Cyclonic Storm Viyaru, operationally known as Cyclonic Storm Mahasen, was a relatively weak tropical cyclone that caused loss of life across six countries in Southern and Southeastern Asia. Originating from an area of low pressure over the southern Bay of Bengal in early May 2013, Viyaru slowly consolidated into a depression on May 10. The depression gained forward momentum and attained gale-force winds on May 11 and was designated as Cyclonic Storm Viyaru, the first named storm of the season. Owing to adverse atmospheric conditions, the depression struggled to maintain organized convection as it moved closer to eastern India. On May 14, the exposed circulation of Viyaru turned northeastward. The following day, conditions again allowed for the storm to intensify. Early on May 16, the cyclone attained its peak intensity with winds of and a barometric pressure of 990 mbar (hPa; ). Shortly thereafter Viyaru made landfall near Chittagong, Bangladesh. On May 17, it moved over the eastern Indian state of Nagaland.
Early in the storm's existence, it brought flooding rains to much of northwestern Indonesia, resulting in significant damage. At least four people died and six others were reported missing. Offshore, a further 11 people went missing and were feared dead. In preparation for the storm, large-scale evacuations were recommended for parts of Myanmar. This resulted with people overcrowding boats to escape, and one or several vessels capsized, causing at least 39 deaths; 42 people were rescued while 19 others were unaccounted for and feared dead. The storm's expansive cloud mass also brought unsettled weather to Sri Lanka, Thailand, and southeastern India. Severe storms in India and Sri Lanka were responsible for at least 16 fatalities and significant damage; one person died in Thailand. Striking Bangladesh in a weaker state than initially expected, the storm caused moderate to severe damage. A total of 95,003 poorly constructed huts were damaged or destroyed, 17 people died, and nearly 1.3 million were affected across the country. Losses to industry reached ৳20 billion (US$257 million). Myanmar was spared damage and further casualties.
Operationally, the storm was referred to as Mahasen; however, this stirred some controversy from nationalists and officials in Sri Lanka. They claimed that the name comes from King Mahasena of Anuradhapura who brought prosperity to the island, thus naming a destructive force of nature after him would be improper. As such, Sri Lankan agencies referred to the system as a nameless cyclone and requested that international agencies do the same. The name had been submitted to be used in the basin by Sri Lanka in 2003. The India Meteorological Department later renamed the system as Viyaru in their final reports and removed Mahasen from all previous archived advisories.
Meteorological history
In early May 2013, an area of disturbed weather formed over the southern Bay of Bengal. Remaining nearly stationary, the system gradually developed. By May 8, organized convection formed around a defined low-pressure area, with banding features present. With conditions favoring intensification, low wind shear, excellent poleward outflow, and unusually high sea surface temperatures (estimated at ), the system was anticipated to become a tropical cyclone over the following days. A pulse in the Madden–Julian oscillation, coupled with a convective Kelvin wave allowed the system, along with its Southern Hemisphere counterpart Tropical Storm Jamala, to further develop. Following additional organization, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert for the low on May 10. Despite an increase in wind shear, causing the low to become dislocated from the deepest convection, the system further intensified and a scatterometer pass from the Oceansat-2 satellite indicated winds up to . In light of this data, the JTWC classified the system as Tropical Cyclone 01B while it was situated roughly south of Chittagong, Bangladesh. At the time, the agency anticipated significant strengthening of the cyclone, forecasting it to attain winds in excess of . The India Meteorological Department (IMD) followed suit hours later, designating the system as Depression BOB 01 and soon upgrading it to a deep depression.