CONVECTIVE FORECAST
VALID Fri 09 Sep 06:00 - Sat 10 Sep 06:00 2005 (UTC)
ISSUED: 09 Sep 06:23 (UTC)
FORECASTER: DAHL

There is a moderate risk of severe thunderstorms forecast across NW Tyrrhenian Sea and portions of NRN Italy.

There is a slight risk of severe thunderstorms forecast across Italy surrounding the MDT risk.

SYNOPSIS

Atlantic upper long-wave trough has closed off into an upper low ... which is ... like the associated SFC low ... expected to move into the Gulf od Biscay region during the forecast period. Downstream ... somewhat smaller upper low currently over the NW Mediterranean will lift NEWD ... reaching the extreme NW Balkans/Austria by Saturday 06Z. Another ... rather stationary upper low is progged to persist across the Black-Sea regions ... as is rather strong meandering NRN stream upper-level frontal zone over the NRN portions of Europe. At low levels ... plume of moist/warm air ... featuring ample instability ... is present over the central Mediterranean.

DISCUSSION

...Italy ... Tyrrhenian Sea...
Friday's 00Z and Thursday's 18Z LIEE ascents reveal rich BL moisture ... supporting CAPE in excess of 2500 J/kg. Other soundings indicate less moisture/instability which is consistent with numerical theta-e analyses ... also ... ascents may have been influenced by extensive convective activity that has been ongoing throughout Thursday evening/night. Indications are that ample CAPE will remain in place within the theta-e tongue ... moving across Italy today.

TSTMS are currently ongoing along the leading edge of the theta-e plume in low-level WAA regime ... and along the cold front/WRN edge of the plume. SAT imagery indicates some clearing between these two convective regimes ... so that insolation may aid in further destabilization of the air mass over Italy ahead of the cold front. Extensive downsream convection in the WAA regime should gradually weaken over the next few hours.

Given early and widespread development ... it seems that majority of the storms may congeal into large MCSs late in the day. But ... deep shear of 15 to 20 m/s should remain in place in the MDT area ... with 0-1 km shear just below 10 m/s per GFS 00Z.

Current thinking is that initial development will favor supercellular storms with an associated large-hail ... severe wind gust and tornado threats. Storms should merge into a linear MCS ... especially towards S Italy where shear should be strongest. This system may still contain mesocyclones given rather large line-normal component of the deep-shear vector. However ... numerical models show bulk of convective precip farther north in weaker shear regime ... consistent with the location of the region of strongest QG ascent.

Hence ... it seems that the greatest severe threat will exist during the next few hours before convection moves into increasingly weaker sheared deep-layer flow. Though threat for organized severe seems to exist as far S as over the Tyrrhenian Sea late on Thursday based on kinematic/thermodynamic fields ... models do not generate convection across this region. At least isolated development is anticipated however.

...Gulf of Biscay ... coast of W France...
Cellular convection may form beneath the Biscayan upper low ... possibly allowing for scattered TSTMS. Deep shear of about 20 m/s may support a small mesocyclone or two ... capable of producing marginally severe hail ... severe wind gusts and maybe a brief tornado ... especially late Friday night. Activity may affect the coastal regions of W France late in the period. Allover severe threat is too low for a SLGT though.

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