Storm Forecast
Valid: Fri 06 Apr 2012 06:00 to Sat 07 Apr 2012 06:00 UTC
Issued: Thu 05 Apr 2012 22:07
Forecaster: TUSCHY

A level 1 was issued for Slovakia mainly for isolated large hail.

A level 1 was issued for the Balearic Islands mainly for an isolated severe wind gust event.

A level 1 was issued for SW-France mainly for heavy rainfall and far NE-Spain mainly for an isolated large hail and heavy rainfall.

A level 1 was issued for far NE-Algeria and far N-Tunisia mainly for large hail, strong to severe wind gusts and heavy rainfall amounts.

SYNOPSIS

Blocking pattern continues with most parts of Europe beneath low geopotential heights. An extensive air mass boundary runs from CNTRL France to S-Germany to N-Ukraine and separates dry air to its north from warm and moist air to its south. Hence scattered showers and thunderstorms are forecast south of that boundary over a broad area.

DISCUSSION

... The Balkan States, Hungary, Slovakia and parts of Romania...

Complex pattern regarding initiation continues. Compared to yesterday, low-levels reveal similar moisture content but EML plume fans out, so expect lower MLCAPE build-up. In general 200-500, locally up to 800 J/kg MLCAPE are forecast in a weakly sheared environment (DLS at or below 10 m/s). Only exception will be Slovakia, where entrance region of a modest mid-level jet results in 15 m/s DLS, enough for a few better organized multicells. This area was placed in a marginal level 1 mainly due to an isolated large hail event or strong wind gusts. Hungary was not added despite strong CAPE signals in model fields. Yesterday's activity, which was confined to the rough topography, and the overestimation of forecast model CAPE keep confidence in anything severe below the level 1 threshold. However, any stronger updraft will be able to produce an isolated large hail event. Otherwise, marginal hail and gusty winds occur with messy clustering. Locally heavy rainfall accompanies activity due to very slow storm motion. Thunderstorms decay until midnight.

... The Balearic Islands ...

First round of storms occurs during the morning hours until noon as mid-level trough axis swings in from the SW. A constantly strengthening wind field will be in line with the trough passage. Forecast soundings show elongated hodographs with strong unidirectional shear. CAPE will be on the lower end with up to 20 m/s DLS. Isolated to scattered thunderstorms cross the islands from SW to NE with marginal hail and strong to isolated severe wind gusts. A small level 1 should cover the isolated severe wind gust risk until 12Z.

Second round of storms is forecast during the evening hours onwards with rapidly cooling mid-levels. However, dry air from E-Spain needs some time to reach critical BL moisture, but at least isolated thunderstorm activity is expected. Nothing severe is forecast, although marginal hail may accompany stronger storms due to the cold nature of the environmental temperature profile.

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

21Z dewpoints over far SW France range from 7-11°C with similar values forecast tomorrow afternoon (HiRLAM and GFS a bit higher). A combination of somewhat steeper lapse rates at mid-levels, favorable upper divergence and temporarily diurnal heating result in an extensive area with low-end to modest MLCAPE (roughly 500 J/kg). The atmosphere is weakly capped, so expect scattered to widespread thunderstorm initiation. Shear will be marginal, so weakly organized multicell clusters with marginal hail, strong wind gusts and heavy rainfall are expected. The level 1 risk mainly covers the rainfall risk. Thunderstorms gradually decay after midnight although storms with connection to the moist inflow from the NW-Mediterranean may last well into the night hours (e.g. S/SE of Toulouse).

A similar risk exists in the level 1 area over far NE Spain, where DLS remains strong (roughly 15-20 m/s). This may result in a few better organized mutlicells with an isolated large hail event. Otherwise, storm clustering causes locally heavy rainfall amounts.

... Far NE-Algeria and N-Tunisia ...

And yet again another day with afternoon activity over the E-Tell region. Overlap of 500-800 J/kg MLCAPE and 20-25 m/s DLS assist in a few well organized multicells with large hail and strong to severe wind gusts. Would even expect supercells, but rough topography and very concentrated swath of enhanced BL moisture preclude long lived thunderstorm activity. Thunderstorms weaken after sunset while moving offshore. A cluster of showers/thunderstorms may result in heavy rainfall over far N-Tunisia until midnight.

... Rest of the thunderstorm areas ...

With either marginal shear or low-end CAPE, no severe risk is anticipated. Onshore thunderstorms gradually decay after sunset with nighttime activity expected in the highlighted areas offshore.