This is the second hurricane in recent weeks to threaten the US Gulf Coast after Hurricane Ida wreaked havoc and killed more than two dozen people.
Heavy rains fell across the Gulf Coast of Texas and Louisiana as tropical storm Nicholas strengthened into a hurricane and made landfall in Texas, bringing the threat of widespread flooding, power outages and storm surges.
Nicholas strengthened to a hurricane on Monday, with meteorologists warning of dangerous levels of rainfall.
US President Joe Biden declared an emergency for Louisiana and ordered federal assistance to supplement local response efforts due to conditions resulting from Nicholas, the White House said.
"It will be a very slow-moving storm across the state of Texas that will linger for several days and drop a tremendous amount of rain," Texas Governor Greg Abbott said.
Abbott declared states of emergency in 17 counties and three cities. He said boat and helicopter rescue teams have been deployed or placed on standby.
Nicholas is the second hurricane in recent weeks to threaten the US Gulf Coast.
Hurricane Ida wreaked havoc in August, killing more than two dozen people and devastating communities in Louisiana near New Orleans.
Houston Mayor Sylvester Turner, citing flood warnings, urged the city's roughly 2.3 million residents to stay off streets and highways.
The Houston Independent School District canceled classes for Tuesday, while dozens of schools across Texas and Louisiana shut down on Monday.
Houston suspended light rail and bus services on Monday evening. Hundreds of flights were canceled or delayed at airports in Corpus Christi and Houston.
Big wind in Freeport TX with Hurricane Nicholas pic.twitter.com/Vm4uLAaQ9V— Reed Timmer (@ReedTimmerAccu) September 14, 2021
Houston, the fourth-most populous US city, was devastated in 2017 when Harvey, a Category 4 hurricane, slammed Texas, dropping up to 40 inches (102 cm) of rain in some sections and killing more than 100 people.
National Weather Service models forecast rainfall totals from Nicholas up to 16 inches for coastal parts of Texas, reaching 20 inches in some isolated areas.
Royal Dutch Shell on Monday began evacuating staff from a US Gulf of Mexico oil platform and other firms began preparing for hurricane-force winds.