An unusual, stubborn low pressure system in the Mediterranean Sea has brought heavy rain and tropical storm-like conditions to parts of Italy and southern France over the past week. Torrential rains have inundated coastal regions and caused disastrous flooding in inland towns at higher elevations.
In southeast France, authorities evacuated thousands of people, as flooded streets and cresting rivers have killed at least three and stranded hundreds of others. Tropical storm force winds brought impressive waves to the coastal city of Nice. Continued heavy rains and wind gusts of up to 75 mph in the worst hit Var region were forecasted through early this morning.
The French weather service, Meteo France, reports that several locations have received the equivalent of 2-3 months of rain in less than a week, with some places reporting over 30 inches.
A sample of rainfall totals:
- 936 mm (36.9 in, or half of annual average precipitation) in Vallerauge
- 719 mm (28.3 in) in Sablières:
- 652 mm (25.7 in) in Loubaresse
- 282 mm (11.1 in) in Draguignan
- 214 mm (8.4 in) in Arles
- 185 mm (7.3 in) in Antibes; with a new all-time record 144 mm (5.7 in) in 24 hours
- 165 mm (6.5 in) in Cannes
- On Nov. 5, Corte (on the island of Corsica) recorded an all-time precipitation record of 210 mm (8.3 in) in 24 hours, of which nearly 6.8 inches fell in just 6 hours.
Heavy rain has also brought widespread flooding to northern and western Italy. In the northern city of Turin, the Po River rose over 13 feet, forcing thousands to evacuate. Farther south, flooded streets in Naples swept away cars and prompted officials to postpone an important soccer match. At least 16 people have died in Italy in the past two weeks, including six who drowned in a flash flood in Genoa last Friday.
What’s causing the heavy rain?
Behind the flooding is an extratropical low (named ‘Rolf’) that developed tropical characteristics after it stalled over the relatively warm waters of the Mediterranean. Wunderground reports that the storm featured sustained 40 mph winds earlier in the week, and its spiral shape and cloud-free center adequately resemble those of a tropical system. Water temperatures of only 17ºC (63ºF) are well below the 26ºC necessary to sustain tropical storm status, however.
The French weather service further explains that a large area of low pressure extending from Iceland to Portugal moved across the eastern Atlantic in late October. As it crossed into southwestern Europe, southerly winds around the low carried warm air into the lower atmosphere. Colder air aloft then triggered the formation of thunderstorms, which intensified once the low crossed the open waters of the Mediterranean. As the humid, unstable air moved north toward the southern coast of France, it was further lifted by the foothills of the southwestern Alps, which resulted in heavy precipitation totals.
Due to its relatively high latitude and narrow dimensions, the Mediterranean basin is rarely conducive to tropical storm development. However, subtropical systems such as this one have occurred in the past. Dr. Jeff Masters provides examples of previous “hybrid lows,” and also discusses whether climate change could lead to stronger Mediterranean storms in the future.
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