Above: Visible satellite image of Invest 91B over the Bay of Bengal taken at 0309Z Friday, May 15, 2020 (11:09 pm EDT Thursday). (RAMMB/CIRA/CSU)
An area of disturbed weather over the southern Bay of Bengal (Invest 91B) is steadily growing more organized and is close to tropical depression status. It has the potential to become a dangerous hurricane-strength cyclone by early next week.
Conditions are very favorable for development for 91B. Wind shear on Friday was moderate, 10-15 knots. Ocean temperatures were exceptionally warm over the Bay of Bengal: 30-31°C (86-88°F), which is over 1°C (1.8°F) above average. Warm waters extended to great depth in the Bay of Bengal, with a Tropical Cyclone Heat Potential (TCHP) over 100 kilojoules per square centimeter over much of the region. This is a tremendous amount of heat energy, and values of TCHP this high are frequently associated with rapid intensification of tropical cyclones.
The Madden-Julian Oscillation (MJO) is also in a phase that will promote rising air and increased chances of tropical cyclone formation over the Bay of Bengal, though the MJO is fairly weak.
Satellite loops on Friday morning showed that 91B had plenty of spin, plus a large area of heavy thunderstorms that was growing steadily more organized. However, a well-defined surface circulation had not yet formed. The system was embedded in a moist atmosphere, though there was some dry air to the west along the east coast of India that could potentially interfere with development. The disturbance formed along the leading edge of the advancing southwest monsoon, and will help pull the monsoon into the southern Indian state of Kerala by June 1—the typical start date of the four-month-long monsoon season—according to the India Meteorological Department (IMD). They are predicting a near-average monsoon season in 2020.
Forecast for 91B
The computer models have come into better agreement on 91B’s future track, though considerable uncertainty in the track can be expected until the storm actually forms a well-defined center of circulation and becomes a tropical depression. Two of our top models for predicting tropical cyclone genesis—the GFS and European models—have been consistently predicting for multiple days that 91B will develop into a tropical storm by Saturday. The 0Z and 6Z Friday runs of these models, along with the 0Z and 6Z runs of the HWRF model, all predicted that 91B would hit Bangladesh between 03Z and 22Z Wednesday. In their 2 am EDT Friday forecast, IMD predicted that 91B would consolidate into a tropical depression by Friday night, and become a named storm by 11 am EDT Saturday. The next name on the list of storms for the North Indian Ocean is Amphan.
Some of the most devastating tropical cyclones in world history have occurred during the May pre-monsoon tropical cyclone season in the Bay of Bengal, and residents of India, Bangladesh, and Myanmar should monitor any potential development of 91B. The 6Z Friday runs of the GFS and HWRF models predicted that 91B would develop into a category 3 or 4 storm capable of driving a devastating storm surge to the coast, and the European model also suggested that 91B could become a major category 3 or stronger storm prior to landfall.
Although Bangladesh currently appears likely to receive the brunt of 91B’s impacts, we cannot yet rule out a landfall in neighboring regions of India or Myanmar.
Tropical cyclone history of the Bay of Bengal
The triangular shape of the Bay of Bengal acts to funnel storm surge waters into Bangladesh, and the very shallow bottom of the bay allows the surge to pile up to very high heights. Thus, there is good reason to be concerned when a hurricane-strength tropical cyclone gets loose in the Bay of Bengal: Twenty-six of the 35 deadliest tropical cyclones in world history have been Bay of Bengal storms, as seen in wunderground's list of the 35 Deadliest Tropical Cyclones in World History (note that since this list was published, research has found that the 1882 Great Bombay Cyclone, which supposedly killed 100,000 people, in reality never occurred). The big killer in all of the most deadly Bay of Bengal cyclones was the storm surge,
During the past two centuries, 42 percent of Earth's tropical cyclone-associated deaths have occurred in Bangladesh, and 27 percent have occurred in India (Nicholls et al., 1995). The deadliest storm in world history, the 1970 Bhola Cyclone of 1970, killed an estimated 300,000 – 500,000 when it made landfall in Bangladesh on November 12, bringing a storm surge estimated at up to 10.4 meters (34 feet) to the coast.