MIAMI, Fla. -

Tropical Storm Cristina is expected to strengthen as it swirls off Mexico's Pacific coast.

The storm's maximum sustained winds early Tuesday are near 40 mph and the U.S. National Hurricane Center says Cristina could become a hurricane by Thursday.

The storm is centered about 155 miles south-southwest of Zihuatanejo, Mexico, and is moving west near 3 mph.

There are no coastal watches or warnings in effect.