Tropical storm Andres was strengthening off the southwestern coast of Mexico early Monday, and forecasters said it could become a hurricane in coming days.

It was too soon to tell if the storm would make landfall, but a tropical storm watch was issued from Zihuatanejo northward to Manzanillo, meaning that tropical storm conditions were possible in the next day or two.

Late Sunday night, Andres became the first named storm of the Eastern Pacific hurricane season, which began May 15 and ends Nov. 30 and is typically busiest between July and September.

Andres was a late arrival, forecasters said. It's been 40 years since it took so long in the Eastern Pacific season for a named storm to come along.

"Normally that season gets underway somewhat earlier," said Richard Pasch, a senior hurricane specialist at the National Hurricane Center in Miami, Fla. "But it's not necessarily an indication that it'll be a quiet year in that basin. We've seen some years starting late and become quite active. We're just going to have to wait and see on that."

Federal forecasters have predicted a near-normal or below-normal season, with the possibility for 13 to 18 named storms, including six to 10 hurricanes.

The National Hurricane Center said Andres' centre as of 5 a.m. ET, (2 a.m. PT) Monday was about 305 kilometres south of Zihuatanejo and 505 kilometres south-southeast of Manzanillo.

Andres was moving slowly toward the west-northwest near six km/h. Maximum sustained winds were near 85 km/h with higher gusts.

Forecasters said Andres could become a hurricane with sustained winds of at least 119 km/h in the next 24 to 36 hours as it turned northwest and skirted along the coast.

The official forecast Monday still didn't indicate landfall, but the storm was close enough to the coast that a tropical storm warning could be required later in the day, Pasch said.