CONVECTIVE FORECAST
VALID Tue 06 Sep 06:00 - Wed 07 Sep 06:00 2005 (UTC)
ISSUED: 05 Sep 23:49 (UTC)
FORECASTER: VAN DER VELDE

There is a slight risk of severe thunderstorms forecast across southern France

There is a slight risk of severe thunderstorms forecast across southern Italy

SYNOPSIS

An old low pressure area and upper cold pool move from south of Ireland to the Iberian Peninsula. A cold front remains stationary over France, where warm humid Mediterranean air is continuously transported northward at its east side as low pressure slides to Spain.
A warm front will pass over the Benelux during the evening with some chance of thunderstorms.
Steep midlevel temperature lapse rates are present over large parts of the Mediterranean Sea and Italy.

DISCUSSION

...southern France...
Frontogenesis is reinforced by strengthening warm humid flow from the Mediterranean and cold air advection at the other side of the cold front. Forced ascent takes place over the southern rim of the Massif-Central. MM5 precipitable water values range into 35-45 mm, while the 12Z Palma sounding shows that steep lapse rates are also advected northward. With colder upper temperatures arriving into the region, today should certainly be no less convective than the past day when storm systems lingered over the area for hours, delivering precip sums already over 60-100mm. Hence, the main threat is that caused by large amounts of precipitation, i.e. flash floods and/or landslides, accompanied by strong lightning activity. In addition, the SLGT area includes strong low-level shear (>10-15 m/s) and storm-relative helicity (>300 m2/s2), moderate deep layer shear (around 15 m/s), and low LCLs (<1000m), that indicate a potential for tornadoes. Marginally large hail should also be possible.

...southern Italy...
Previous day soundings have shown ample instability (>1000 J/kg MLCAPE) created by a fairly deep layer with steep lapse rates over a moister layer. In addition, GFS/NMM models forecast moderate deep layer shear, and some modest SREH enhancement over Sicily. This instability/shear combination is enough for organized convective storms with in particular a threat of large hail and some severe gusts.

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