Category 4 Giovanna battering Madagascar

Earth's most dangerous storm of 2012 is Tropical Cyclone Giovanna, which is bearing down on Madagascar as a powerful Category 4 storm with 145 mph winds. Giovanna is predicted to hit a heavily populated portion of the east coast of the island near 22 GMT tonight as a Category 3 storm, then move inland, passing near the capital of Antananarivo as a Category 1 storm on Tuesday morning. The outer spiral bands of the storm have already moved over the island, bringing heavy rains and gusty winds.

Figure 1. Visible image from NASA's Terra satellite of Tropical Cyclone Giovanna approaching Madagascar, taken at 6:35 UTC Monday February 13, 2012. At the time, Giovanna was a Category 4 storm with 145 mph winds. Image credit: NASA.

Figure 2. Webcam view of the skies over the Andila Beach Hotel in northwest Madagascar, taken at 6:15pm local time on Monday February 13, 2012. Image credit: Andilana Beach Hotel.

The forecast: not good
Recent microwave satellite imagery (Figure 3) shows that Giovanna has concentric eyewalls, and it likely that the inner eyewall will collapse today as the storm undergoes an eyewall replacement cycle. This process should gradually weaken the storm, and I expect Giovanna will weaken slightly to a still very dangerous Category 3 storm with 125 - 130 mph winds at landfall. However, the eyewall replacement cycle will spread out the storm's hurricane-force winds over a larger area, increasing the storm surge. A 70-mile long swath of the coast that is heavily populated will receive sustained hurricane-force winds tonight. Rainfall amounts in excess of eight inches in a 24-hour period are expected along the center of Giovanna's path. These rains will cause extensive flooding and major damage to the country, and the storm is likely to be one of the top three most expensive disasters in Malagasy history. The damage potential is higher than for previous storms of similar intensity, due to the considerable deforestation Madagascar has experienced over the past 30 years. Madagascar lost 8.3% of its forest cover between 1990 and 2010 and is now just 22% forested, according to mongabay.com. Flood waters run off quicker from deforested land, reach higher heights, and cause greater damage.

Figure 3. Microwave satellite image of Tropical Cyclone Giovanna taken at 12:30 UTC (7:30 am EST) Monday, February 13, 2012. The echo-free eye is surrounded by two concentric eyewalls, the sign of a storm undergoing an eyewall replacement cycle. Image credit: Navy Research Lab, Monterrey.

Madagascar's tropical cyclone history
The strongest and deadliest tropical cyclone ever recorded in Madagascar was Tropical Cyclone Gafilo, which hit the northern end of the island on March 7, 2004, as a Category 5 storm with 160 mph winds. Galfilo dumped up to 20 inches of rain on the island, and its winds and flooding rains killed 363 people and did $250 million in damage, making it the deadliest and second most expensive storm in Madagascar's history. Gafilo's central pressure of 895 mb made it the second strongest tropical cyclone ever recorded in the Southern Hemisphere, behind the 890 mb central pressure of Tropical Cyclone Zoe of December 2002, which affected Fiji and the Solomon Islands. With a central pressure of 937 mb, Giovanna is a much less intense storm than Gafilo was.

Dr Jeff Masters - Weather Underground