Storm Forecast
Valid: Sun 12 Oct 2014 06:00 to Mon 13 Oct 2014 06:00 UTC
Issued: Sat 11 Oct 2014 16:09
Forecaster: TUSCHY

A level 2 was issued for parts of the W/NW Mediterranean mainly for excessive rain, large to very large hail, severe to damaging wind gusts and tornadoes.

A level 1 surrounds the level 2 for less intense but similar risks.

A level 1 was issued for NW Portugal mainly for an isolated tornado risk.

A level 1 was issued for S-UK mainly for an isolated tornado risk.

SYNOPSIS

A deep trough approaches the Iberian Peninsula during the morning and slows down while moving east. A break-up into numerous troughs with shorter wavelengths is forecast, which affect an area from Spain and Portugal to Great Britain.
Downstream ridge over the CNTRL Mediterranean features very unstable but capped air, as EML spreads north over the moist marine air mass. Interface of both synoptic features will be the area of most concern for organized severe.
A quasi-stationary cold-core low is centered over W-Turkey and the Aegean Sea with another round of active onshore thunderstorm activity forecast.
Wintry air mass covers far N-Europe. Cold profiles and meager low-tropospheric moisture suppress convection, excluding offshore regions, where warmer SSTs (9-14 °C) offer some fuel for initiation. The same for SE Sweden, where better BL moisture is present.

DISCUSSION

... CNTRL, N-France, Benelux, W/NW Germany and S UK ...

A couplet jet configuration over NW France allows cyclogenesis of moderate strength over the N Bay of Biscay (e.g. 5-10 hPa pressure drop/24 h). It sends a warm front over France to the north and will be found over SE UK and the Netherlands during the end of the forecast (already occluding). An eastbound moving cold front comes on its heels and should affect CNTRL France during the night. In addition, numerous mid-level vort maxima will cross the warm sector during the forecast and insert additional lift for scattered CI.

All models agree in rapid and extensive moisture return due to the dynamic nature of this event. Moisture will be advected northbound around the Massif Central with orographic channeling between the Alps/Jura and the Massif Central. Most models bring an healthy theta-e ridge all the way towards CNTRL France, which increases confidence in adequate low-tropospheric moisture content. Warm sector should see some post-frontal clearing as somewhat drier mid/high tropospheric air filters in from the west, but not sure how cirrus clouds from ongoing thunderstorms over S-France (see next paragraph) will affect the warm sector. However we should see at least a few breaks in the clouds, so with mid-layer lapse rates in excess of 7 K/km, roughly 800 J/kg MLCAPE should be realistic...mainly west and north of the Massif Central.

Initiation is forecast as eastbound moving cold front interacts with available CAPE tongue during the afternoon hours onwards...e.g. W/CNTRL France. 20 m/s DLS, augmented LL directional/speed shear and LCLs around 1 km allow initiating storms to become well organized with large hail, strong to severe wind gusts and an isolated tornado risk. If storms can manage to form next to the warm front, an isolated strong tornado can't be excluded as LL shear maxes out. However, rapid upscale growth into a N-S aligned MCS is forecast due to the near front-parallel low/mid-tropospheric flow and excessive rain due to temporal training is possible. After sunset, a gradual decrease in thunderstorm activity occurs although a few elevated storms remain possible all night long.

Bound to the northward surging warm front/occlusion, elevated thunderstorms spread N/NE during the night and affect W/NW Germany and Benelux. Elevated nature should keep this activity non-severe.

S-UK however is placed next to the still deepening cyclone with strong S/SE-erly flow. Models differ in degree of CAPE build-up, but there are some hints that even an isolated near-surface based thunderstorm can occur during the night. With maximized LL shear along/just south of the synoptic front an isolated tornado event is possible. Marginal hail and gusty winds accompany this activity.

... S-France, NW Italy and parts of the far NW Mediterranean ...

