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 31 °C (88 °F)), 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 65 km/h (40 mph).
In light of this data, the JTWC classified the system as Tropical Cyclone 01B while it was situated roughly 1,950 km (1,210 mi) south of Chittagong, Bangladesh.
At the time, the agency anticipated significant strengthening of the cyclone, forecasting it to attain winds in excess of 155 km/h (100 mph).
The India Meteorological Department (IMD) followed suit hours later, designating the system as Depression BOB 01 and soon upgrading it to a deep depression.

Situated to the south of a subtropical ridge, the storm tracked west-northwestward to northwestward.
Early on May 11, the IMD upgraded the system to a cyclonic storm and assigned it the name Mahasen.
Deep convection, with cloud tops estimated as cold as −85 °C (−121 °F) developed near the storm's center.
Despite moderate to strong wind shear, Viyaru's prominent outflow offset the negative influence of the shear, allowing a central dense overcast to form.
Later that day, the cyclone's motion matched that of the shear, further decreasing the effects of it.
This allowed Viyaru to intensify, with the JTWC estimating one-minute sustained winds reaching 95 km/h (60 mph).
By May 12, the central dense overcast broke apart into fragmented banding features that wrapped into the center.
Dry air soon began to flow into the circulation, disrupting convection and causing the low to relocate eastward and become partially exposed.
Later that day, the system began turning northward as it approached the western edge of the subtropical ridge.
By this time, the circulation had become broad and ill-defined, indicating that the storm weakened.

On May 13, steering currents weakened around the cyclone due to a shortwave trough over India, causing Viyaru to slowly move northwestward.
Wind shear also increased once again as the outflow degraded.
By May 14, the exposed and elongated circulation of Viyaru turned northeastward as the ridge became more pronounced.
The majority of convection remained sheared to the west, though upper-level conditions were anticipated to become somewhat favorable for restrengthening.
As the system approached Bangladesh, a large area convection and rebuilt over the center.
Slight strengthening took place on May 15, with the IMD reporting winds reaching 85 km/h (50 mph).
Viyaru also began to accelerate somewhat as a trough became established to the west.
Early on May 16, the system attained its peak intensity with a barometric pressure estimated at 990 mbar (hPa; 29.23 inHg).
Around 0800 UTC (1330 IST), Viyaru made landfall in Bangladesh between Feni and Chittagong.
Within hours of moving ashore, rapid weakening ensued as the circulation deteriorated and convection became shallow.
In light of this, the JTWC issued their final advisory on the cyclone.
The IMD downgraded Viyaru to a deep depression shortly after as it moved over Mizoram, India.
The system further degraded as it moved over mountainous terrain and was last noted as a well-marked area of low pressure over Nagaland on May 17.
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 31 °C (88 °F)), 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 65 km/h (40 mph). In light of this data, the JTWC classified the system as Tropical Cyclone 01B while it was situated roughly 1,950 km (1,210 mi) south of Chittagong, Bangladesh. At the time, the agency anticipated significant strengthening of the cyclone, forecasting it to attain winds in excess of 155 km/h (100 mph). The India Meteorological Department (IMD) followed suit hours later, designating the system as Depression BOB 01 and soon upgrading it to a deep depression.
Situated to the south of a subtropical ridge, the storm tracked west-northwestward to northwestward. Early on May 11, the IMD upgraded the system to a cyclonic storm and assigned it the name Mahasen. Deep convection, with cloud tops estimated as cold as −85 °C (−121 °F) developed near the storm's center. Despite moderate to strong wind shear, Viyaru's prominent outflow offset the negative influence of the shear, allowing a central dense overcast to form. Later that day, the cyclone's motion matched that of the shear, further decreasing the effects of it. This allowed Viyaru to intensify, with the JTWC estimating one-minute sustained winds reaching 95 km/h (60 mph). By May 12, the central dense overcast broke apart into fragmented banding features that wrapped into the center. Dry air soon began to flow into the circulation, disrupting convection and causing the low to relocate eastward and become partially exposed. Later that day, the system began turning northward as it approached the western edge of the subtropical ridge. By this time, the circulation had become broad and ill-defined, indicating that the storm weakened.
On May 13, steering currents weakened around the cyclone due to a shortwave trough over India, causing Viyaru to slowly move northwestward. Wind shear also increased once again as the outflow degraded. By May 14, the exposed and elongated circulation of Viyaru turned northeastward as the ridge became more pronounced. The majority of convection remained sheared to the west, though upper-level conditions were anticipated to become somewhat favorable for restrengthening. As the system approached Bangladesh, a large area convection and rebuilt over the center. Slight strengthening took place on May 15, with the IMD reporting winds reaching 85 km/h (50 mph). Viyaru also began to accelerate somewhat as a trough became established to the west. Early on May 16, the system attained its peak intensity with a barometric pressure estimated at 990 mbar (hPa; 29.23 inHg). Around 0800 UTC (1330 IST), Viyaru made landfall in Bangladesh between Feni and Chittagong. Within hours of moving ashore, rapid weakening ensued as the circulation deteriorated and convection became shallow. In light of this, the JTWC issued their final advisory on the cyclone. The IMD downgraded Viyaru to a deep depression shortly after as it moved over Mizoram, India. The system further degraded as it moved over mountainous terrain and was last noted as a well-marked area of low pressure over Nagaland on May 17.
Cyclone Viyaru