By Jon Herskovitz
AUSTIN, Texas (Reuters) - Large parts of Texas were under a flash flood warning on Monday after a storm system dumped as much as 20 inches (51 cm)of rain in areas near Houston, causing hundreds of canceled flights, closed roads and massive power outages.
The National Weather Service also issued a flash flood watch extending through much of central Texas, into the Houston area as well as western Louisiana and Arkansas.
In Houston, more than 70 residential subdivisions reported flooding and Mayor Sylvester Turner canceled his State of the City speech planned for Monday, instructing all non-essential city employees to stay home.
"If you do not have to be out, please stay put. This not the day to be on the roads in the city of Houston," Turner told reporters, adding seven area rivers have exceeded their banks.
The city has turned one shopping mall into an evacuation center.
The city said on its Twitter feed there have already been 115 high water rescues as of Monday morning.
There were more than 100,000 customers without power in the Houston area on Monday morning due to the severe weather, CenterPoint Energy reported.
As of 9:30 a.m. CDT (1430 GMT), more than 650 flights at major airports in Texas were canceled, according to tracking service FlightAware.com.
The rains were expected to cause rivers to crest later in the week, bringing floods to downstream areas, the weather service said.
Some of the hardest hit areas have been in western Harris County, which includes Houston.
In the San Antonio and Austin areas, torrential rains have closed streets, caused traffic accidents and led to dozens of people being rescued from cars stuck in high water.
In Bastrop County, east of Austin, 20 families had to evacuated from their homes due to flooding, official said.
(Reporting by Jon Herskovitz; Additional reporting by Jim Forsyth in San Antonio; Editing by Jeffrey Benkoe and Marguerita Choy)