What was a category 5 hurricane on Monday heading for Mexico's Pacific Coast has now been downgraded to a tropical storm, but looks set to bring heavy rain to the U.S. East Coast.

Willa recently made landfall as a category 3 and has rapidly weakened, but torrential rains are falling over west-central Mexico.

The US National Hurricane Center has warned of potentially life-threatening flash flooding and landslides. It also warned of large waves and rough surf conditions on the coast of the states of Sinaloa and Nayarit and continued strong winds.

It hit land 15km from the town of Escuinapa in Sinaloa state, where it damaged the roof of the local hospital.

Juan José Flores of Mexican berry association Aneberries told Fresh Fruit Portal that growers seemed to have escaped unscathed.

"Fortunately there will be no problems for the fruit," he said, noting the blueberry plantations in Sinaloa are in the north of the state and were not in the path of the hurricane.

Willa is expected to enter the U.S. later this week and could affect an area stretching from Texas to New England.

"The storm will encounter colder air, which will transform from a tropical area of low pressure to a more wintertime-like storm system," CNN meteorologist Brandon Miller was quoted as saying.

"If the low pressure tracks up the East Coast of the US, it could become the season's first nor'easter. This would bring significant amounts of rain and wind Saturday and Sunday in cities like Washington, D.C., Philadelphia, New York City and Boston."