The first tropical depression of the Pacific hurricane season has formed off the coast of Mexico.

The National Hurricane Center in Miami said the tropical depression was centered about 400 miles south-southwest of the Mexican city of Mazatlan around midday.

Maximum sustained winds are near 30 mph. Forecasters say it could strengthen to a tropical storm before nearing the Mexican coast by Friday night or Saturday.

The depression was moving toward the north at about 9 mph and was expected to turn gradually north-northeast late Friday.

Mexican authorities have issued a tropical storm watch for the Pacific coast from Topolobampo south to El Roblito and for the Islas Marias.