Authorities issued a tropical storm warning from Costa Rica to El Salvador.
Costa Rican authorities evacuated low-lying areas and set up more than 160 storm shelters after Alma dumped rain over the country for 24 hours. A few highways were blocked by landslides.
The National Hurricane Center in Miami said the storm had maximum sustained winds of 45 mph and was expected to strengthen before hitting Nicaragua's northwest corner later Thursday. The storm was moving north at 6 mph.
The center predicted it would plow through the southern border region of El Salvador and Honduras early Friday.
Forecasters warn that Alma could dump as much as 20 inches of rain in places.
The eastern Pacific hurricane season began May 15.