Storm Forecast
Valid: Fri 14 Nov 2014 06:00 to Sat 15 Nov 2014 06:00 UTC
Issued: Thu 13 Nov 2014 13:16
Forecaster: TUSCHY

A level 2 was issued for S-France and NW Italy for excessive rainfall amounts.

A level 1 surrounds the level 2 mainly for an isolated tornado threat, but also for large hail and severe wind gusts (coasts) and excessive rain.

A level 1 was issued for far NW Spain and NW Portugal mainly for excessive rain and an isolated tornado threat.

A level 1 was issued for far SW Turkey mainly for excessive rain and an isolated tornado threat.

SYNOPSIS

Wave train of intense troughs continues to affect W-Europe. Yet another one digs south/southeast and enters the far western Mediterranean during the end of the forecast.
Downstream ridging builds north from Italy all the way to the Czech Republic. Quiescent conditions regarding organized DMC activity are forecast. Another focal point for an active thunderstorm pattern exists over the Aegean Sea, Greece and Turkey, where a closed upper low drifts east.

DISCUSSION

... Greece, the Aegean Sea and SW-Turkey ...

Cyclonic vortex continues to weaken in the low/mid-levels but keeps a rather cold core signature through the forecasting period. This ensures ongoing temperature spread between SSTs (17-21°C over the Aegean Sea from north to south and 21-24 °C from Crete to S-Turkey) and mid-layers (500 hPa temperature -20 °C). A diffuse LL depression with tendencies to open up into a wave or at least form a W-E elongated surface pressure channel will cross the area of interest.

From Greece to Crete to W-Turkey, scattered thunderstorms occur beneath the center of the upper low. Shear at all levels is weak, so the main focus will be stratification of the lowest 2 km AGL. Augmented LLCAPE is forecast and local models show signals of stronger convergence f.ex. over NE Greece. A few waterspout reports are possible. Offshore flow and advection of a stable and dry air mass from Greece to the south should lessen that risk over S /SE Greece.

For far SW Turkey, an excessive rainfall risk exists as long as the vortex remains to the west. This should be the case until the late afternoon hours, before the vortex starts to leave the forecast area to the east. 15 m/s southerly low-tropospheric flow advects a moist and unstable air mass towards SW Turkey, where repeatedly onshore moving storms pose an heavy rainfall risk. A rather weak LLJ and early backing of the LL winds should lower the risk for backbuilding storms. Nevertheless a temporal training effect can't be ruled out. Tail-end storms could produce an isolated tornado. Due to numerous inhibiting effects we kept this event in a level 1.

... S-France and NE Spain ...

And yet another excessive rainfall event is expected.

The upper trough approaches from the NW during the day, so mid-layer heights start to decrease. A pronounced jet streak of 45 m/s at 500 hPa circles its base and reaches the Balearic Islands during the overnight hours. Its left exit region overspreads the far NW Mediterranean and ensures the development of a gradually deepening vortex over the Golfe du Lion (along a SW-NE aligned baroclinic zone). Models still have some difference in the handling of how far it will deepen, but any cyclogenesis will substantially delay the eastbound motion of the fronts. In addition enhanced flow between the W-Alps and the approaching front causes a strong low/mid-level jet over S-CNTRL France. Its right entrance region affects S-France.

Despit previous frontal passages, mixed-layer mixing ratios in excess of 10 g/kg exist ... both due to SSTs in excess of 20 °C over most of the W-Mediterranean and due to the approaching modified subtropical air mass from the E-Atlantic. Negative tilt of the trough allows offshore air mass to become moderately unstable during the night over the Golfe du Lion, where up to 1000 J/kg MLCAPE is forecast. Earlier in time but also further east, CAPE build-up remains lower with 300-600 J/kg.

Shear, weak during most of the daytime hours, increases significantly during the night, as the trough and attendant jet streaks approach. Any storm along the coast would see adequate shear for rapid organization with all kind of severe. Of interest is the better overlap of onshore CAPE/intense shear compared to previous events. Any discrete storm (e.g. tail-end storm) could acquire supercellular characteristics with an isolated tornado and severe wind gust threat.
The main threat however will be excessive rain from the Golfe du Lion to the Massif Central to the SW-Alps. This time the level areas were expanded far inland due to the northward extent of the MUCAPE plume. Numerous models show QPFs of 100 l/qm or more in 24 hours, which should lead to flash flood problems (keeping amount and extent of past rainfall events in mind).

During the night, this risk shifts east towards NW Italy, where the W-Apennine Mountains and coastal areas will become the new foci. Despite the late arrival and limited time for a level 2 verification during this forecast period, we expanded the level 2 area all the way to NW Italy. Past events already proved that training bands of convection can easily produce local rainfall amounts, which match our criterion. The excessive rainfall risk continues during the following day!

Dependant on the final shape and placement of the developing low over the far W-Mediterranean, a short period of time for organized convection could exist over NE Spain due to backed low tropospheric flow. Interaction of onshore advected moisture with the eastbound moving front/trough could support a few storms during the afternoon and evening hours. Organized multicells/isolated supercells are possible due to very strong shear with an isolated large hail/severe wind and tornado risk. This risk ends around 18-21 Z from W to E, as the front moves offshore.

...Parts of Portugal and NW Spain, the Bay of Biscay and the W-English Channel ...

Active marine convection is forecast, as cold mid-levels cover the warm sea. Repeatedly onshore moving showers and thunderstorms with graupel/marginal hail and gusty winds are forecast. A small leve 1 was issued, where some onshore CAPE overlaps with enhanced LL shear. An isolated tornado event is possible. Heavy rain also occurs with numerous onshore moving strong showers/thunderstorms.