Storm Forecast
Valid: Mon 30 Sep 2013 06:00 to Tue 01 Oct 2013 06:00 UTC
Issued: Sun 29 Sep 2013 22:45
Forecaster: TUSCHY

A level 1 was issued for the E coasts and adjacent areas of the Adriatic, Ionian and Aegean Sea mainly for an isolated tornado risk, large hail, strong to severe wind gusts and excessive rain.

SYNOPSIS

Big double-barreled vortex centered at 48N, 39W and 52N, 28W steers a true tropical moisture towards the Iberian Peninsula. Satellite derived blended TPW product shows a tongue of 40-50 mm (high positive anomaly) spreading east and being in contact with newly formed TD11 at 27N, 47W. Despite a strengthening E-W aligned and deepening baroclinic zone, transient mid-level ridging and increasing thickness keep this air mass mostly deactivated for most of the forecast.
A stable mid-level wave crosses the C Mediterranean to the east/southeast. With increasing baroclinity ahead, a gradual spin-up of the vortex is forecast also in the lower troposphere. An active thunderstorm day will be in store for the C/SE Mediterranean. Rest of Europe experiences dry/stable conditions beneath a strong Scandinavian high pressure area.

DISCUSSION

... Italy to W-Turkey to Crete ...

Two foci exist for CI. The first one beneath the base of the wave with scattered to widespread DMC over Italy and surrounding areas. Modest shear/CAPE limits storm longevity and strength, but unstable airmass over offshore areas should insert and isolated waterspout risk for any stronger updraft which moves/evolves atop a mesoscale convergence zone (numerous strong signals are present in local WRF products). Compared to yesterday, storm motion vectors increase by roughly 5 kt in average with the upper trough remaining progressive through the period. Local back-building can't be ruled out especially towards the coasts with heavy to excessive rainfall amounts, but the overall risk remains lower compared to 24 h ago. Still, isolated flash flooding is possible over Italy.
The rainfall risk increases along the E-coast of the Adriatic Sea at least until 15Z as long as this region remains downstream of the consolidating low-tropospheric vortex. Stronger LL S- to SW-erly winds in the BL may support temporal training from W-Croatia towards NW Albania. Local QPFs indicate 50-100 mm/24h and a level 1 was added.

The second focus and probably the one with the highest organized DMC chances exists along the SE-ward moving cold front, which crosses the Ionian Sea and Greece from W to E. Weak isallobaric gradients support a slow shift to the E/SE. Ongoing MCS event is expected over Albania with a slow shift to the south ... towards W-Greece. Shear/CAPE support organized storms and the general set-up supports a V-shaped MCS event with supercell structures along its tail-end. Large hail, strong to severe wind gusts and an isolated tornado event are possible all the way from W-Albania to W-Greece. Excessive rain will be another hazard, especially along the coastal areas and just inland. The level 1 was expanded inland over Greece to cover a temporal risk for surface based convection during the afternoon hours. Similar hazards are possible, although thick cloud coverage and lower CAPE should keep this risk in check. Heavy rain of W-coast storms will be another concern.
The severe thunderstorm risk also increases over the Aegean Sea and W-Turkey during the overnight hours. Organized multicells/supercells are capable of producing all kind of severe, inlcuding an isolated tornado event and excessive rainfall amounts. It has to be noted however that QPFs differ significantly regarding southward extent of convection. We added Crete to the level area as history showed that numerical data often underestimates DMC, which builds into a CAPE tongue.

Despite ominous looking shear profiles especially over S-Greece, expected type of convection (a solid but training MCS) keeps confidence in a more widespread severe event on the low to moderate side. Especially rainfall amounts could locally exceed a level 1 criterion however.

... Spain to Ireland ...

Elevated convection is forecast from Ireland to W/NW France all day long. Nothing severe is expected.

Over Spain/Portugal, a strengthening baroclinic zone will be the region to expect scattered showers/isolated thunderstorms. Some models expect a weak wave to evolve along that feature, which could shift the boundary a bit to the south during the day. Nevertheless, very high TPWs and effetive warm rain process signals point to a regional flash flood risk with that convection. Weak mid-level lapse rates / warm mid-levels should limit graupel production and hence lightning activity. Only a low-end lightning area was added. An isolated tornado risk exists over Portugal, where enhanced LL shear, very low LCLs and somewhat enhanced LL CAPE overlap.