The blast was so powerful it apparently blew the woman's clothes off her body. A char mark was visible on the trunk of the tree.
Meanwhile, another woman was killed in the Big Bear area of the San Bernardino Mountains when a lightning bolt struck a tree, which then toppled onto her truck as she was driving underneath.
The vehicle was a total loss, as the tree crashed through the roof of the truck.
Neither woman has been identified.
Lightning blasts also started six fires in the San Bernardino Mountains and Riverside County. While none of the fires has so far turned into anything major, firefighters have been battling them as fast as they are reported.
One fire burning in the foothills above San Dimas burned about 11 acres and was being fought mostly by aircraft.
The aircraft were delayed for a while as they waited for lightning activity in the region to ease.