MANZANILLO, MEXICO: Hurricane Patricia grew into a monster Category 5 storm and bore down on Mexico's central Pacific Coast late Thursday for what forecasters said could be a devastating blow, as officials declared a state of emergency and handed out sandbags in preparation for flooding.

Steady rain began to fall after dark in Manzanillo, one of the country's principal ports, ahead of an expected landfall Friday. Luis Felipe Puente, Mexico's civil defense coordinator, said schools would be closed in Colima state, which is home to Manzanillo.

"We are calm," said Gabriel Lopez, a worker at Las Hadas Hotel in the city. "We don't know what direction (the storm) will take, but apparently it's headed this way. ... If there is an emergency we will take care of the people. There are rooms that are not exposed to wind or glass."

The US National Hurricane Centre in Miami warned that preparations should be rushed to completion, saying the storm could cause coastal flooding, destructive waves and flash floods.

"This is an extremely dangerous, potentially catastrophic hurricane," Centre meteorologist Dennis Feltgen said.

Luz Adriana Limon Rojas of Colima state's civil defense agency said the area has problems with drainage during storms.

"The neighborhood leaders have come for sacks to fill with sand," she said.

The federal government declared a state of emergency for 56 municipalities in the storm's projected path, in the states of Colima, Nayarit and Jalisco.

By late Thursday night, Patricia's maximum sustained winds had increased to 160 mph (260 kph) — a Category 5 storm, the highest designation on the Saffir-Simpson scale used to quantify a hurricane's wind strength.

Patricia was centred about 200 miles (320 kilometres) south-southwest of Manzanillo and was moving northwest at 13 mph (20 kph) on a projected track to come onshore between Manzanillo and Puerto Vallarta sometime Friday afternoon or evening.

Some weakening was forecast before then, but the Hurricane Centre said it would still be "extremely dangerous" when it makes landfall.

A hurricane warning was in effect for the Mexican coast from San Blas to Punta San Telmo, a stretch of coast that includes Manzanillo and Puerto Vallarta. A broader area was under hurricane watch, tropical storm warning, or tropical storm watch.

The Hurricane Centre said Patricia was expected to bring rainfall of 6 to 12 inches, with isolated amounts of up to 20 inches in some locations. Tropical storm conditions were expected to reach land late Thursday or early Friday, complicating any remaining preparation work at that point.

Feltgen, the meteorologist, said Patricia also poses problems for Texas. Forecast models indicate that after the storm breaks up over land, remnants of its tropical moisture will likely combine with and contribute to heavy rainfall that is already soaking Texas independently of the hurricane, he said.

"It's only going to make a bad situation worse," he said.