Storm Surge Threat Remains Extreme for Eastern India, Bangladesh as Amphan Approaches

May 19, 2020, 8:45 AM EDT

Above: NASA’s Aqua satellite passed over Tropical Cyclone Amphan, located in the Bay of Bengal, at 0740Z (3:40 am EDT) Monday, May 18, 2020. Aqua found the highest concentrations of water vapor (brown) and coldest cloud top temperatures were around the clear eye. (NASA/NRL)

Though down from its previous Category 5 strength, Tropical Cyclone Amphan continues to pose a threat of catastrophic storm surge as it moves through the northern Bay of Bengal. The kinetic energy in Amphan’s winds is spreading over a broad area, pushing immense amounts of water toward the river deltas of far eastern India and Bangladesh.

As of 03Z Tuesday (11 pm EDT Monday), the Joint Typhoon Warning Center pegged Amphan’s top winds at 140 mph, making it a Category 4 equivalent on the Saffir-Simpson scale. Increasing wind shear and intrusions of dry air have degraded the storm’s structure considerably since its peak at Category 5 strength, with its eye no longer crisply visible on satellite imagery. Amphan will likely make landfall as a Category 2 equivalent, perhaps a Category 3. Nevertheless, a major storm surge is almost certain given the storm’s trajectory and the geography of the Bay of Bengal (see below).

Amphan continues on a general north-northeast bearing and is expected to accelerate slightly prior to a landfall in India’s West Bengal state, possibly near Kolkata, late Wednesday afternoon local time (Wednesday morning EDT). Such a track would drive the biggest storm surge into the mangrove forests of the Sundarbans of far eastern India and Bangladesh. The Indian Meteorological Department warned that a surge of 4-5 meters (13-16 feet) above astronomical tides could engulf low-lying areas from just east of Kolkata to the Sundarbans. (Similar to Houston, Kolkata sits about 50 miles inland.)

Although Amphan’s high winds will wreak havoc, and its torrential rainfall will cause inland flooding—a major concern in itself, given the storm’s vast envelope of moisture—the most serious threat posed by Amphan is potentially catastrophic storm surge. Even if Amphan's top winds weaken further, the storm surge threat will likely remain extreme. Amphan is a large cyclone that is already pushing a tremendous amount of water northward into the Bay of Bengal, which exerts a funneling effect on northward-moving cyclones. There is a great deal of momentum in the water pushed by large, powerful storms when their peak winds weaken but their overall wind fields expand, as evidenced by 2008's Hurricane Ike in Texas and 2012's Hurricane Sandy in New Jersey and New York.

A massive evacuation effort is under way

Motivated by having endured some of the deadliest cyclones in world history (see our last post for details), Bangladesh has assembled a formidable system of cyclone shelters and evacuations that will go a long way toward reducing the impact of Amphan. The Dhaka Tribune reported that more than 12,000 cyclone shelters have been prepared for evacuations on Tuesday, including 5000 dedicated shelters and more than 7000 schools being brought into service. As many as 5 million people could be accommodated. The bolstered number of shelters will help maintain social distancing, said Enamur Rahman, Bangladesh’s minister for disaster preparedness and relief. All evacuees will be asked to wear masks while in shelters.

It remained unclear how many people might avoid seeking shelter because of the coronavirus crisis or because of perceptions the shelters are poorly maintained. “In the [Bholda] district, there are 21 risky remote islands, on which around three lakh [300,000] people are living. Most of them don't want to go to the cyclone shelters. But we will try to bring in as many as possible [to the shelters]," Mohammed Masud Alam Siddique, deputy commissioner of Bhola, told the Daily Star.

The world’s largest refugee camp—the Rohingya settlement on the Bangladesh-Myanmar border, with close to a million residents—will likely be just far enough east to avoid the worst of Amphan and its surge. Relief agencies are in close contact with the Bangladeshi government to take action as needed.

Sidr and Amphan

One of the closest recent analogs for Amphar is Cyclone Sidr, which approached landfall in western Bangladesh as a Category 4 storm in 2007. An estimated 3 million people were evacuated to safety. Even so, the cyclone killed 4235 people and caused $1.7 billion of damage, destroying an estimated 1.5 million houses. The eye fortunately came ashore in the Sundarbans, the world's largest forest of mangrove trees and the least populated coastal area in the country. On its current track, Amphar will make landfall just west of Sidr’s landfall point, which could still put the worst of the surge in the Sundarbans.

Ted Fujita to be profiled Tuesday on "American Experience"

A documentary on the life and work of famed tornado researcher Tetsuya Theodore Fujita will be aired Tuesday night on PBS as part of the "American Experience" series. For an in-depth look at this episode, be sure to check out the special Category 6 review contributed by Sean Potter, long-time author of the "Retrospect" column in Weatherwise.

Dr. Jeff Masters contributed to this post.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

Bob Henson is a meteorologist and writer at weather.com, where he co-produces the Category 6 news site at Weather Underground. He spent many years at the National Center for Atmospheric Research and is the author of “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”
 

emailbob.henson@weather.com

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