Category 5 Super Typhoon Bopha bearing down on the Philippines

Extremely dangerous Typhoon Bopha is bearing down on the Philippine island of Mindanao as a Category 5 storm with 160 mph winds. Bopha completed an eyewall replacement cycle on Sunday and has been steadily intensifying today, and will make landfall on Mindanao in the early morning on Tuesday local time. Mindanao rarely gets hit by typhoons, since the island is too close to the Equator, and the infrastructure of Mindanao is not prepared to handle heavy typhoon rains as well as the more typhoon-prone northern islands. Bopha is potentially a catastrophic storm for Mindanao. The typhoon is following a similar track to last year's Tropical Storm Washi, which hit Mindanao on December 16, 2011 with 60 mph winds and torrential rains. Washi triggered devastating flooding that killed 1268 people. Washi was merely a tropical storm, and Bopha is likely to hit at Category 4 or 5 strength, making it the strongest typhoon ever recorded in Mindanao.


Figure 1. Super Typhoon Bopha at 01:45 UTC on December 2, 2012. At the time, Bopha had top sustained winds of 150 mph, as was just below its peak intensity of 155 mph, which it reached from 06 - 12 UTC on December 2. Image credit: NASA.

Bopha: the 2nd most southerly typhoon on record
Bopha became a tropical depression unusually close to the Equator, at 3.6°N latitude. Tropical cyclones rarely form so close to the Equator, because they cannot leverage the Earth's rotation to get themselves spinning. According to hurricane expert Dr. Paul Roundy of SUNY Albany, Bopha got its spin from a large-scale atmospheric wave called a mixed Rossby gravity wave. Because of the lack of atmospheric spin so close to the Equator, it took Bopha over four days to intensify into a typhoon, and it stayed a relatively small storm. Bopha became the 2nd most southerly typhoon ever recorded in the Western Pacific at 06 GMT on November 30, when the storm was at 3.8°N latitude. The Joint Typhoon Warning Center lists Typhoon Vamei of 2001 as the most southerly typhoon on record, at 1.5°N. However, other meteorological agencies do not credit Vamei with reaching typhoon strength, so this record is disputed. The previous most southerly typhoon was Typhoon Kate of 14 - 25 October 1970, which reached typhoon intensity at 4.3°N, 137.4°E. Bopha continued intensifying over the weekend, becoming the second most southerly super typhoon ever recorded (150 mph winds) at 00 GMT on December 1, when it was at 6.1°N latitude. The record most southerly super typhoon was Kate, which reached super typhoon intensity at 6.0°N, 126.3°E. Kate struck the Philippine island of Mindanao as a Category 4 storm, killing 631 people. Bopha further intensified into a Category 5 typhoon on Monday at 7.4°N latitude, becoming the second most southerly Category 5 typhoon on record, next to Typhoon Louise of 1964, which was a Category 5 storm at 7.3°N. According to NOAA's Coastal Services Center, there have been only 4 previous typhoons of at least Category 4 strength to track within 200 nautical miles of Mindanao Island, dating back to 1945: Mike ("Ruping" ) in 1990, Ike ("Nitang") in 1984, Kate ("Titang") in 1970, and Louise ("Ining" ) in 1964.


Figure 2. Tracks of all Category 4 typhoons to affect the southern Philippine Island of Mindanao since 1945. Image credit: NOAA Coastal Services Center.

The closest a tropical cyclone has ever been to the Equator is 0.7°N, by Tropical Cyclone Agni in the North Indian Ocean in November 2004. Agni got counter-clockwise spin from the presence of the summer monsoon circulation in the Indian Ocean. The closest a Western Pacific tropical cyclone has been to the Equator is 1.4°N latitude, by Tropical Storm Vamei on December 27, 2001. Vamei hit Singapore after Christmas in 2001, at a latitude of 1.5°N.


Figure 3. This MET-5 visible satellite image taken at 0400 UTC November 28, 2004, shows Agni as a developing tropical storm just north of the Equator in the Indian Ocean. Image credit: Navy Research Lab, Monterey.

Jeff Masters