"Trump declared that an emergency exists in the State of Louisiana and ordered Federal assistance to supplement State, tribal, and local response efforts", the White House said in a statement.
Louisiana Gov John Bel Edwards wrote to President Trump on Sunday requesting that he declare an Emergency Disaster for the state.
The nation's fourth-largest city remained largely paralyzed by one of the largest downpours in USA history and there was no relief in sight from the storm that spun into Texas as a Category 4 hurricane, then parked itself over the Gulf Coast.
Edwards said 40 people from Louisiana's Department of Wildlife are in Texas are helping with rescue efforts, and added that Louisiana has not received a specific request from Texas or Houston to take in evacuees.
"Harvey's circulation is crawling eastward and the center of the storm is forecast to track through the state of Louisiana later this week", said Fox News Meteorologist Janice Dean.
Edwards was expected to visit Lake Charles Monday afternoon as Tropical Storm Harvey brings heavy rain to the area.
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Of the 126 people who died in flash and river floods in the United States in 2016, 46 percent were killed in a auto, likely as they tried to cross an inundated road, according to National Weather Service data.
The Federal Emergency Management Agency has been in the state for the past week leading up to Harvey.
FEMA Administrator Brock Long said Monday that more than 450,000 people were expected to seek disaster assistance due to flooding from Harvey.