Mahasen formed as a tropical storm over the northern Indian Ocean on May 10, 2013. On May 13, the U.S. Navy’s Joint Typhoon Warning Center (JTWC) reported that Mahasen was moving northward over the Bay of Bengal, headed in the direction of Bangladesh and northeastern India. The storm had maximum sustained winds of 50 knots (95 kilometers per hour) and gusts up to 65 knots (120 kilometers per hour). Within 72 hours, the JTWC reported, sustained wind speeds were expected to increase to 70 knots (130 kilometers per hour).
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this natural-color image on May 13, 2013. Mahasen lacked a distinct eye, but still sported a rough apostrophe shape typical of strong storms. Clouds extended along the coast of India and over Sri Lanka.
The JTWC reported that the storm was located roughly 630 nautical miles (1,170 kilometers) south-southwest of Kolkata (Calcutta). The JTWC forecast map showed Mahasen moving northward over the Bay of Bengal before turning slightly toward the northeast, making landfall around May 16.