Storm Forecast
Valid: Fri 12 Jun 2015 06:00 to Sat 13 Jun 2015 06:00 UTC
Issued: Thu 11 Jun 2015 17:24
Forecaster: TUSCHY

A level 2 was issued for SW-UK mainly for excessive rain.

A level 2 was issued for S-France, NW Italy, Corsica and Sardegna mainly for significant hail, severe to damaging wind gusts, excessive rain and numerous tornadoes. A significant tornado event or two is possible.

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

A level 1 was issued for Switzerland to W-Germany mainly for excessive rain and an isolated large hail risk.

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

A level 1 was issued for CNTRL/NE Poland mainly for large hail, severe wind gusts and an isolated tornado event.

SYNOPSIS

Large-scale cyclonic vortex remains on site over the far W-Bay of Biscay. Weaker disturbances circle that feature and affect W-Europe during the forecast.
A compact cold-core low over NE Germany lifts E/NE and induces thunderstorm probabilities over Poland.
Low thickness values over SE Europe indicate a flat trough with adequate mid-level lapse rates for instability build-up over a broad area. This pattern results in a very active thunderstorm day for many European regions.

At lower levels a complex frontal pattern runs from Spain to France and further east with an extensive warm sector covering most of S/CNTRL Europe. The air mass in this warm sector features rich BL moisture and at least modest mid-level lapse rate overlap and therefore widespread moderate to strong MLCAPE build-up.

DISCUSSION

... France, W-Mediterranean, N-Italy, Switzerland, W-Austria, parts of Germany, Benelux and S UK ...

A thundery day occurs over S-France and the W-Mediterranean with widespread severe possible.

During the start of the forecast, strong to severe thunderstorms already affect SW France with all kind of severe possible. Main risk however is excessive rainfall due to a strong (15-20 m/s) LLJ and an undisturbed fetch of very rich BL moisture offshore. V-shaped MCS with training convection continues to spread east during the day. Progressive nature of this event may preclude extreme rainfall amounts, nevertheless more than 100 l/qm of precipitation are possible over S-France with higher amounts, where upslope flow enhances LL lift. During the evening and overnight hours, this activity also affects NW Italy.
Tail-end storms and more discrete storms offshore but also well ahead of the eastbound moving MCS may organize rapidly due to overlap of more than 1kJ/kg MLCAPE and 20 m/s DLS. Impressive LL shear is also forecast with more than 15 m/s LL shear and in excess of 200 m^2/s^2 SRH-1. In fact, environmental setup looks favorable for numerous well organized supercells in this level 2 with significant hail events (hail diameter in excess of 5 cm), severe to damaging straightline winds and numerous tornadoes ... a significant event or two can't be ruled out especially over SE France, NW Italy and Corsica.

DMC continues all night long over N-Italy, but also further west over far S-France with ongoing SW-erly flow regime. Less shear diminishes the severe risk, although 10 m/s and 1000 J/kg MLCAPE remain still supportive of organized convection with all kind of severe.

Further north, overall picture becomes more messy due to uncertainties how clouds from DMC activity to the south may inhibit diabatic heating but also due to the potential suppress of the northbound advection of a very moist BL air mass. Current idea is that fronts will serve as foci for CI (e.g. northbound moving warm front over W-Germany and a leisurely eastbound moving cold front over E-France). DLS of 10 m/s or less should limit the overall severe risk, although effective PWs in excess of 30 mm and low storm motions indicate an augmented excessive rainfall risk. More discrete storms during initiation may also produce an isolated large hail event and gusty winds. Betimes (during the overnight hours), clustering storms tend to produce mainly an excessive rainfall risk from Switzerland to W-Germany.

Over S-UK, an unseasonably moist air mass becomes entrenched in a broad low pressure channel, which runs from S-UK to Belgium/the Netherlands. Widespread CI occurs and slow moving storms will be rather effective rainfall producers. Corfidi vectors indicate training/back-building being a distinct possibility with storm motions of less than 5 kt. Excessive rain is forecast on a more widespread scale especially over far SW-UK and a level 2 was issued for the region with highest BL convergence and persistent signals of slow moving DMC activity. An isolated tornado event is also possible, given LCLs well below 1000 m and locally enhanced LL shear.

... NE Germany to Poland ...

Eastbound moving cold-core low sparks numerous thunderstorms due to its interaction with a more unstable air mass over Poland. Current idea is that non-severe storms over NE Germany spread E towards CNTRL Poland. There, a gradual overlap of 15-20 m/s DLS, enhanced LL shear and more than 1000 J/kg MLCAPE occurs and organized DMC is forecast mainly over CNTRL/NE Poland. Thunderstorms will be capable of producing large hail, severe wind gusts and an isolated tornado event. Weakening occurs until midnight with loss of CAPE.