Forecast Update
Valid: Fri 17 Sep 2010 11:00 to Sat 18 Sep 2010 06:00 UTC
Issued: Fri 17 Sep 2010 10:50
Forecaster: TUSCHY

A level 2 was issued for parts of Slovenia and Croatia mainly for excessive rainfall amounts.

A level 1 surrounds the level 2 and expands far west to NW-Italy mainly for isolated large hail, an isolated tornado event and strong to isolated severe wind gusts.

A level 1 was issued for E-Spain and another one for SW-Spain mainly for isolated large hail, strong wind gusts and heavy rainfall.

A level 1 was issued for NE-Spain mainly for excessive rainfall amounts, isolated large hail and strong to isolated severe wind gusts. A few tornado events are also possible mainly along the coast of NE-Spain and offshore. Those hazards are also forecast over the Balearic Islands later-on, but more conditional in nature.


SYNOPSIS

Please refer to the first outlook, issued at Thu 16 Sep 2010 10:35 UTC.

DISCUSSION

... Parts of Slovenia and Croatia ...

A potential serious multi-day heavy rainfall event is about to start / locally already ongoing but not yet coupled to any significant thunderstorm activity in my forecast period. This may change during the upcoming hours, as environment sets up for some potential instability build-up. Cyclonic vortex somewhere over NW/N-central Italy remains diffuse in nature, but seems to have increased in vertical extent with local sounding stations indicating good backing up to 700 hPa, which highlights persistent WAA towards the NE-coast of the Adriatic Sea. 06Z data from Udine also reveals a deep column of high moisture content, which is reflected in model data. This air mass remains in place all day long with a quasi-stationary, deep baroclinic zone placed over the Alps and a constantly re-developing, diffuse depression over N-Italy ( with slight weakening tendencies during the night hours). Deep westerlies with a near orthogonal angle to the N-Dinaric Alps assist in an effective orographic rain event during the forecast period. In addition (ref. to 06Z Udine sounding), deep moist convection becomes increasingly likely during the following hours mainly offshore and along the coast, which only enhances the already effective environment for heavy rainfall amounts. Local model data assist in 12-h sums of 100 l and more, so we decided to upgrade. In addition, enhanced SRH 1 along the coast and best chance for more discrete storm structures nex to the coast (tail-end storms) keep tornado probabilities augmented.

No major change for the rest of the level 1 with similar risks as mentioned in the first outlook.

... Spain ...

We went ahead and issued a level 1 for parts of SW-Spain due to higher CAPE build-up to what was anticipated in the main outlook. Vigorous convection is alread ongoing, probably with large hail, strong to severe wind gusts and heavy rainfall.

Nevertheless, the bulk of action will shift to the NE, where conditions still support organized thunderstorms. We expanded the level 1 far to the south where dewpoints remain in the upper tens and thunderstorms will spread to the N/NE, also affecting the E/SE-coast of Spain. Isolated large hail and strong wind gusts accompany that activity.

Best CAPE/shear overlap still expected over NE-Spain, where conditions for a back-building line of storms (building towards the coast) still look promising. Locally excessive rainfall will be the main hazard with training/slow moving storms, next to isolated large hail and strong wind gusts. Also, the tornado risk remains in place along the NE-coast of Spain and offshore, now expanding all the way to the Balearic Islands, which were upgraded, too.