More Heavy Rain—and a Medicane—Are Possible as Cyclone Numa Churns Near Greece

At least 16 people died early Wednesday in Greece as major flash flooding tore through several towns on the northwest edge of the Athens metropolitan area. Mudslides poured through the region after torrential overnight rainfall, inundating roads with bright red-orange soil and tossing vehicles like toys. In the town of Nea Peramos, some 1000 homes were flooded, according to deputy major Stavros Fotiou.  All of those reported killed by the floods were between ages 45 and 70, according to Greece’s national TV network ERT (as reported by the Guardian).

The heavy rains across Greece were associated with a large, cold upper low centered over Sicily on Wednesday. The storm has been dubbed Numa by the Free University of Berlin, whose vortex-naming practices have become the default choice for extratropical storms in Europe.

Cut off from the jet stream far to its north, Numa has been moving very slowly, weakening in place while pumping moist southerly flow into Greece. This moist air is pushing against the steep hillsides of the Balkan Peninsula, exacerbating the flood potential. The website Severe Weather Europe warned early Thursday that 48-hour rainfall amounts of 200-400 mm (8 – 16”) were possible in parts of central and northern Greece.
 

Figure 1.  This wundermap satellite image from 1647Z Thursday, November 16 (6:49 pm Thursday in Greece) shows the circulation around Cyclone Numa centered southeast of Sicily. Heavy showers and thunderstorms were clustered over parts of central and northern Greece.

Figure 2: MODIS satellite image of Numa in the Mediterranean Sea taken on Thursday morning, November 16, 2017. Greece is at the upper right of the image. Image credit: NASA.

Numa could become a “medicane”

When the Atlantic hurricane season begins to quiet down in late October and November, it’s time to cast an eye toward the Mediterranean Sea for “medicanes”--a nickname for storms that develop tropical characteristics just off the coast of southern Europe. Medicanes aren’t considered full-fledged tropical systems, since the waters of the Mediterranean aren’t extensive or warm enough to sustain a true hurricane. And despite the implication embedded in the name, very few medicanes achieve sustained winds as strong as a Category 1 hurricane. However, it’s quite possible for an existing center of low pressure in the Mediterranean to briefly take on tropical characteristics, including a symmetric structure and a small core of warm air.

Numa could become a medicane over the next couple of days. Sea surface temperatures are only around 20°C, far too cool for classic hurricane development, but medicanes can still form over such waters if upper-level temperatures are cold enough to make the air unstable. Phase-space diagrams from Florida State University, based on GFS model output from early Thursday, suggested that Numa could briefly become a shallow, symmetric warm-core system. Such development would most likely happen on Friday or Saturday local time, as Numa carries out a tight cyclonic loop over the Ionian Sea just west of Greece.

There is a fair amount of model disagreement on how Numa will evolve, as explained in a detailed discussion on Thursday morning from Severe Weather Europe. France’s global model (ARPEGE) predicts that Numa could reach maximum intensity while approaching the west coast of Greece this weekend, while the Max Planck Institute’s ICON model shows Numa weakening in the western Ionian Sea.

Figure 3.  The surface low associated with Numa was predicted by the 12Z Thursday run of the GFS model to strengthen as it circles over the Ionian Sea east of Italy and west of Greece from Friday into Saturday. Winds at the standard near-surface height of 10 meters (33 feet) were predicted by the GFS to exceed 35 knots (40 mph) on the west side of Numa by Friday evening local time. Image credit: tropicaltidbits.com.

Where to watch for medicanes

Medicanes are most likely to develop in two hot spots, according to a long-term climatology published in 2014 in Climate Dynamics. Interpolating from long-term atmospheric data for the period 1948-2011, the authors estimated that medicanes occur once or twice per year, on average, but with much year-to-year variability. None of the world’s meteorological agencies are tasked with monitoring medicanes, so countless such storms have no doubt gone unrecognized, especially before routine satellite monitoring began.

The most favored area for development is in the western Mediterranean, especially between Spain and the islands of Sardinia and Corsica. The other hot spot is the Ionian Sea, especially just southeast of Malta and the Italian peninsula, which is where Numa could become a medicane. In both areas, medicanes become more likely in autumn, peak in winter, and decrease in spring, according to the study, although the western area has a broader “season.”

Since the waters of the Mediterranean aren’t warm enough to support conventional hurricane development, medicanes rely on colder air aloft, typically brought in as part of an upper-level low that decays over the Mediterranean. Wind shear relaxes as the upper low decays, and the contrast between the cold air aloft and the relatively warm sea surface temperatures can stimulate the formation of showers and thunderstorms. These, in turn, may congeal around a weak surface low and help give it a symmetric, warm-core structure--and sometimes even a cloud-free, eye-like feature. Often a medicane’s warm core will be enveloped within broader cold-core features, which makes it more akin to a hybrid or subtropical storm than a tropical storm.

Recent medicanes

Our last medicane occurred October 28 – November 1, 2016, and was called 90M or Trixie. Trixie/90M peaked with sustained winds of 50 mph, and did minor damage to Malta before passing over Crete and dying. It appears the system briefly took on the symmetric warm-core features typical of a tropical storm.

A powerful medicane in early November 2014 dubbed Qendresa produced wind gusts as high as 96 mph on the north coast of Malta. Winds at the Luqa, Malta, airport looked suspiciously like what one would observe with a tropical storm passing overhead--a double peak with a near-calm in between, with the pressure falling to 984 mb during the calm. Wunderground member Zivipotty, a meteorology student in Hungary who analyzed Qendresa, concluded it was primarily subtropical in nature, as it weakened rapidly once it became detached from its parent frontal system.

In early November 2011, another noteworthy storm named Rolf took shape in the western Mediterranean. Rolf was the only medicane to be officially monitored by NOAA, whose Satellite Analysis Branch named it 01M and tracked it for two days. Zivipotty found Rolf to be more tropical in nature than Qendresa at its peak. 

Figure 4.  Satellite image of Rolf spinning south of the French Riviera in early November 2011. Image credit: Courtesy Dávid Hérincs.

Could bona fide hurricanes develop in the Mediterranean later this century?

According to research published in 2007, an increase in ocean temperatures of 3°C (5.4°F) in the Mediterranean by the end of the century could lead to hurricanes forming there. Miguel Angel Gaertner of the University of Castilla-La Mancha in Toledo, Spain, ran 9 different climate models with resolutions of about 50 km and found that some (but not all) of the models simulated hurricanes in the Mediterranean in September by the end of the century, when sea surface temperatures there could reach 30°C (86°F). Though the Mediterranean could start seeing hurricanes by the end of the century, these storms should be rare and relatively short-lived for three reasons:

• The Mediterranean is quite far north and is subject to strong wind shear from jet stream activity.
• The waters are shallow, and have relatively low heat content. There is no deep warm water current like the Gulf Stream.
• The Mediterranean has a number of large islands and peninsulas poking into it, increasing the chances that a tropical storm would weaken when it encountered land.

Dr. Jeff Masters co-wrote this post.