Hong Kong Braces for Category 3 Typhoon Usagi

Published: 4:25 PM GMT on September 21, 2013

Dangerous Category 3 Typhoon Usagi is charging through the Luzon Strait between Taiwan and the Philippines on its way towards China, where landfall is expected early Monday morning local time (near 2 pm EDT on Sunday) near Hong Hong. The typhoon battered the northern Philippine Batanes Islands overnight with wind gusts of up to 155 mph (250 kph), ripping down power lines and damaging crops, according to the South China Morning Post. Torrential rains of over a foot (305 mm) have fallen in 24 hours over eastern Taiwan, where Usagi's counterclockwise flow of moist air rode up over the high mountains of the island. Usagi reached its peak strength on Thursday, taking advantage of low wind shear and very warm waters 30°C with high heat content, to intensify to a Category 5 super typhoon with 160 mph winds. On Friday, Usagi began an eyewall replacement cycle that the typhoon is still attempting to complete. This process, where the inner eyewall collapses and a new, larger-diameter eyewall forms from an outer spiral band, typically causes a reduction in intensity by one Saffir-Simpson category, but spreads out the storm's hurricane-force winds over a larger area. Satellite images show that Usagi has lost its symmetry and the cloudtops have warmed, indicating weakening; this weakening is likely due to disruption of the low-level inflow by the high mountains of Taiwan. Wind shear is low, near 10 knots, but ocean temperatures have cooled to about 29°C, and the heat content of the waters is much lower than on Friday.

Figure 1. MODIS satellite image of Typhoon Usagi, taken at approximately 04:30 UTC on September 21, 2013. At the time, Usagi was a Category 4 typhoon with 140 mph winds. Image credit: NASA.

Figure 2. Rainfall in Taiwan for the 24-hour period ending at 12 pm EDT (midnight in Taiwan) on Saturday, September 21, 2013. Rainfall amounts in excess of 300 mm (11.81", yellow colors) fell in less than 24 hours in several regions. Image credit: Central Weather Bureau.

Forecast for Usagi
Continued slow weakening of Usagi is likely as the storm tries to complete its eyewall replacement cycle as ocean temperatures cool to 28°C and the heat content of the water diminishes. By the time Usagi reaches the coast near Hong Hong in the early morning hours on Monday (local time), the storm should be at Category 2 strength. This is still strong enough to pose a formidable storm surge, wind, and heavy rain threat to China, and Usagi is likely to be one of the five strongest typhoons to affect Hong Hong in the past 50 years. If the eye of the storm hits just west of Hong Kong, a large storm surge capable of causing over a billion dollars in damage will inundate portions of the coast along the bay that Hong Kong, Macau, and Shenzhen share. Wunderground's weather historian, Christopher C. Burt, has a new post, Hong Kong's typhoon history, detailing the most notable storm's in Hong Hong's history. The most notable typhoon to hit Hong Kong in the past 50 years was Typhoon Rose of 1971, which sank over 300 boats and killed 110 people.

Figure 3. Super Typhoon Usagi, as seen by the VIIRS instrument on the Suomi satellite on Thursday, September 19, at 1635 UTC (375-m I-band 5). At the time, Usagi was a Category 5 super typhoon with 160 mph winds. The eye is at its narrowest at the surface, and slopes outward with altitude (like being inside a stadium), so that the cloud temperatures measured in infrared light in this image change dramatically surrounding the eye. The warmest brightness temperature inside the eye in this image is -11°C, which suggests that we are seeing a mid-level cloud deck in the center of the eye. Image credit: Dan Lindsey, NOAA/RAMMB/CIRA.

Figure 4. Visible light image from VIIRS taken under the full moon at the same time as the above infrared-light image. Image credit: Dan Lindsey, NOAA/RAMMB/CIRA.

Usagi Links
Southeast China radar

The University of Wisconsin CIMSS Satellite Blog has a nice animation showing the trochoidal (wobbling) forward motion characteristic of intense tropical cyclones.

Video 1. Typhoon chaser James Reynolds caught video of some impressive surf from Typhoon Usagi impacting Taiwan on September 21, 2013. His Twitter feed is here. He is now in Hong Hong to document the storm's arrival there.

Vulnerability assessment of storm surges in the coastal area of Guangdong Province, a 2011 journal article by Li and Li.

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.