Newly formed Tropical Storm Roslyn gained strength off Mexico's southern Pacific coast, and forecasters expected it to be a hurricane when it makes a likely weekend landfall between the resorts of Puerto Vallarta and Mazatlan.

The U.S. National Hurricane Center said Roslyn's maximum sustained winds had increased to 50 mph late Thursday night. It was centered about 230 miles south-southeast of Manzanillo and moving northwest at 8 mph.

Forecasters said Roslyn was expected to reach hurricane force Friday night. They said it would gradually shift to a more northerly course and begin moving toward land in the coming days.

The hurricane center said tropical storm-force winds extended out to 60 miles from Roslyn's core.

Mexico issued a tropical storm warning from Manzanillo to Cabo Corrientes, which is near Puerto Vallarta. The National Water Commission said rains from Roslyn could cause mudslides and flooding.

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