A very dynamic patter is unfolding over the central and western Mediterranean this weekend, bringing severe thunderstorms and torrential rainfall to parts of southern France, the Balearic Islands, Corsica, Sardinia, parts of Italy and the Adriatic region. We break down the setup and take a closer look.
A broad upper low ejects a longwave trough across the Iberian peninsula into the Mediterranean, pushing across the Alpine region and north Italy by Monday. The system is rounded by a moderately strong (30-50 kt) westerly to southwesterly mid-level (500 mbar) jetstreak.
A surface low has developed over SW France / NW Mediterranean by early on Saturday, pushing eastward. Ahead of it strong southeasterly to easterly surface winds have established over the W-CNTRL Mediterranean between the Balearic Islands and Italy; in particular, easterly low level winds will be particularly strong along the southern coast of France, reaching sustained speeds of 50-70 km/h, gusting up to 100 km/h! Strong to locally extreme instability will be in place in the warm sector: up to 2000-3000 J/kg MLCAPE across the Balearic Islands towards Sardinia, pushing into 3000-4500 J/kg MLCAPE between Sardinia, Sicily and the coast of Algeria and Tunisia on Saturday and early on Sunday!
On Sunday a complex situation will have developed, with several surface lows under the broad trough. One surface low will likely develop over north Italy, pushing into the northern Adriatic sea. A second cutoff low will push across the Tyrrhenian sea, pushing into the southern Adriatic and the Ionian sea by late on Monday. Strong to extreme instability will build up across the Tyrrhenian sea on Sunday, with up to 2000-3000+ J/kg MLCAPE. On Monday the northern Adriatic will see the build up of moderate instability, up to 1000 J/kg MLCAPE, while the central and southern part of the Adriatic will go up to 2000 J/kg and parts of the Ionian sea up to 2000-3000 J/kg MLCAPE. Strong southerly to southeasterly winds will be in place across warm sectors of these lows.
Severe weather outlook for Saturday, September 21st
Expect major rainfall in parts of southern France and extreme NE Spain. Expect torrential combined convective and orographic rainfall, locally reaching 100-200 mm by the end of Saturday and continuing into Sunday, for a total of up to 200-300 mm locally by Sunday afternoon. Expect locally intense flooding in these areas. Expect a mesoscale convective system to form along the frontal boundaries of the surface low, slowly pushing ENE-wards across the NW Mediterranean between the coasts of south France, northeast Spain and the Balearic Islands throughout the day. With sufficient deep-layer shear, expect (some) rotating updrafts. Strong low-level shoutheasterlies to easterlies, with strong vertical veering wind profile will provide up to 20-30 kt 0-1km shear and 1km SRH in 200-300 m2/s2 range, and sharp low level lapse rates will support waterspouts, a strong event is possible. The coast of extreme NE Spain and to a lesser extent the coast of southern France may see severe thunderstorms with waterspouts making landfall. Large hail and severe straight line winds are also possible with these storms.
Severe weather outlook for Sunday, September 22nd
Detailed forecast on the link below: