(CNN)-- Tropical Storm Carlotta neared hurricane strength as it churned in the Pacific Ocean early Friday, heading toward Mexico's southwestern coast, forecasters said.

"Carlotta is forecast to become a hurricane by Friday afternoon," the National Hurricane Center said. "These rains could cause life-threatening flash floods and mudslides."

The storm was about 220 miles (355 kilometers) southeast of Puerto Angel, Mexico, and 440 miles from the resort town of Acapulco early Friday, according to forecasters.

The storm, with sustained winds near 70 mph (110 kph), was moving toward the northwest at 10 mph, the center said.

A storm becomes a hurricane when it produces sustained winds of 74 mph.

A hurricane warning is in effect for parts of the Pacific coast of Mexico, from the seaport city of Salina Cruz to Punta Maldonado.

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