Gov. Greg Abbott is warning of the "potential for very dramatic flooding" from Harvey.
He's expanded a state declaration of emergency from the state's original 30 counties to 50.
Abbott says it's too early to speculate as to how much property damage the storm has caused.
Hurricane Harvey Preps on Friday
Texas Gov. Greg Abbott is warning people that Hurricane Harvey "is going to be a very major disaster."
Abbott said Friday that he's asked President Donald Trump for a federal disaster declaration.
Harvey made landfall in Texas at about 10 p.m. Friday as a Category 4 storm.
Abbott has activated about 700 members of the Texas National Guard in preparation for Harvey. The storm is set to be the first hurricane to make landfall on the Texas coast since Hurricane Ike in 2008.
Aside from savage winds and storm surges, the system was expected to drop prodigious amounts of rain. The resulting flooding, one expert said, could be "the depths of which we've never seen."
President Donald Trump says he's keeping a close watch on Hurricane Harvey.
On Twitter Friday, Trump said he "Received a #HurricaneHarvey briefing this morning" from top federal officials.
In another statement on Twitter, Trump said he had spoken with Texas Gov. Greg Abbott and Louisiana Gov. John Bel Edwards. He added: "Closely monitoring #HurricaneHarvey developments & here to assist as needed."
Trump tweeted Thursday, encouraging people to be prepared.
The mayor of the popular Texas Gulf Coast city of Galveston says the city is expected to be inundated with water from Hurricane Harvey.
He says a leading concern is that high tide will occur Saturday morning, shortly after Harvey makes landfall, and the water is not expected to recede for three or four days.
Forecasters now say there's a good chance Hurricane Harvey may hit Texas twice, worsening projected flooding.
The National Hurricane Center's official five-day forecast Friday has Harvey slamming the central Texas coast, stalling and letting loose with lots of rain.
Then forecasters project the weakened but still tropical storm is likely to go back into the Gulf of Mexico, gain some strength and hit Houston next week.