Heavy rains lashed Texas and Louisiana as Hurricane Nicholas weakened into a tropical storm, bringing the threat of widespread floods and power outages as it swept down the US Gulf Coast.

It is the second major storm to threaten the region in recent weeks after Hurricane Ida killed more than two dozen in August and devastated communities in Louisiana near New Orleans.

Nicholas should weaken further and become a depression by tomorrow, the National Hurricane Center (NHC) said. It could still cause life-threatening flash floods across the Deep South in the next couple of days, the agency warned.

Nicholas was about 25km south-southwest of Houston by 12pm Irish time, heading north-northeast with maximum sustained winds of 95km/h, the NHC said in a bulletin.

President Joe Biden declared an emergency for Louisiana and ordered federal assistance for local responders because of the effects of Nicholas, the White House said.

Nicholas could knock out electricity and hamper restoration efforts after Ida knocked out power in Louisiana.

Early today, more than 95,000 customers in Louisiana had lost power, according to the website PowerOutage.us, while in the Houston area alone, more than 415,000 customers faced outages, utility CenterPoint Energy said.

Texas Governor Greg Abbott declared states of emergency in 17 counties and three cities, with boat and helicopter rescue teams being deployed or put on standby.

Crowdsourse Rescue, a disaster-response organisation inTexas, said on Twitter that its swift-water teams reported no calls for rescue overnight.

"But we'll take a mediocre night of sleep in a truck then no sleep in a boat," it said. "Knocking on wood, but so far so good?"

Volunteers with the organisation fanned out across the coastal areas to do damage assessments and found minimal debris and flooding.

Some 35cm of rain fell in Galveston while Houston got almost 15cm of rain overnight and into the morning, the National Weather Service reported.

Galveston Mayor Craig Brown told the Daily News that there was heavy street flooding in low-lying areas early this morning.

"What caught us a little by surprise was the speed of the winds," Mr Brown said. "They were a little heavier with more force than we thought."

Northwest in Houston, many residents, especially on the south and southeast sections of the city, awoke to power outages, downed trees and water in their homes after Nicholas passed over the area.

Houston, the fourth most populous US city, was devastated in 2017 when Harvey, a Category 4 hurricane, slammed Texas, dropping up to 102cm of rain in some areas and killing more than 100 people.

The Houston Independent School District along with dozens of schools districts across both states cancelled classes today.

The METRO in Houston resumed limited light rail and bus service, but hundreds of flights were cancelled or delayed at airports in Corpus Christi and Houston.

The storm was expected to move slowly to the northeast throughout the day, and then turn eastward by tomorrow over Louisiana, the National Weather Service (NWS) said.

Louisiana Governor John Bel Edwards warned against flash floods triggered by the heavy rain as drainage systems were still clogged with debris from Ida and other storms.

NWS models forecast rainfall totals from Nicholas from 40cm in coastal parts of Texas to 50sm in some spots.

Its northeast sweep was expected to pummel parts of south-central Louisiana and southern Mississippi with up to 25cm of rain.