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NEW: The Category 3 hurricane is 180 miles southwest of Manzanillo, Mexico

NEW: It may strengthen before it makes landfall

Tourists say they won't let Hurricane Jova ruin their trip

Emergency officials are opening shelters and positioning troops

Puerto Vallarta, Mexico CNN  — 

Emergency officials and residents rushed to prepare as a weakened Hurricane Jova slowly spun toward Mexico’s southwestern coast.

Emergency officials scrambled to open shelters as forecasters warned that the Category 3 hurricane may strengthen before it makes landfall Tuesday.

It could become a Category 4 hurricane as early as Monday night or early Tuesday, forecasters said.

Jova was about 180 miles southwest of the resort town of Manzanillo at 8 p.m. ET Monday, according to the National Hurricane Center in Miami. It was moving northeast at about 7 mph.

Forecasters predicted it would make landfall Tuesday afternoon or evening.

A hurricane warning was in effect from Punta San Telmo north to Cabo Corrientes, near Puerto Vallarta, Mexico. A tropical storm watch was in effect for the coast, north of Cabo Corrientes to San Blas.

“Our main concern is the welfare of the population,” said Trinidad Lopez, civil protection director in Jalisco state. “We’re doing everything in our power to protect people.”

At least 100 shelters were open, Lopez said. Workers distributed food, cots and blankets, and crews positioned heavy machinery in strategic locations throughout the state, he added.

Crews positioned heavy machinery in strategic locations throughout the state, Lopez said.

Mexico’s federal government was providing assistance as well. More than 300 soldiers had been deployed and members of the Mexican navy in Puerto Vallarta were on alert, Lopez said.

While officials prepared for the storm’s arrival, tourists heading to the popular resort area said they are determined not to let the storm ruin their trip.

“We already have plans. Everything is paid for. We’re going. It’s not going to bother us,” said Doris Milburn, who was on her way to Puerto Vallarta for a week-long trip with her daughter, her grandson and the grandson’s girlfriend.

The Arkansas resident said her family was prepared to have fun, even if rainy weather threatens.

“We’ll just walk around, have a margarita or two, (a) bloody Mary and read our books,” she said as she waited for a flight at the Dallas airport.

A Carnival cruise ship originally scheduled to arrive in Manzanillo Monday headed to Cabo San Lucas instead, cruise line spokeswoman Joyce Oliva said.

Mexico’s National Meteorological Service warned boaters off the country’s Pacific coast to prepare for increasing rain, waves and wind.

High surf warnings were in effect, with forecasters warning that swells generated by Jova were striking parts of Mexico’s southwest coast.

“These swells are likely to cause life-threatening surf and rip current conditions,” the hurricane center said. “Jova is expected to produce total rain accumulations of 6 to 12 inches over the states of Michoacan, Colima and Jalisco and Nayarit, with possible isolated maximum amounts of 20 inches.”

A tropical storm warning also was in effect in an area near Punta San Telmo, stretching south toward Lazaro Cardenas.

Meanwhile, Tropical Storm Irwin, which is farther off the Pacific coast of Mexico and was temporarily a hurricane, continued to weaken Monday. Forecasts at the end of last week indicated it could follow Jova’s path onto shore, but the latest predictions are that it will turn south on Thursday or Friday, before it reaches land.

CNN’s Dave Alsup contributed to this report.