The hurricane's maximum sustained winds this morning were near 215 kph, down from a peak of 250 kph yesterday, making it the most powerful May hurricane since the mid-1960s, when reliable records began to be kept.
The US National Hurricane Center said rapid weakening is expected over the next two days.
The hurricane was centered about 1,095 kilometres south of the southern tip of Mexico's Baja California peninsula and was moving north-northwest near 7 kph.
While Amanda was forecast to stay out to sea, Mexico's National Meteorological Service said rains associated with the storm were likely to drench much of western and central Mexico.
The Eastern Pacific hurricane season officially started May 15. The Atlantic hurricane season starts June 1.