It was upgraded to a Category 3 storm on Friday. Increasing wind shear and cooler waters will lead to steady weakening Saturday but Aletta will likely maintain its major hurricane status over the next 24 hours with some fluctuation in strength.
This is the first tropical storm of the 2018 eastern North Pacific season.
Aletta is not a direct threat to land and will instead move out to sea and weaken over the next several days.
It was about 555 miles (895 kilometers) south of the Baja California Peninsula. Infrared satellite images show the storm has a distinct, 20-mile wide eye, with deep convection.
It was centered about 255 miles south of Manzanillo, Mexico, and was moving northwest at 9 mph.
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Forecasters are also keeping an eye on another area of low pressure to the east of Aletta.
AccuWeather Hurricane Expert Dan Kottlowski said: "Since this system will track over and through an environment very similar to what Aletta experienced, there's a good chance that it will become a hurricane".
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Mexico's National Water Commission (Conagua) is predicting 32 hurricanes for this season, four more than average, with 18 in the Pacific and 14 in the Atlantic.