According to outage tracker PowerOutage.US, there were 503,638 customers out of power in Texas and 93,583 in Louisiana.
The storm, downgraded from a Category 1 hurricane, made landfall early Tuesday along the Texas coastline and was moving slowly toward metropolitan Houston at 7 a.m. CT.
Nicholas was forecast to shift eastward over Louisiana by Wednesday and the National Hurricane Center warned of life-threatening flash flooding across the Deep South over the next few days.
While the tropical storm is predicted to weaken to a tropical depression by Wednesday, its maximum sustained winds were 60 mph with higher gusts on Tuesday and tropical-storm-force winds extended outward up to 125 miles from the storm's center.
"A tornado or two" will also be possible through Tuesday along the upper Texas Coast and southern Louisiana and a storm surge warning, tropical storm warning and storm surge watch were issued for parts of Texas and Louisiana.
A combination of a dangerous storm surge and the tide, impacted by Nicholas, is predicted to cause normally dry coastal zones to be flooded with 3-5 feet from Sargent, Texas, to High Island and Galveston Bay.
Nicholas is also expected to produce additional rainfall of 5-10 inches from the upper Texas coastal area into central and southern Louisiana, far southern Mississippi and far southern Alabama.
Isolated storm totals of 20 inches are likely across central to southern Louisiana.
On Tuesday morning, trees were reported to have toppled over as the 14th named storm of the 2021 Atlantic hurricane season forced Lousiana Gov. John Bel Edwards to declare a state of emergency on Sunday.
Just two weeks ago, Hurricane Ida wreaked havoc on Louisiana and East Coast. While Louisiana has been slowly recovering, power had yet to be restored for many there.
President Biden approved an emergency declaration for the state on Tuesday, freeing up federal emergency aid for response efforts to emergency conditions in areas affected by Nicholas.
Texas Gov. Greg Abbott said authorities have placed rescue teams and resources in and around Houston and along the coast and Houston Mayor Sylvester Turner said high-water rescue vehicles had been deployed and barricades had been set up throughout the city.
"This city is very resilient. We know what we need to do. We know about preparing," Turner said.
The Associated Press contributed to this report.