AUSTIN, Texas (AP) — Harvey has so far killed at least 70 people who drowned in floods, got crushed by trees and died during power outages — a surprisingly low toll that experts say reflects heeded warnings, swift action by first responders and volunteers, and no small amount of luck.

"It was astounding that we didn't have a much larger loss of life," said Phil Bedient, co-director of a Rice University effort to research severe storms and evacuations. "It is a relatively low number for as big a storm as this was."

The system intensified from an ordinary storm to a Category 4 hurricane in just...