Severe Cyclonic Storm Onil was first identified as an area of convection early on September 30, 2004 situated roughly 465 km (290 mi) southwest of Mumbai, India.

Situated over warm water and within an area of moderate wind shear,[1] the Joint Typhoon Warning Center (JTWC) assessed the system's chances of developing into a tropical cyclone as "fair".

[3] Despite a decrease in convection later on September 30,[1] the IMD upgraded the cyclone to a deep depression, stating that three-minute sustained winds had reached 55 km/h (35 mph).

[1] Roughly 24 hours after being named, the system attained its peak intensity as a severe cyclonic storm with winds of 100 km/h (60 mph) and a barometric pressure of 990 mbar (hPa; 29.23 inHg) according to the IMD with.

[2] As the storm neared the Indian coastline, dry air quickly entered the circulation, causing most of the convection associated with Onil to dissipate.

By October 7, the system was reclassified as a tropical depression by the JTWC as it stalled several hundred kilometres southwest of Gujarat, India.

[1] In Hyderabad, heavy rains amounting to 98 mm (3.9 in) caused a 10 ft (3.0 m) section of a drainage system to collapse, prompting the evacuation of several hundred residents.

[8][9][10] Throughout Sindh, an estimated 607 square kilometres (234 sq mi) of land was inundated by flood waters, destroying roughly 70% of the cotton crop.

[15] On October 3, Imtiaz Ahmed Shaikh expressed his grief over the damage caused by Cyclonic Storm Onil and stated that he would be touring the affected region within the following days.

All officers in the Hyderabad Development Authority who were initially put on leave ahead of the cyclone were told to resume work to assist in clearing drainage systems.

[19] Several days after the storm, reports indicated that there were more than 300 instances where the city's drainage system collapsed across the area, resulting in standing water in many structures.

In response to these protests, Shaukat Hayat Bhutto suspended Assistant Engineer Sewage manager, Qamar Memon, for his negligence on draining flood waters.