By Jim Forsyth

SAN ANTONIO (Reuters) - Texas Governor Greg Abbott on Monday described the flash flooding that had killed at least three people in his state as "a relentless wall of water that mowed down huge trees like they were grass."

Abbott declared states of disaster in 24 counties and flew over the area south of Austin to assess the damage caused by tornadoes, heavy rainfall, thunderstorms and flooding that forced evacuations and rooftop rescues and left thousands of residents without electrical power.

"This is the biggest flood this area of Texas has ever seen," Abbott said.

"It is absolutely massive - the relentless tsunami-type power of this wave of water," the governor said.

He described homes that were "completely wiped off the map" by the dangerous weather system that struck Texas and Oklahoma.

Widespread severe thunderstorms were forecast to continue on Monday in north-central and northeast Texas and southern Oklahoma, likely bringing destructive winds, tornadoes and hail, the National Weather Service said.

The bodies of a 14-year-old boy and his dog were found in a storm drain on Monday morning in the Dallas suburb of DeSoto, police said. Two other people killed in the storm were described as an unidentified man found dead from the flooding in San Marcos, Texas, and in Oklahoma, a firefighter who was swept into a storm drain.

A possible fourth person killed was reported by the New York Times, which said a Tulsa woman died on Saturday after her automobile hydroplaned on a highway.

Twelve people were listed as missing, including eight from an extended family from Corpus Christi who has been vacationing in a home in Wimberley that was washed off its foundation into the raging Blanco River, according to local officials and the family's church in Corpus Christi. One family member found alive was hospitalized.

Wimberley schools will be closed on Tuesday and a curfew extended for a second night on Monday in Wimberley and San Marcos, Hays County officials said.

Parts of the area have received more than 1-1/2 feet of rain since May 1, six times what he area typically receives in all of May, Accuweather.com said.

The governor's office said the severe weather could continue through the week.

Meteorologists said soil was saturated from heavy rainfall over the past three weeks, leading to the dangerous flash floods.

(Additional reporting by Lisa Garza in Dallas and Ellen Wulfhorst in New York; editing by Barbara Goldberg, Richard Chang, Peter Cooney and Diane Craft)