HOUSTON/MIAMI (BLOOMBERG, REUTERS) - Storm Nicholas made landfall in Texas, bringing torrential rainfall that threatens to unleash flooding in Houston and parts of Louisiana still recovering from Hurricane Ida two weeks ago.

Nicholas, downgraded to a tropical storm after briefly being rated a hurricane, roared ashore at about 12:30am local time on Tuesday (Sept 14) near the Matagorda Peninsula.

It has top winds of 110 kilometres per hour, the National Hurricane Center said in its latest advisory on Tuesday. It's the eighth tropical cyclone to hit the US this year.

"Nicholas could cause life-threatening flash floods across the Deep South during the next couple of days," NHC said.

While it will mostly bypass the Gulf of Mexico's oil and natural gas platforms, it could dump as much as 46 centimetres of rain, posing a threat to coastal refineries and petrochemical facilities.

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.

Louisiana Governor John Bel Edwards also declared a state of emergency and Texas Governor Greg Abbott ordered emergency crews to get ready for the storm.

A hurricane warning stretches from Port O'Connor to Freeport, Texas, and the heaviest rain will likely fall from Monday to Wednesday, according to the Weather Prediction Center.

The storm surge could reach 1.5m at Galveston Bay in Houston.

"Houston is in the crosshairs," said Steve Silver, a senior meteorologist with Maxar, noting that precipitation poses a bigger threat than wind.

Nicholas will be "a significant rainfall event."

The storm's slow forward advance adds to the flooding risk as it passes over the region.

Nicholas is the Atlantic's 14th storm in 2021. Half of the storms so far have hit the US, and Ida was the season's worst, crashing into the Louisiana coastline before devastating New York with rain and floods that killed more than 40 people.

On Monday, AIR Worldwide updated its projected losses from Ida, saying the storm probably caused US$20 billion (S$26.9 billion) to US$30 billion in insured losses. Earlier estimates ranged around US$18 billion.

The latest storm is expected to hit areas of Louisiana still recovering from last year's Hurricane Laura and is likely to bring heavy rain to areas slammed by Ida, Edwards said. That could also disrupt efforts to restore power.

Nicholas might also disrupt restoration efforts of Gulf of Mexico oil platforms and pipelines that have remained offline since Ida.

Nearly 50 per cent of oil supply is still down in the Gulf and the volume of shut-in output might start growing once again.

US Gulf Coast physical crude prices could surge if supplies aren't returned promptly. An average Atlantic season produces 14 storms by the time it ends in November, so 2021 is ahead of pace.

Flights cancelled

The Houston Independent School District cancelled 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 cancelled or delayed at airports in Corpus Christi and Houston.

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 sections and killing more than 100 people.

National Weather Service models forecast rainfall totals from Nicholas up to 40.6cm for coastal parts of Texas, reaching 50.8cm in some isolated areas.

As the storm moves north-east, it was expected to pummel parts of south central Louisiana and southern Mississippi with up to 25.4cm of rain.

The National Weather Service issued storm surge, flood and tropical storm warnings and watches throughout the region, calling it a "life-threatening situation".

"We want to make sure that no one is caught off guard by this storm," Louisiana Governor John Bel Edwards said at a news briefing on Monday.

Mr Edwards warned that drainage systems still clogged with debris from Ida and other storms may be deluged by the heavy rain, triggering flash floods.

Nicholas could also knock out electricity and hamper restoration efforts as more than 119,000 homes and businesses remain without power from Ida, he said.

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.