Tropical Cyclone Amphan made landfall south of Kolkata, India, on Wednesday as a Category 2 storm, with maximum sustained winds of 100 mph.

While it had weakened considerably from its once fearsome Category 5 intensity, Cyclone Amphan still directed a significant storm surge to the coastline of one of the most vulnerable regions on Earth to coastal flooding.

In addition, the storm is dumping heavy rains and damaging winds on Kolkata, a city whose metropolitan area is home to 15 million people.

Nearly 3 million people in India and Bangladesh evacuated their homes ahead of the storm, going to emergency shelters that presented their own risks as coronavirus infections rise.

The region where the storm came ashore, located at the top of the Bay of Bengal, where India and Bangladesh meet, has a history of deadly storms. A cyclone in 1999 killed 10,000 people in India, though in recent years both countries have made strides in evacuating vulnerable populations and putting storm shelters to use.

The shape of the land in this region, as well as bathymetry of the ocean floor extending off the coast, serve to heighten storm surge flooding from tropical cyclones by funneling water into a smaller area. The Indian Meteorological Department forecast a storm surge of up to 16 feet above astronomical tide levels from Amphan. The storm surge is the storm-driven rise in water above normally dry land.

The full extent of the storm surge-related damage will not become clear for a few days, but it likely pushed high water into both West Bengal and portions of low-lying Bangladesh, which is located on the right-hand side of the storm. In this region, onshore winds are maximized because of the storm’s forward speed and counterclockwise circulation.

Because Cyclone Amphan was once a “super cyclonic storm” at the pinnacle of storm intensity, it has pushed water toward shore that is more consistent with a much stronger storm. Water is a tropical cyclone’s greatest killer, both from storm surge and inland flooding due to heavy rainfall.

Like many powerful storms in recent years, Tropical Cyclone Amphan underwent a period of rapid intensification Sunday night, feasting upon the warm waters of the Bay of Bengal and the absence of wind shear that could have interfered with its circulation, as well as abundant moisture surrounding the storm.

Based on data from the typhoon warning center, Cyclone Amphan intensified by 110 mph in just 36 hours. “That would certainly put it in a rare class of [rapid intensification] events globally,” said meteorologist Alex Lamers via Twitter. It also jumped in intensity from a Category 1 storm to a Category 5 monster in just 24 hours, according to Bob Henson of Weather Underground.

Rapidly intensifying storms constitute one of the impacts scientists have seen from human-caused global warming, with studies pointing increasingly to the dangers posed by storms that make a sudden leap in intensity categories.

Globally, the odds of the strongest tropical cyclones, such as Category 3, 4 and 5 storms, are increasing because of climate change, according to a study published Monday. That study did not find clear trends in the North Indian Ocean Basin.

Joanna Slater and Niha Masih contributed reporting from New Delhi.