Storm Forecast
Valid: Fri 28 Nov 2014 06:00 to Sat 29 Nov 2014 06:00 UTC
Issued: Thu 27 Nov 2014 17:25
Forecaster: TUSCHY

A level 2 was issued for parts of SW France mainly for excessive rain.

A level 2 was issued for parts of Corsica mainly for excessive rain.

A level 1 surrounds both level 2s mainly for excessive rain and an isolated tornado, hail and severe wind gust threat.

A level 1 was issued for NE Spain mainly for excessive rain.

A level 1 was issued for the E-Adriatic Sea mainly for excessive rain and an isolated tornado, hail and severe wind gust threat.

A level 1 was issued for SW Portugal mainly for an isolated tornado (waterspout) risk.

SYNOPSIS

A strong cut-off low affects the Iberian Peninsula while downstream ridge (weak strength with pretty low geopotential heights) still covers most parts from the CNTRL Mediterranean to the North Sea. For E-Europe, hostile (cold/dry) conditions preclude organized and long-lived convection.

DISCUSSION

... Portugal and SW/NE Spain ...

Scattered to widespread cold-core convection occurs beneath the upper low's center. Shear is weak and with 300-600 J/kg MLCAPE to work with, mainly disorganized thunderstorms are forecast. Still, a few waterspouts are possible along the coasts due to better LL instability. Slow moving storms will be capable of producing heavy rainfall ... especially over SW and S-Portugal during the night, when LL southeasterly winds increase due to a deepening vortex just offshore.

Thunderstorms and heavy rainfall probabilities gradually increase over NE Spain during the night, but meager CAPE and elevated nature of convection keeps the general severe risk low. Heavy to excessive rain is possible however mainly along the upslope areas of the mountains. A level 1 was issued due to the rainfall risk.

... SW France ...

Very consistent high QPFs for an area Massif Central to Montpellier. 24 h rainfall amounts of 100-250 l/qm (with more extreme local peaks) are forecast.

Persistent onshore flow occurs with LL winds increasing to 25-30 m/s onshore. Trajectories show undisturbed SE-erly flow with long offshore fetch, although LL moisture remains unimpressive (ML mixing ratios of 10 g/kg or less). With somewhat enhanced mid-layer lapse rates atop, abundant offshore CAPE of 1 kJ/kg will be realized, which fuels coastal storms. A sharp mid/high tropospheric trough passes by until the early afternoon hours and exits to the north while weakening. This feature supports upper divergence over S-France until the afternoon hours.

Organized cluster of storms continues from the previous night. Forecast soundings in the level 2 area show LL jet maximum (roughly 800-850 hPa) of 60 kt with slightly weaker winds aloft/below. Hodographs also reveal a pronounced kink at the lowest levels, so environmental setup favors slowly forward propagating or temporarily back-building convection. Deep warm cloud depth and previously mentioned moist/unstable inflow probably support excessive rainfall amounts.
During the afternoon hours the upper-level support weakens with the departing trough. Also low-tropospheric winds start to shift from SE-erly more to an E-erly direction. Nevertheless, this transition will be a gradual one, so heavy rain continues until the evening hours hours/first half of the night. Flash flooding (probably life-threatening!) will be the major concern. Tail-end storms along the Gulf of Lion see impressive shear/CAPE overlap for an isolated tornado, hail and severe wind gust risk.

... N / CNTRL France ...

Would not be surprised about a few elevated storms that far north due to strong WAA and steep mid-layer lapse rates. Therefore the general thunderstorm area was expanded far to the north. Nothing severe is forecast.

...Corsica to SE France to NW Italy ...

Diffluent / slightly divergent high-level flow pattern affects those regions, too. Main time-frame of concern will be until the evening/early night hours, when 15-20 m/s SE-erly winds at 850 hPa occur. Winds show gradual weakening tendencies during the late afternoon onwards. With quite weak mid/upper level winds and a prolonged period of moist/unstable/undisturbed inflow from the Tyrrhenian Sea, slow moving/training convection is possible (e.g. E-coast of Corsica and coastal areas of SE France and to a lesser extent over NW Italy). Excessive rainfall amounts are possible beneath such bands of thunderstorms and QPFs inidcate 24 rainfall amounts of 100-200 l/qm. Such high-end rainfall amounts occur on a small scale however and therefore it is hard to place a level 2. Corsica has the highest backup by models and most unstable air to work with, so we upgraded this area to a level 2. Elsewehere, model discrepancies remain too large for a level 2 (e.g. SE France with spread of rainfall amount of 10 to 200 l/qm/24 h in numerical models). However, as soon as convection starts to evolve along a longer-lived coastal convergence zone, local level 2 conditions due to excessive rain can be expected.

Again, tail-end storms could produce all kind of severe on an isolated scale (hail/tornado/wind).

... E-Adriatic Sea ...

During the night, increasing CAPE and enhanced convergence along the E-coast should induce scattered CI. Despite rather favorable environmental conditions (effective PWs of 30 mm or more and marginal storm motions) QPFs remain rather unimpressive. Would not be surprised about training convection with excessive rainfall, especially as a 15-20 m/s LLJ approaches from the south during the night. Late timing precludes a level 2, so a broad level 1 was issued for now. Isolated large hail and an isolated tornado event are also possible.