Harvey made landfall as a Category 4 storm late Friday
between Port Aransas and Port O'Connor, Texas, about 150 miles southwest of Houston, the nation's fourth largest city with a metropolitan-area population of 5 million people.
A flash-flood watch was in effect Saturday for the Houston area, according to the National Weather Service. More than a foot of rain could fall in the city over the next five days, with higher tallies possible in localized areas, CNN meteorologists said.
By 10 a.m. ET, some parts of Houston already had gotten more than 3 inches of rain over the prior 24 hours, the weather service reported.
And more heavy rain was expected, with Harvey due to stall for days over southeast Texas. The system could maintain tropical storm strength through early Wednesday, then weaken into a tropical depression, the weather service predicted
"This is going to be a major rainmaker," Houston Mayor Sylvester Turner told CNN Saturday morning. "This is just day one. We anticipate four to five days of this, so we're asking people to stay off the roads if they don't need to be on the roads."
"Our high-water teams are ready and strategically placed," he said. "The first responders are out and prepared to go."
Sylvester has asked Houston residents to "exercise common sense," and stay off the roads this weekend.
Houston public schools have canceled classes on Monday, which was scheduled to be the first day of the academic year. The city's parks and main zoo also are closed. The Houston Dynamo, the city's Major League Soccer team, canceled its Saturday game, and Coldplay
canceled a concert Friday night.
Voluntary evacuation orders were in place Saturday for several communities around Trinity Bay in Harris County, which is also home to Houston, county officials said.
Flat land means flood risk
Houston's main concern from Harvey remained the flood risk, with one estimate setting the potential for damage at $20 billion in metropolitan Houston alone
, analytics firm CoreLogic said.
Strong storms last spring
claimed at least eight lives and caused at least $5 billion in property damage
in Harris County. Damaging flooding from heavy rain also socked Houston in the fall of 2015
Houston floods easily because of how flat it is, CNN meteorologist Jennifer Varian said.
"They start flooding between 2 and 3 inches" of rain over a short time, Varian said, adding that the amount of rain predicted in Houston from Harvey is "going to be far exceeding that."
Many have compared Harvey with Tropical Storm Allison, which in 2001 killed 23 people and caused billions of dollars in damage in Texas, according to the National Hurricane Center
. Allison stalled over land and dumped more than 30 inches of rain on Houston, causing catastrophic flooding.
Compared with Harvey, Varian said, "Allison was a measly little storm."
CORRECTION: An earlier version of this story stated that deadly flooding last hit Houston in May. It was April 2016.