Slow-moving and powerful Extratropical Storm Cleopatra (called Ruven by the Free University of Berlin) dumped prodigious rains over the Mediterranean island of Sardinia on Monday, triggering floods that have killed at least eighteen people. According to weather records researcher Maximiliano Herrera, the highest 24-hour rainfall recorded at an official station was 467 mm (18.39") measured at Orgosolo (Nuoro province.) Note that media reports on Tuesday incorrectly reported the deluge as occurring in less than two hours. Monday's deluge was at most the 3rd greatest 24-hour rainfall event for Sardinia. An October 16, 1951 storm brought 544 mm (21.42") in 24 hours to Sicca d' Erba, causing the greatest 20th Century flood in Sardinia history. Second place goes to Villagrande Strisaili on December 6, 2004, when 517mm (20.35") fell.
Figure 1. Extratropical Storm Cleopatra/Ruven over the Mediterranean at night, taken at approximately 6 UTC (1:30 am EST) November 19, 2013. Image credit: Navy Research Lab, Monterrey.
Slow moving hybrid storm to blame Monday's extreme rainfall event was caused by a slow-moving extratropical low pressure system that lingered over relatively warm 19 - 20°C waters Mediterranean waters long enough to develop a shallow warm core, according to cyclone phase space diagrams from the GFS and UKMET models available from Florida State University, and an analysis from the European Storm Forecast Experiment. As the low deepened to a central pressure near 990 mb, bands of heavy thunderstorms developed over the mountainous terrain of Sardinia and dumped torrential rains. Ocean temperatures were about 0.5°C (1°F) above average, which helped intensify the storm's rains. While the storm's shallow warm core meant that Cleopatra/Ruven had a hybrid nature and was able to pull heat energy out of the ocean like a hurricane does, this was not the primary cause of the heavy rain, which would have occurred without the storm having a shallow warm core. Hybrid storms that bring extreme weather to the Mediterranean happen regularly; I blogged in November 2011 about a hybrid storm (Rolf) that brought heavy rains and 95 mph wind gusts to Southern France.
The jet stream set-up for this extreme event was very much like the ones that spawned this year's destructive floods in Calgary, Canada, Boulder, Colorado, and Central Europe--a slow-moving upper low trapped to the south of a blocking ridge of high pressure. A NOAA chart showing the amount of "blocking"--where on the planet ridges of high pressure are getting "stuck"--shows pronounced blocking near 0°E (over the UK). The plot also shows a major block over Alaska, which has brought some remarkable warmth there (see Chris Burt's post on this, More Crazy Weather in Alaska). Slow-moving lows stay in place longer, allowing greater rains to fall over a localized area, increasing the risk of catastrophic flooding. A big question is how climate change may affect the incidence of blocking. The latest 2013 IPCC report has this to say about the subject: "Increased ability in simulating blocking in models and higher agreement on projections indicate that there is medium confidence that the frequency of Northern and Southern Hemisphere blocking will not increase, while trends in blocking intensity and persistence remain uncertain." It seems like we've been getting more than our fair share of these slow-moving "blocked" storms that generate extreme rainfall events, though; I discussed the possibility that climate change may be at work in this regard in a post on the $22 billion flood that hit Central Europe this summer.