Storm Forecast
Valid: Mon 01 Nov 2010 06:00 to Tue 02 Nov 2010 06:00 UTC
Issued: Sun 31 Oct 2010 19:33
Forecaster: DAHL

A level 1 was issued for the British Isles for severe wind gusts and tornadoes.

A level 2 was issued for NE Italy mainly for excessive precipitation.

A level 2 was issued for S Italy mainly for severe wind gusts and tornadoes and to a lesser extent for large hail and excessive rainfall.

SYNOPSIS

A seasonably strong meandering upper frontal zone is stretching from the Atlantic across the northern parts of Europe into Russia. An intense jet streak imbedded in this frontal zone, and an attendant SFC cold front are expected to cross the British Isles late in the period. Meanwhile... the western European long-wave trough will be closing off into a cut-off cyclone, migrating into the central Mediterranean on Monday.

DISCUSSION

British Isles

In along the cold front that is to cross the British Isles on Monday and Monday night ... a strongly-forced line of convection is anticipated. For the given situation, the models indicate an unsusually clear signal in the CAPE/ICAPE fields. Confidence is thus rather high that a line of convection will indeed form. As usually happens with such quasi-linear convective systems ... widespread strong/severe wind gusts are to be expected, along with tornadoes associated with bow echoes/line segments. It currently seems that the majority of the gusts will be only marginally (sub)severe, so that a LVL1 should be sufficient. Comparatively shallow cellular convection is anticipated in the wake of the front. Strong wind gusts may accompany the strongest of these showers/thunderstorms.

Central Mediterranean

There seem to be two foci for potentially severe weather. The northern focus appears to be tied to warm advection and possibly also to orographic forcing. CAPE and shear should be rather weak, but strong mesoscale ascent should support large amounts of rain, augmented by imbedded convective cells. The latest WRF runs place the highest precip/flood threat over extreme NE Italy.

Farther south ... 25 m/s deep shear will be in place, along with 10 to 15 m/s low-level shear. Overall, CAPE seems to be seasonably weak (a few 100 J/kg), but given decent large- and mesoscale forcing as well as the favorable shear, well-organized storms are anticipated.
Forecast hodographs show partly strong low-level curvature, suggesting that chances for persistent supercells exist. However, rather strong forcing and weak capping suggest that storms may rapidly coagulate into clusters and/or lines or line segments. The main threats should thus be severe wind gusts and probably a few tornadoes, along with excessive convective rainfall. Any persistent isolated supercell would in addition pose a threat for large hail. The severe threat may last well into Monday night.