Storm Forecast
Valid: Fri 17 Sep 2010 06:00 to Sat 18 Sep 2010 06:00 UTC
Issued: Thu 16 Sep 2010 10:35
Forecaster: TUSCHY

A level 1 was issued for NE Spain mainly for excessive rainfall, large hail and strong to isolated severe wind gusts. An isolated tornado event is possible along the coast and offshore.

A level 1 was issued for N-Italy, coasts of Slovenia and Croatia mainly for large hail, strong to severe wind gusts and an isolated tornado. The level 1 was expanded well inland to account for the heavy rainfall risk.

SYNOPSIS and DISCUSSION

A large cyclonic vortex is situated over N-Europe and remains in place throughout the forecast. A smaller-scale but still significant upper trough approaches Portugal and Spain from the west and crosses the Iberian Peninsula eastwards. A third but much weaker upper trough is placed over SE-Europe and remains quite inactive regarding strong convection.

Scattered convection /mainly daytime driven onshore with weaker diurnal signals offshore/ keeps going over the highlighted areas over N-Europe. As it is often the case with such occluded vortices, best energetic air mass is already displaced far away from the center, which keeps CAPE peaks quite marginal. Modest LL CAPE is expected but with no spots, where significant LL CAPE will be realized. Shear is strong, especially at LL, so showers/short-lived thunderstorms may pose a strong to severe wind gust/marginal hail risk. Risk for water spouts remains limited despite improving thermal BL structure, but background shear is still strong with no distinct convergence zones seen in latest model data. Hence, no level area was included.

... Portugal, Spain and the Balearic Islands ...

Scattered to widespread thunderstorm development is forecast over the Iberian Peninsula, already starting during the early morning hours in a weakly capped air mass. For Portugal and most parts of Spain, the atmosphere remains weakly sheared beneath pool of lower geopotential heights, so rapidly clustering pulse storms/weakly organized multicells are forecast with marginal hail, strong wind gusts and locally heavy rainfall. This activity decreases during the night, as CAPE wanes.

Conditions for organized thunderstorms improve over E/NE Spain, as prolonged period of moist onshore flow is expected east of the broad cyclonic vortex (centered over W-Spain). This moist inflow next to 20 m/s DLS creates a favorable CAPE/shear overlap and organized multicells/an isolated supercell are forecast. Capping of the onshore air mass is quite weak, too, so for now, expect more discrete storms along the NE-coast of Spain and just offshore, where 850 hPa temperatures remain a tad higher. Nevertheless, the main risk will be excessive rainfall with slow moving synoptic features, prolonged period of modest onshore flow and rich BL moisture just waiting offshore. In addition, upper divergence improves during the afternoon hours, as a mid-/upper jet evolves parallel to the E-coast of Spain. We went with a level 1 for that risk, as no real focus for extremely excessive amounts can be seen and MCS propagation vectors reveal only weak signals for backbuilding. Also no well structured LLJ is seen. Next to that, isolated large hail and strong wind gusts are expected.

The night hours remain a bit more puzzling regarding initiation. Environmental conditions would be supportive for organized thunderstorms, but forcing is missing due to the slow movement of the upper trough. If later data indicate conditions more supportive for initiation, an upgrade may become necessary.

... NE-Adriatic Sea ...

Ill defined LL depression is present south of the Alps and assists in a persistent influx of moisture from the south. Atop, strong westerlies overspread the area with some weak cooling at mid-/upper levels forecast. The main risk will be rapidly clustering stroms with heavy rainfall (NE-Adriatic Sea). However, more discrete storms are forecast over N-Italy and also along the coast of Croatia/Slovenia (tail-end storms of clusters) in an environment with 20-25 m/s DLS and locally modest CAPE up to 800 J/kg. Large hail, severe wind gusts and an isolated tornado are possible, especially along the coast, where moist onshore flow causes peaks in the CAPE field and enhanced SRH 1. After sunset, the risk vanishes due to decreasing CAPE magniudes. The level 1 was expanded to the east mainly to account for the heavy rainfall risk.

Due to the early release of this outlook, an update may become necessary tomorrow, if new model data confirm significant deviations from the current schedule.