continue well into the night with a gradually decreasing severe risk.
Approaching major trough from the west could induce another round of convection over the W-Alps during the night.

... Italy to the W-Ukraine ...

Daytime driven pulsating thunderstorms are forecast. Weak shear but MLCAPE in excess of 1 kJ/kg should result in numerous high based thunderstorms with an isolated large hail and severe downburts risk. In addition, if locked to the orography, locally very high rainfall amounts could induce flash flooding. A level 1 was issued where confidence in CI will be the highest, e.g. over most of Italy, Slovenia and Slovakia.

... Finland to W-Russia ...

Weak shear and CAPE of 400-800 J/kg cause pulsating storms and a few multicells with isolated large hail and heavy rain. An isolated tornado event can't be ruled out in the southern part of the 50-% lightning area, where low LCLs and enhanced LL shear overlap. The latter risk heavily depends on the overlap of CAPE/shear however, which is different in numerical models. Hence no level 1 was issued. The thunderstorm risk shifts east during the forecast and weakens after sunset.