Amphan Weakens To Category 4, But Indian/Bangladeshi Authorities Expecting 13-16-Foot Storm Surge

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.)


Surge forecast for Amphan issued by India’s National Centre for Ocean Information Services (INCOIS) at 0030Z (6 am IST) Tuesday, May 19, 2020. The largest surge is expected just south and east of Kolkata, India. (INCOIS)

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.

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https://www.wunderground.com/cat6/storm-surge-threat-remains-extreme-for-eastern-india-bangladesh-as-amphan-approaches?cm_ven=hp-slot-1