Hourly Forecast Detailed. 1  0am:The Westerville, OH forecast for Aug 16 is 74 degrees and Partly cloudy. 2  3am:The Westerville, OH forecast for Aug 16 is 68 degrees and Mist. 3  6am:The Westerville, OH forecast for Aug 16 is 68 degrees and Partly cloudy. 4  9am:The Westerville, OH forecast for Aug 16 is 75 degrees and Sunny. There is 88 percentage chance of rain and 4 mph winds from the East. 12am:The Westerville, OH forecast for Aug 16 is 84 degrees and Patchy rain possible. There is 72 percentage chance of rain and 5 mph winds from the Southeast.