Above: VIIRS image of Invest 91B over the Bay of Bengal taken on Thursday afternoon, May 14, 2020. Credit: NASA Worldview.
An area of disturbed weather over the southern Bay of Bengal (Invest 91B) is steadily growing more organized, and it has the potential to become a dangerous hurricane-strength cyclone by early next week.
Conditions are very favorable for development. Wind shear on Thursday 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 Thursday afternoon showed that 91B had plenty of spin, plus a large area of heavy thunderstorms that was growing steadily more organized. 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 India Meteorological Department (IMD). They are predicting a near-average monsoon season in 2020.
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. In their 2 am EDT Thursday forecast, IMD predicted that 91B would consolidate into a tropical depression on Friday, and become a named storm by 8 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 12Z Thursday runs of the GFS and European models predicted that 91B would develop into hurricane-strength Tropical Cyclone Amphan in the Bay of Bengal early next week, and make landfall in the northern Bay of Bengal on Wednesday.