Storm Forecast
Valid: Fri 07 Sep 2012 06:00 to Sat 08 Sep 2012 06:00 UTC
Issued: Thu 06 Sep 2012 21:12
Forecaster: TUSCHY

A level 1 was issued for NE Morocco and NW Algeria mainly for large hail and strong to severe wind gusts. Heavy rain may accompany slow moving storms.

A level 1 was issued for SE Spain mainly for large hail.

SYNOPSIS

Intense westerlies spread eastwards during the forecast, affecting most of N-Europe. High pressure to the south keeps weather conditions more quiet. Main focus for DMC arises over far SW-Europe, due to a NW-ward moving cold-core upper low.

Side-note (review):

Until Sept. 4th, the described depression organized east of Sardinia/Corsica, producing copious amounts of rain along the eastern fringes of those islands. Pulsating but strong DMC evolved along its center with the most concentrated DMC along its northern quadrant (mainly forced by strong LL convergence). ASCAT scans revealed gale force winds in the outer fringes of the circulation with much weaker winds along its southern quadrant (roughly 20-25 kt). However, strongest winds were well displaced from the center and evolved mainly due to increasing pressure gradients and channeled flow. A few synop data over E-Corsica/Sardinia support sustained winds in excess of 30kt. On the 5th, DMC decayed, but a well defined low level circulation center (LLCC) was seen in HVIS data which again acquired more DMC around landfall during the morning hours of the 6th (north of Napoli, Italy). Given the well established LLCC and the temporarily better organized DMC on the 4th and around landfall, a subtropical depression may be verified in later analysis.

As often seen in similar events, not the final strength, but the degree of organization is the most dangerous part of such systems, as organized bands of showers and thunderstorms may be capable of producing intense rainfall amounts (e.g. 24h rainfall amounts well in excess of 100 mm over parts of Sardinia).

DISCUSSION

... N-Morocco/Algeria, parts of Spain and Portugal ...

Cold mid-levels accompany gradually evolving cold-core low over / just west of the Iberian Peninsula. This low temporarily manages to built the vortex down to the surface, which in response assists in weak WAA over the SW-Iberian Peninsula. Hence, better LL moisture spreads ashore with moderate onshore CAPE build-up. Offshore, strong but weakly to moderately capped 1-1.5 kJ/kg MLCAPE will be available. DLS remains very weak over Spain/Portugal and increases up to 20 m/s towards NW-Africa due to a stronger mid-level flow.

Pulsating storms are forecast with isolated large hail and strong wind gusts. The overall severe risk increases over N-Morocco and NW-Algeria due to better shear, but models vary significantly in respect of CI. Nevertheless, a level 1 was introduced. Despite locally more roubst CAPE over S-Portugal, no level was yet added due to the bad kinematics. A second level 1 was issued for SE Spain mainly due to marginal better shear and locally more robust CAPE. Slow storm motion increases the risk of locally heavy rainfall.

Thunderstorms gradually decay after sunset with some offshore storms continuing all night long.

Sporadic thunderstorm activity is forecast over the C-Mediterranean all day long. Strongw ind gusts and locally heavy rain will be the main hazard.

... N-Europe ...

With fronts, sneaking beneath the strong westerlies, patches of low-end CAPE evolve over a broad area. With peak activity during the afternoon hours, sporadic to isolated thunderstorms may evolve. Expected slim CAPE preclude organiced DMC despite strong DLS. No severe risk is forecast and a quite optimistic lightning area was added.