Widespread Surge Threat as Fani Moves toward Northeast India

Still a Category 3 storm as of Wednesday, Tropical Cyclone Fani is hurtling toward landfall in northeast India on a track that could spread a significant amount of storm surge as far as Bangladesh, well east of Fani’s center. Update (10:40 AM EDT Thursday): Fani has intensified to top-end Category 4 strength as of Thursday morning, with top sustained winds of 155 mph according to the Joint Typhoon Warning Center. Millions are being evacuated in the Indian state of Odisha, where landall is expected. Impacts of a storm this strong in the upper Bay of Bengal could be catastrophic. We will have a full update by midday Thursday.

As of 12Z (8 am EDT) Wednesday, Fani was packing top sustained winds of 120 mph, according to the Joint Typhoon Warning Center (JWTC). The cyclone was located about 600 miles south-southwest of Kolkata, moving just east of due north at about 8 mph. Fani’s shower and thunderstorm activity (convection) had weakened considerably as a result of strong wind shear, so the top sustained winds may drop by Wednesday night and Thursday, although the official JTWC forecast was calling for slight strengthening. Fani’s wind field is likely to expand over time even if the top winds decrease.

Wow, the impact of mid-level shear on Cyclone #Fani's inner core really turned out spectacularly. Episodes of nearly full eyewall disruption may occur periodically thru Friday.

Fani remains a significant danger to India, regardless of its exact intensity by the time of landfall pic.twitter.com/wQqMx1ipqj

— Levi Cowan (@TropicalTidbits) May 1, 2019

Between now and Saturday, Fani will accelerate north-northeast on a fairly straightforward path, driven by the approach of a weak upper-level trough. Complexities around landfall are mainly because of the angle of the Indian coast. Fani will be moving largely parallel to the coast, which will bring it close to land as much as 12 hours before the center actually makes landfall. The setup is familiar to residents of the U.S. East Coast, where tropical storms and hurricanes often nudge ashore rather than slamming headlong into the coast. Fani’s angle of approach will limit the reach of its strongest winds, but it also means a large swath of northeast India will experience torrential rains.

In its 12Z Wednesday advisory, JWTC predicted that Fani would make landfall east of Puri around 6Z Friday (2 am EDT or 11:30 am India time) as a Category 2 storm. The winds associated with the stronger right-hand side of Fani will likely rake parts of the Jagahtsingpur, Kendrapara, and Bhadrak coastal districts of India’s Odisha state on Friday afternoon and evening local time, with sustained winds topping hurricane force in some areas.

By Friday night, Fani is expected to pass just west of Kolkata as a weakening Category 1 storm or high-end tropical storm. This trajectory could push sustained tropical-storm force winds across the metropolitan area of more than 14 million people.  The Kolkata office of the India Meteorological Department warned of a wide array of potential wind-related impacts in coastal and near-coastal parts of West Bengal state, including the Kolkata area:

—Total destruction of thatched houses
—Extenstive damage to kutcha houses (huts made of mud, wood, straw, and dry leaves)
—Bending and uprooting of power and communication poles
—Major damage to roads and flooding of escape routes
—Widespread damage to standing crops and plantations

See weather.com for frequent updates on preparation for Fani. In addition, the India desk of weather.com has a continually updated page with details on Fani from an Indian perspective.

The storm-surge threat

The Bay of Bengal is one of the world’s most notoriously dangerous locations for storm surge. The India Meteorological Department warned Wednesday that storm surge could reach 1.5 meters (4.9 feet) across the Ganjam, Khurda, Puri, and Jagatsinghpur districts of India’s Odissa state. Model-based surge guidance issued at 0Z Wednesday shows the potential for surge further to the northeast as far as western Bangladesh, perhaps exceeding 2 meters (6.6 feet) in places (see Figure 1).

Figure 1. Potential storm surge, in meters, as predicted by modeling from the Indian National Center for Ocean Information Services as of 0Z Wednesday, May 1, 2019. The amounts shown here are on top of astronomical tides, so the total inundation level (storm tide) could be higher or lower than shown here depending on Fani’s timing. Image credit: India Meteorological Department.

