MEXICO CITY — Tropical Storm Nate gained force as it sped toward Mexico's Yucatan Peninsula Friday after drenching Central America in rain that was blamed for at least 21 deaths. Forecasters said it was likely to reach the U.S. Gulf Coast as a hurricane over the weekend.Louisiana and Mississippi officials declared states of state of emergency and Louisiana ordered some people to evacuate coastal areas and barrier islands ahead of its expected landfall Saturday night or early Sunday. Evacuations began at some offshore oil platforms in the Gulf.Mississippi's government said it would open 11 evacuation shelters in areas away from the immediate coast, with buses available for people who can't drive.The U.S. National Hurricane Center warned that Nate could raise sea levels by 4 to 7 feet (1.2 to 2.1 meters) from Morgan City, Louisiana, to the Alabama-Florida border.