U.S. forecasters say Miriam has strengthened into a Category 3 hurricane well off Mexico's Baja California peninsula.
Forecasters at the National Hurricane Center in Miami said Miriam is packing top sustained winds of 120 mph (165 kph), making it what's classified as a major hurricane.
The hurricane formed a day earlier in the eastern Pacific and was centered about 410 miles (655 kms) south-southwest of the southern tip of the Baja peninsula.
The center says Miriam was moving northwest at 12 mph (19 kph). No coastal watches or warnings are in effect, but swells will hit Baja's southern and western shore during the next couple of days.
In the open Atlantic, Tropical Storm Nadine was meandering far from land with top sustained winds of about 50 mph (85 kph).