Cyclone Fani Threatens to Generate Extensive Storm Surge in India

Tropical Cyclone Fani has increased in strength on Wed May 1, as it tracked northward in the Bay of Bengal, east of India. As of Wednesday evening, local time, the cyclone packed maximum sustained winds of 105 kts (121 mph, 194 kph), according to the Joint Typhoon Warning Center (JTWC).

 

Water vapor satellite imagery of Cyclone Fani on Wed May 1, 2019. 

If this cyclone were in the Atlantic Ocean it would be classified as a category-3 hurricane on the Saffir-Simpson Hurricane Wind Scale.

The cyclone is forecast to remain powerful as it approaches the northeast Indian coast, although it may weaken slightly before landfall. While Fani is forecast to make landfall in Orissa, severe impacts will be felt throughout Orissa and West Bengal.

Cyclone Fani's forecast track from the Joint Typhoon Warning Center

Fani’s alongshore track will enable the cyclone to generate dangerous storm surge levels along a widespread area of coastline. This occurs when cyclones move along a coastline as the center of the storm moves towards the direction of onshore winds. Another way to visualize such cyclones is by picturing them turning to the "right" in the Northern Hemisphere and the "left" in the Southern Hemisphere.

Hurricane Flossy took an alongshore track near the U.S. Gulf Coast in 1956, generating a large storm surge well to the "right" of the landfall location. However, locations much closer to the hurricane's center, but on the "left" side of the storm observed much lower storm surge levels.

For example, Flossy's eye tracked just south of Pensacola, Florida, but the maximum storm tide there only reached 2.85 feet (0.87 m). Yet, in Cedar Key, Florida, more than 400 miles from the landfall location, storm tide levels exceeded 7 feet (2.13 m). 

Hurricane Flossy (1956) generated an extensive storm surge to the "right" of the storm track. Flossy's alongshore track as well as the shallow waters of Florida's coast, provided this setup.

Marked-up version of the same graphic.

A big exception to this rule was found over the Louisiana Delta, where onshore winds "wrapped around" Flossy and pushed storm surge levels exceeding 12 feet (3.66 m) high.

The impacts of Fani's storm surge may be substantial, as saltwater flooding and "compound flooding" from both storm surge and heavy rainfall should impact densely populated areas, like Calcutta. Models also suggest that flooding may be pronounced along the densely populated Hooghly River.

Fani may produce extensive flooding along the highly-populated Hooghly River.

Although Fani will make landfall in India, coastal flood impacts could be felt as far east as Bangladesh. The coastline of northeast India and Bangladesh are extremely susceptible to storm surge flooding, and this area of the world has observed the world's highest storm surge levels.

A tropical storm in 1981 made landfall near the India/ Bangladesh border with sustained winds of 60 mph (100 kph) and generated a storm surge of 2.2 m (7.2 ft) (Dube et al. 1997). Fani should be more intense than the 1981 cyclone and has the potential to generate more extensive flooding.

REFERENCES

Dube, S.K., A. D. Rao, P.C. Sinha, T.S. Murty, and N. Bahulayan, 1997: Storm surge in the Bay of Bengal and Arabian Sea : The problem and its prediction. Mausam, 48, 283-304.