Mesoscale Discussion
Valid: Wed 12 Jul 2023 18:00 to Wed 12 Jul 2023 22:00 UTC
Issued: Wed 12 Jul 2023 18:32
Forecaster: GROENEMEIJER

Large supercells with a history of producing very large hail are tracking eastward across the lower Rhône Valley in an environment with 2500 - 4000 J/kg of CAPE. They have an excellent and intense representation in radar and satellite imagery. The first of two intense cells is now struggling as it traverses the higher elevations of the western Alps. A second intense storm crossed the Rhône river near Montélimar before 18 UTC, and is followed by newly initiating cells. Given the very high CAPE, low LCL height, and radar representation of a hook echo and strong rotation, there is a non-neglibile risk of a tornado with these storms.

The evolution during the next hours includes the continuation of cell development near the Rhône valley with possible new very intense storms producing very large hail, severe winds and possibly a tornado. In addition, some storms will (re)form over northwest Italy and extreme southern Switzerland.

Hence, across both the French, Swiss, and Italian parts of the indicated areas a considerable risk of large an very large hail, well in excess of 5 cm diameter will exist through the next hours. In addition, locally damaging winds and very heavy rainfall are likely, and isolated tornadoes not ruled out.

Guidance by convection-allowing and global models both suggest that, across North Italy, storm activity should continue well into the evening. It is possible that the storms will gradually turn into more linear modes, with a lower risk of hail, but a higher probability of severe winds. That said, the cooling boundary layer will probably mitigate the wind risk somewhat.