Storm Forecast
Valid: Fri 19 Sep 2014 06:00 to Sat 20 Sep 2014 06:00 UTC
Issued: Thu 18 Sep 2014 21:10
Forecaster: TUSCHY

A level 2 was issued for NE-Spain, S/CNTRL-France, parts of Italy and surroundings mainly for excessive rain, large to very large hail and severe wind gusts. An isolated significant tornado event can't be ruled out.

A level 2 was issued for NE France, Belgium and parts of W-Germany mainly for large hail and excessive rain.

A level 1 surrounds both level 2s mainly for excessive rain, large hail and severe wind gusts.

A level 1 was issued for parts of UK and S-Ireland mainly for excessive rain and large hail.

SYNOPSIS

Rex block continues as E-Atlantic trough retrogrades a bit to the westsouthwest. Numerous short-waves eject out of its base with the most pronounced one leaving NE Spain to the NE while weakening. Downstream ridge remains in place over the CNTRL Mediterranean and caps a very unstable air mass over the sea. Complex trough/ridge pattern occurs further north but none of the troughs will play a serious role for our forecast. Either they're cut off from the moisture or they initiate storms out of our forecast area (e.g. Turkey).

At the surface, a wavy front runs from the E-Iberian Peninsula to France and towards Germany. Crossing mid-layer waves and orographic effects induce an E-W oscillation of that front which serves as focus for enhanced deep moist convection.


...NE-Spain, Balearic Sea, Ligurian Sea, S/CNTRL-France N/CNTRL Italy to the N-Adriatic Sea ...

Many uncertainties exist due to supposable ongoing convection from the previous night, rather thick cloud cover and a decaying short-wave which passes by during the forenoon hours. Expect bulk of activity along the southern coast of France, which already has the chance to be severe as 15 m/s DLS and
roughly 1kJ/kg MLCAPE intersect.

Thereafter models diverge substantially. Weak short-waves circle the ridge, which itself reveals lowering thickness (e.g. over and north of Corsica). Capped air mass features unseasonably high BL mixed-layer mixing ratios in excess of 16 g/kg. Plume of mixed mid-layer air from the Atlas mountains over this air mass pushes MLCAPE to 2-3 kJ/kg. Shear oscillates a bit and peaks during the passage of any wave but remains aoa 20 m/s. Big question remains where and when new storms initiate. Otherwise expect favorable parameter overlap until the end of the forecast. With decreasing thickness over offshore areas, CI during the night becomes increasingly likely further south towards Corsica and Sardinia.

Latest model data indicates two peaks of DMC acitivty, one during the afternoon/evening hours and another one during the night. This time coincides with the passage of mid-layer waves.

Aside from ongoing uncertainties, confidence is high that any storm could become severe with very large hail (hail diameter in excess of 5 cm), excessive rain (selective QPF maxima in excess of 100 mm/12 h) and an augmented tornado risk (a significant event is possible as 200 m^2/s^2 and LCLs of 800 m or less overlap over SE-France and NW Italy). In addition, clustering storms will see enough CAPE/shear for long swaths of damaging wind gusts. We expanded the level 2 far east to account for such an overnight cluster, which rides along the CAPE tongue to the east. The level 2 was expanded south, too, to account for organized convection, developing over Corsica and racing to the E.

A similar setup evolves over NE Spain, where isolated DMC could evolve into supercells with all kind of severe. Despite uncertainties of thunderstorm coverage, adequate confidence for CI exists to upgrade.

... Rest of France, S-UK, Benelux and parts of Germany ...

Persistent advection of the marine layer from the W-Mediterranean towards France ensures deep and moist low-tropospheric profiles during the day. Hence, diurnal mixing should be suppressed and with mid-layer lapse rates of 7-8 K/km, MLCAPE forecasts of 1-2 kJ/kg seem reasonable. A forecast sounding just SW of Paris has MLCAPE of 2600 J/kg, PWs of 28 mm and less than 10 m/s of 6 km bulk shear. Differential heating helps to diminish CIN whereas residual outflow boundaries from yesterday's storms and the passage
of a weakneing mid-layer wave during peak heating induce enough forcing for widespread CI. Slow moving storms with torrential rain will be the main risk ... especially as storms grow upscale into numerous slow moving clusters. Initiating storms could bring large or very large hail and severe downbursts (LCLs above 1 km and augmented delta theta-e).An isolated tornado event can't be excluded due to enhanced LLCAPE along any convergence zone. This activity diminishes during the night, althoguh the air mass still supports severe events (large hail and excessive rain).

A similar kinematic/thermodynamic set-up extends towards Belgium and W/CNTRL Germany, where the same risks are
forecast. Concerns exist that with so much CAPE, an organized/long-lived cluster (MCS) could evolve over
CNTRL-France. SW-erly steering flow could bring any cluster towards W/NW Switzerland or SW/W-Germany. Aside from that, an E-W aligned convergence zone from N-France to W-Germany will be another focus for slow moving storms with excessive rain.

Further north, over S-UK, forecast soundings indicate less CAPE (1-1.5 kJ/kg), which is still noteworthy for this region. Persistent LL convergence signals, PWs in excess of 20 mm, weak upper divergence and storm motions of 5 kt point to an augmented risk for slow moving multicells with isolated large hail and excessive rain. High resolved models indicate selective amounts of 50 mm/3h, which would bring a flash flood threat to those areas. This risk also extends to S-Ireland and shifts north during the night, conform to the northbound advection of the unstable air.

... Portugal and parts of Spain ...

Beneath the upper low, scattered thunderstorms evolve. Weak lapse rates/warm mid-layers and thin CAPE profiles within the hail growth zone lower the risk for electrified convection. Heavy rain and marginal hail will be the main hazard. A few spout-type tornado events are possible along the coasts.