Storm Forecast
Valid: Sat 13 Sep 2008 09:00 to Sun 14 Sep 2008 06:00 UTC
Issued: Sat 13 Sep 2008 08:46
Forecaster: DAHL

SYNOPSIS

Feature of main interest this period will be a compact upper-level low centered over the north-central Mediterranean. This feature and the attached SFC low along with the deep-layer cold front at its eastern periphery, are expected to make some eastward progress during the day. The SFC cold front should be moving from the Tyrrhenian Sea into the Adriatic until early Sunday morning. The air mass east of this cold front is exhibiting very strong instability, with the Trapani 00Z launch revealing more than 4000 J/kg MLCAPE. Otherwise, a large SFC high pressure area is covering the northern parts of Europe.

DISCUSSION

... central Mediterranean region ...

Favorable setup for the evolution of organized severe thunderstorms is present over the central Mediterranean region amidst a very unstable and moderately sheared air mass ahead of an intense upper low.

i) Thermodynamic evolution:

It seems that the depth and strength of the low-level moisture is rather variable across Italy, with the Trapani 00Z ascent probably showing the maximum possible CAPE that might be available today. Model guidance hints at a concentration of large instability values immediately ahead of the cold front, with ICAPEs exceeding 3000 kJ/m^2.

ii) Kinematic evolution:

Indications are that at least 15 m/s 0-6 km shear, and probably around 20 m/s 0-3 km shear will exist in the immediate pre-frontal environment ... decreaseing to the east with greater distance from the front. Some mesoscale variation to the deep-layer wind field appears likely, e.g. resulting in locally augmented 0-3 km SRH as advertised by GFS, simulating 300 m2/s2 in the late afternoon hours over S Italy, which is tied to a mesoscale 700 hPa wind maximum. The exact timing of such features is somewhat uncertain, but confidence is high that such variations do exist, apt to create quite a favorable kinematic environment at least across in a limited spatio-temporal scale.

iii) Evolution of the vertical-motion field:

Though appreciable DCVA exists ahead of the upper low along and ahead of the cold front already, model guidance suggests that large-scale DCVA-forced ascent will become strongest late in the period. This is also reflected by the low-level frontogenesis field, which becomes increasingly well-organized as the period progresses. A mesoscale region of low-level convergence is present ahead of the front, which may turn out to be the main focus for convective evolution, as well as orographically-induced circulations over Italy.

iv) Storm evolution:

Convective initiation is underway already, tied to the pre-frontal low-level wind-shift line as well as to forced ascent along orographic features over Italy. Storm coverage should gradually increase over the next few hours, with rapid upscale growth anticipated especially across a strip from NE Tunesia to the S Adriatic Sea. A negative may be too fast evolution so that the environment may be deteriorating ... still, some of the cells should become quite well-organized.
Main storm mode will likely be well-organized multicells and supercells, posing a threat for isolated very large hail and destructive wind gusts. An isolated tornado cannot be discounted either. Storm activity should last well into, and probably increase towards, Saturday night and early Sunday morning.

In the post-frontal air mass near the center of the upper-level thermal low, quite steep low-level lapse rates are likely to develop late in the period, suggesting that waterspouts will be possible.