The megacity of Kolkata sits almost 100 miles inland from the coast, on the east side of the Hooghly River. The coastal plain from Kolkata to the bay is lined with tidal creeks and riverine channels that allow the impacts of a major surge to reach well inland.

The latest storm surge forecast from IMD is concerning. It shows a low-lying coastline with a population in the millions potentially receiving a storm surge in excess of 2 meters in some locations, including parts of the Hooghly River system. Such a surge would displace a lot of people and cause significant damage, and it could prove deadly if a successful evacuation effort does not occur. Heavy rains of 4 - 8" will cause flooding of the rivers that drain into the Bay of Bengal, resulting in compound freshwater and storm surge flooding that will significantly disrupt life in the most vulnerable parts of West Bengal state. Much of this rain will precede the arrival of the storm surge, exacerbating the potential for compound flooding.

Further to the east, the surge should be considerably less in Dhaka, Bangladesh (another megacity, with a metro area of 19 million), and heavy rains are less likely to precede the surge there.

In a blog post on Wednesday, storm surge expert Hal Needham pointed to a rough U.S. analog for the far-flung surge potential of Fani:

Kolkata's flooding crisis

With its low-lying topography and massive population growth, Kolkata is considered one of the most flood-prone major cities on Earth. A 2019 paper in Applied Water Science led by Ravinder Dhiman (Indian Institute of Technology) notes:

The [Kolkata] region is prone to flooding primarily due to the flat terrain and the drainage outfalls which are tide-dominated. Flooding occurs during excess rainfall on the catchment as well as during high-tidal conditions. The drainage sewer system in the city was designed during the British era and is not capable of handling the current run-off scenario. The sewer system also suffers from siltation and a consequent reduction in conveyance capacities irrespective of the efforts by the Kolkata Municipal Corporation (KMC) in renovating the drainage pipelines (Sen 2013). The population increase...rapid urbanization and the corresponding land-use changes have substantially reduced the flood-resilient efficacy of the region in recent years.

In September 2018, Kolkata became the first of several Indian cities to launch a comprehensive city-level flood forecasting and early warning system, supported by the Asian Development Bank. Real-time data from 400 sensors around the urban area will be sent to a cloud-based dashboard, allowing for quick alerts to residents through radio, TV, and smartphones. Kolkata’s heaviest rains typically occur during the summer monsoon season, so Fani may pose the first big test of the new system.

Figure 2. Natural-color satellite image of the tropical wave in the Bahamas as of 1845Z (2:45 pm EDT) Wednesday, May 1, 2019. Image credit: www.tropicaltidbits.com.

Slight chance of tropical development east of Florida

In a special outlook issued Wednesday morning, the National Hurricane Center highlighted a cluster of showers and thunderstorms over the northwest Bahamas that will drift northwest toward Florida over the next couple of days. As it recurves northeastward into the open Atlantic late this week, there is a small potential for this wave to organize into a tropical cyclone. NHC gave the system a 20% chance of developing into at least a tropical depression at some point between this coming Friday and Monday, when upper-level westerlies will be pushing it away from Florida.

If the wave manages to become a named storm, it will be the fifth year in a row that the Atlantic season has begun before its official start date of June 1. Jon Erdman has more details in a weather.com feature.

Figure 3. Tracks of the 10 Atlantic Basin named storms that have formed before the official June 1 start of the hurricane season since 2000. (Black tracks denote those parts of the storm's lifetime during which it was either a tropical wave or remnant.)

Is hurricane season getting earlier?

As ocean temperatures warm in our changing climate, it stands to reason that the period of time supportive of Atlantic hurricanes might be growing longer. The Atlantic hurricane season does appear to be lengthening in the region south of 30°N and east of 75°W, with the earliest formation dates getting earlier and the latest dates getting later, according to a 2008 paper by Dr. James Kossin of the University of Wisconsin in Geophysical Research Letters titled, "Is the North Atlantic hurricane season getting longer?" A 2016 analysis by Dr. Ryan Truchelut of WeatherTiger also supported this idea, with the earliest 5 percent of the hurricane season trending significantly earlier. However, Juliana Karloski and Clark Evans of the University of Wisconsin-Milwaukee found no trend in tropical cyclone formation dates when looking at the entire Atlantic, for the period 1979–2014.

Dr. Jeff Masters contributed to this post.