CONVECTIVE FORECAST - UPDATE
VALID 15Z WED 01/10 - 06Z THU 02/10 2003
ISSUED: 01/10 15:47Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across southern Portugal, parts of western and southwestern Spain.

General thunderstorms are forecast across The Iberian peninsula, the Bay of Biscay, much of France and extreme western Switzerland.

SYNOPSIS

To the southwest of an upper low located northwest of Iberia 15Z... a strong westsouthwesterly mid/upper flow is present over Iberia into southern and eastern France. A shortwave trough developing southwest of Portugal is expected to affect the weather over southern Iberia late this evening and overnight. A ridge is located over the central Mediterranean, while a trough is present over Scandinavia.

DISCUSSION

...southwestern Iberia...
A few thunderstorms are currently going on over northern Portugal and northwestern Spain. Strong rising motions are expected on the approach of the trough, which will likely induce a comma-like disturbance. Convection associated with the system is forecast to affect the Iberian mainland starting approximately around midnight. MUCAPE will likely be on the order of a few 100's of J/kg near the coast and decrease inland. Despite the low instability, strong (50-65 kt 0-6km) deep-layer shear and very strong low-level shear (25-40 kt 0-1km) in combination with strong forcing for upward motion will aid storms becoming severe. Expect linear organised convection, that may contain rotating updrafts/embedded supercells. Main threats will be strong gusts. High low-level helicity and low LCL heights indicate a few tornadoes may occur with any embedded supercell, while they also pose a minor threat of large hail. Apart from this, a few places within the SLGT RISK area may see rainfall amounts in excess of 50 mm.

...Eastern France, Jura, Upper Rhine Valley...
The categorical risk has been cancelled, because the amount of forcing was low than expected and does not seem to be able to initiate widespread thunderstorms. Although a few isolated storms may affect the area during the next few hours, the risk that they will be severe will be small as low level shear is expected to decrease this evening and instability is low.

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