CONVECTIVE FORECAST
VALID 06Z TUE 30/09 - 06Z WED 01/10 2003
ISSUED: 30/09 19:16Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across parts of northeastern Spain, the French Mediterranean Coast, the lower Rhone Valley, the western Alps, the Jura and parts of the upper Rhine Valley.

General thunderstorms are forecast across extreme southern England, France, the western Alps, and parts of northern Italy, Spain and the western Mediterranean Sea.

SYNOPSIS

At 06Z...on the southwestern flank of a mid/upper low approx. 500 km NW of Spain, a quite strong westerly mid/upper flow is forecast over Iberia and moving over the western Alps and eastern France. A ridge is located over the central mediterranean, while a trough is present over Scandinavia.

DISCUSSION

...Spain, southeastern france...
Within the southwesterly flow over southwestern Europe, a mid-level speed maximum is expected to be near the Spanish east coast and be advected northeastward during the day. Strong rising motions are forecast in its left exit region leading that will likely have intiated an MCS over northeastern Spain at 06Z. Quite strong low-level (0-1km) shear (20-30 kts) and some 50-60 kts of deep-layer shear (0-6km) will be present. One or two linear MCS's will likely move into southeastern France during the morning hours. These will be capable of producing severe gusts and --given high LCLs and low level shear-- one or two tornadoes are possible especially over the Languedoc and Rhone delta areas. A minor threat of large hail also exists. Convective activity will likely start to back-build over the Rhone Valley, which may result in a few isolated flash flood events.

...east-central France, Jura, Upper-Rhine Valley...
In the afternoon and evening a few 100's of J/kg CAPE will likely have formed over the area. Convective activity will affect the area from the southwest after noon. Expect than one or two linear MCS's will affect the area with a main threat of high winds as orographic effect will likely enhance low-level shear quite strongly. Also, one or two tornadoes may form over the area along the leading edge of the MCS. A few isolated large hail events are possible as well.

...Portugal, southwestern Spain...
About 20 kt of low-level shear, and strong deep-layer shear are expected to be present during the day, while a marginally unstable air-mass is advected into the Iberian Peninsula. Some of the convection will likely be organised in small bows and possibly isolated mini-supercells. This implies that a small chance of strong gusts, weak tornadoes, and a chance of large hail will be present over the area. If low-level shear and/or instability will turn out to be larger than currently anticipated an upgrade to SLIGHT RISK may be needed.

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