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Weakening Typhoon Chan-hom Still a Major Storm Surge Threat for Shanghai

By: Jeff Masters 4:08 PM GMT on July 10, 2015

Category 3 Typhoon Chan-hom is steadily weakening as it heads northwest at 10 mph towards China. The storm has slowed down and turned more to the north as it "feels" the steering influence of a trough of low pressure to its north, and the latest round of computer model forecasts have nudged the track of Chan-hom to the east, and it is possible that the center of Chan-hom will not make landfall in China. The 10 am EDT Friday forecast from the Joint Typhoon Warning Center (JTWC) and 8:50 am EDT Friday forecast from the Japan Meteorological Agency (JMA) predicted that Chan-hom would graze the coast of China and pass close to or just offshore from Shanghai on Saturday evening local time (Saturday morning in the U.S.) On this path, Shanghai, China's most populous city with 23 million people in the metro area, would be on the weak side of the storm, and receive only modest wind damage and heavy rain. However, Chan-hom would still drive a large storm surge into Shanghai, and this storm surge could be one of the highest ever observed, equivalent to a 1-in-200 year flood. Even though Chan-hom is weakening due to cooler waters and interaction with land, part of the weakening is due to an eyewall replacement cycle, where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall. While this process weakens the peak winds near the center, it spreads the typhoon-strength winds over a larger area, increasing the size of the storm surge.

Figure 1. Typhoon Chan-hom as seen by the MODIS instrument on NASA's Aqua satellite at approximately 10 pm EDT Thursday, July 9, 2015 (02 UTC.) At the time, Chan-hom was a weakening Category 4 storm with winds of about 130 mph. Image credit: NASA Worldview.

Chan-hom's storm surge
Chan-hom is a very large typhoon with tropical-storm force winds that extended outwards up to 310 miles from the center, which will pile up a large storm surge throughout the Yellow Sea, from China to the Korean Peninsula. Since the Yellow Sea is shallow and enclosed on three sides, this water will be forced up onto land over Shanghai as Chan-hom makes its closest approach. In their 10 am EDT Friday forecast, JTWC predicted that Chan-hom would be a Category 1 storm with 80 mph winds (1-minute average) at 12 UTC Saturday, which would make it just 5 mph weaker than the strongest landfalling storm to hit within 200 miles of Shanghai in the past 35 years, Typhoon Winnie of August 1997. As I discussed in detail in my previous post, the storm surge from Winnie was only 5.5" (14 cm) below the top of the 19.2-foot (5.86 meter) Suzhou Creek floodgate that protects downtown Shanghai.

Low tide in Shanghai is at 07:07 UTC Saturday, and high tide is at 12:48 UTC Saturday, at a time when the center of Chan-hom is predicted to be 20 - 80 miles south of the city. Thus, the counter-clockwise circulation around the center will be pushing water into the city at high tide. Fortunately, this high tide is not a very high one--high tides late next week will be more than two feet higher than this. Though Chan-hom will be weakening as it approaches Shanghai during Saturday's high tide, the typhoon will be capable of pushing a record-size storm surge into the city during this 12:48 UTC Saturday high tide. I've read several studies explaining how storm surge propagation in the Yellow Sea is extremely complicated, so I am unsure just how the great the risk is from this storm without seeing data from a sophisticated real-time storm surge model.

Figure 2. Predicted swath of winds (top) and precipitation (bottom) for Typhoon Chan-hom, made by the 06 UTC (2 am EDT) Friday, July 10 run of the HWRF model. The model predicted that Chan-hom would graze the coast near Shanghai as a strong tropical storm, bringing rainfall amounts of 4 - 8" near the coast (dark yellow colors), and 2 - 4" farther inland. The typhoon was also forecast to bring large areas of 4 - 8" of rain to North Korea, which is suffering one of its worst droughts on record. Image credit: NOAA.

Chan-hom's rains and winds
With the latest round of model runs showing the Chan-hom will not penetrate far inland, wind damage is looking to be less of a concern, since most of the land areas affected will be on the weaker (left) side of the eye. The prospect for heavy damage due to flooding from torrential rains is also looking lower, as Chan-hom may only dump heavy rains of 4 - 8" along the immediate coast (Figure 2.) As Chan-hom turns to the north, it is expected to track over North Korea, which could use the rain--they have reportedly been suffering through their worst drought in 100 years, though rains in June have likely eased the drought.

Figure 3. Tracks of all typhoons with at least 75 mph winds (10-minute average winds as rated by the Japan Meteorological Agency) to pass within a 230-mile diameter circle (light shaded region) near Shanghai, China. Typhoon Winnie is labeled in white. Ten-minute average winds of 75 mph are roughly equivalent to 85 mph winds for the one-minute averaging time winds used for the U.S. Saffir-Simpson scale. All of the storms in this plot had sustained 10-minute average winds of 75 mph or less when they made landfall. Image credit: NOAA.

Strong typhoons hitting near Shanghai: a rare occurrence
China gets hit by about nine tropical cyclones (tropical depression, tropical storms, or typhoons) each year (Chen, 2000), but these strikes occur primarily in the southern portion of the country. The Jiangsu Province where Shanghai lies received only seven landfalls in the 50-year period 1947 - 1999, so the region does not have a lot of typhoon experience. Since 1979, no typhoon with winds in excess of about 85 mph (75 mph winds using a 10-minute averaging time) has made landfall within about 200 miles of Shanghai (Figure 2.) Historically, the strongest typhoon to affect the city in the past century may be Typhoon Gloria of July 24 - 25, 1949, whose storm surge overwhelmed the city's flood walls and left much of Shanghai a flooded ruin, with over 250,000 people homeless (See David Longshore's Encyclopedia of Hurricanes, Typhoons, and Cyclones). Note that Typhoon Wanda of 1956 was at Category 3 strength when it hit the coast of China about 100 miles south of Shanghai. Wanda killed 2000 people in China. China has had four typhoons that have killed at least 37,000 people each--most recently in 1975, when torrential rains from what had been Super Typhoon Nina caused the Banqiao Dam to fail, killing 90,000 - 230,000 people.

The new Japanese Himawari satellite has some spectacular imagery of Chan-hom (Sector 4 in Band 3=visible, and Sector 6 in Band 13=IR.)

Chan-hom satellite imagery from NOAA/NESDIS.

Weather radar from China.

Shanghai webcams (thanks to wunderground member fuzed for posting this link.)

Wunderblogger Steve Gregory has the latest on the status of El Niño in his latest post.

Jeff Masters


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