Another round of a dangerous and potentially life-threatening thunderstorm activity is forecast for the coastal areas.

With favorable flow towards/over the Pyrenées, lowering leeside pressure is forecast over far SW France. This time the dynamic nature of this event (e.g. deepening cyclone over the Bay of Biscay) results in a strong/deep LLJ over the NW Mediterranean, which affects all of the coasts of S-France with winds in the lower troposphere in the range of 25-40 kt! Two wind maxima are forecast ... one between the morning and afternoon hours and another one during the evening hours until midnight, before the synoptic cold front approaches from the west.

Trajectories emerge from the NW Mediterranean, where a capped and very unstable air mass resides. Therefore expect a prolonged period with onshore flow (and upslope flow along the Massif Central). Training MCSes along any LL convergence zone is likely as LL flow temporarily exceeds mid-level flow. Main uncertainty will be where CI occus, as lowering thickness well offshore my assist in CI between the Balearic Islands and S-France. This could lower the rainfall risk as inflow for coastal storms becomes less unstable. However, this is not yet foreseeable, so we issued a broad level 2 for excessive and probably life-threatening rainfall amounts. Furthermore intense shear and CAPE of more than 1 kJ/kg also support tail-end storms or any discrete storm to become supercellular with large to very large hail, severe wind gusts and an enhanced tornado threat. In fact with roughly 25 m/s DLS and SRH-1 in excess of 300 m^2/s^2, a significant tornado event is possible along the coasts.
Despite a temporal lull in the late afternoon hours, another round of intense DMC is forecast ahead of the cold front, which should slowly shift east during the night.

SE France and NW Italy see an upswing in training DMC from the late afternoon hours onwards. Similar life-threatening conditions exist due to excessive and flash flood producing rains. The rest of severe is also well possible and hence the level 2 was expanded far east.

For the Balearic Islands to Corsica: CI remains conditional during the daytime hours due to weaker forcing and a strong cap. Nevertheless, back-building storms from the Gulf of Lion may affect those islands. Impressive CAPE/shear would favor well organized supercells or bowing segmentes with all kind of severe or very severe.
Scattered CI is forecast for the Balearic Islands during the evening and night as the cold front/strong vorticity maximum approach from the west. Also, convection from E-Spain spreads E/NE and affects the Balearic Islands during that time. Models indicate a prefrontal convergence zone, which could lower CAPE substantially, but most models still offer up to 1 kJ/kg during the cold front passage. 0-3 km shear increases to 25 m/s, which would indicate a chance for bowing segments with swaths of damaging winds...this risk spreads NE towards Corsica during the second half of the night. Tornadoes, large hail and excessive rain will be once again very well possible. Therefore the level 2 was expanded far south.

... The Iberian Peninsula ...

Scattered thunderstorms occur over Portugal and SW Spain, but they will be non-severe in most of the cases. An isolated large hail and heavy rainfall event may be possible with strongest storms. Also an isolated tornado event along the coasts (augmented LLCAPE) or over NW-Portugal (strong LL shear) will be possible. In general shear/CAPE fields remain too inconspicuous for organized DMC. Marginal level 1s were added.

Organized DMC however becomes likely as eastbound moving cold front interacts with the moist/unstable air mass over E-Spain around noon. Initiating storms grow upscale into well organized multicells and supercells due to 30 m/s DLS and up to 20 m/s 0-3 km shear. Large or very large hail (exceeding 5 cm in diameter) and severe wind gusts will be possible. In case of the development of bow echoes, swaths of damaging winds could be the result of today's activity. Augmented LL shear along the front and along the coast may also add an isolated tornado risk. This activity spreads E/NE and exits E-Spain during the night. More organized convection is possible during the night long the NE coast of Spain, where residual unstable air and strong DLS overlap.

... Rest of the lightning areas ...

Either weak shear and/or meager CAPE preclude organized DMC. Marginal hail/graupel and gusty winds accompany stronger storms.