The National Hurricane Center says Amanda formed today as a Category 1 hurricane and is strengthening rapidly.
The storm is centred about 1,035 kilometres southwest of Manzanillo, Mexico, with sustained winds of 120 kph.
The centre says Amanda was moving northwest at 11 am EDT today at about 7 kph.
Forecasters say Amanda could become a major hurricane sometime tomorrow but no coastal watches or warnings are in effect.
The centre said the storm poses no immediate threat to land.