Storm Forecast
Valid: Sat 02 Oct 2010 06:00 to Sun 03 Oct 2010 06:00 UTC
Issued: Sat 02 Oct 2010 02:09
Forecaster: VAN DER VELDE
A level 1 was issued for the southwestern Black Sea for waterspout-type tornadoes.
A level 1 was issued for NW France and S UK for a small chance of severe convective gusts and tornadoes.
SYNOPSIS
A frontal band in the southwestern circulation around a large low south of Iceland drags over the northwestern tip of the Iberian peninsula, northwestern France, southern UK and the Benelux. Along the front, strong winds transport moist air northward. In nearly neutrally stratified warm airmass, tiny amounts of CAPE could be embedded. More importantly, several models predict cyclogenesis and wave structures along the frontal boundary. A wave amplifies around 18Z near Bretagne and then rapidly moves over the southern UK. Around this feature 1-3 km wind speeds will reach 25-30 m/s, wind profiles show very large 0-1 km shear (20 m/s) and 0-3 km SR helicity values (250 mē/sē). While the model shows very little CAPE (higher along the front to the southwest), it is possible that strong mesoscale forcing near the occlusion point might trigger convection which could profit from the shear and produce a tornado or small bow echo with severe gusts.
On the other side of an elongated ridge between Italy and Finland, an upper low drifts from Ukraine into the Black Sea later in the period. Increasing conditional instability and lapse rates in the lower 3 km are predicted, as well as very steep SFC-500m lapse rates. Convective cell motion is weak and a convergence zone develops which can initiate convection and focus local vertical vorticity. Waterspouts are the likely result.
In most places on the map, convective cloud tops are either hardly cold enough, CAPE is too marginal, too capped, or too much under influence of negative QG motion (Ebro area, Greece) to warrant even a 15% thunder line.
In Galicia, convection could just reach the coast by the end of the period, while cloud tops appear to be warming, but low-level shear and background winds are strong so that eventually some threat of convective gusts and tornadoes may exist.
