Kerr and Webb briefly shared the lead early in the round, but Sorenstam moved in front for good with a birdies at the 11th and 12th. She added birdies at the 15th and 16th.
"I think you probably don't want to be too much more than three behind going into Sunday," said Webb. "It's just going to depend on the weather and the wind."
Defending champion Meg Mallon, who faltered late in the opening round, rebounded with a 66 and was six shots off the lead.