The hurricane center's forecast for Helene's rapid transition from unnamed storm to Cat 3 hurricane may be the fastest ever.

Tropical Storm Helene was rapidly strengthening in the Caribbean Sea and expected to become a hurricane Wednesday while moving north along Mexico's coast toward the U.S., prompting residents to evacuate,

The storm could come ashore as a dangerous Category 3 or 4 hurricane with a surge over 10 feet, winds over 110 mph and flooding rain.