Soldiers and police were moving 600 residents from the fishing village of Punta Allen in Mexico's Quintana Roo state, where authorities opened emergency shelters and began preparing for the evacuation of other low-lying coastal settlements.
The heart of the storm was expected to hit south of Cancun and the Riviera Maya, though strong rain and winds were likely there, and officials also prepared shelters there as a precaution.
Ernesto was 220 miles (354 kilometers) east of Chetumal, Mexico, with maximum sustained winds of 65 mph (105 kph) late Tuesday morning, according to the U.S. National Hurricane Center in Miami, and it was moving to the west-northwest at 14 mph (22 kph).
Forecasters said it was expected to hit the coast late Tuesday.
The storm that entered the Caribbean on Saturday was driving through the sea parallel to the Honduran coast, though officials there said the threat had passed without any damage or injuries.
Nicaragua had evacuated hundreds of people living along the coast, but it too apparently was spared significant damage.
Forecasters said the storm was expected to grow to hurricane force before moving ashore near the Belize-Mexico border early Wednesday, passing near the jungle Mayan ruins of Calakmul and eventually entering the southern Gulf of Mexico and hitting the Gulf coast near the city of Veracruz.
Hurricane warnings were posted for the entire coast of Belize and the southern half of Mexico's Caribbean Yucatan coast.
Tropical storm warnings were issued for the northern part of the Yucatan coast up to Cancun.
Mexican authorities warned of possible flooding in an area where swollen rivers in the past have swept away houses, livestock and people and collapsed mountainsides. In a landslide last year, 31 people were buried in the Chiapas state town of Juan del Grijalva.
Meanwhile, Tropical Storm Gilma formed in the Pacific Ocean, 620 miles (1,000 kilometers) west of Mazanillo, Mexico, with winds of 40 mph (64 kph). The storm was not expected to threaten land.