A tropical storm churning north along Central America killed at least 20 people in Nicaragua, Costa Rica and Honduras on Thursday, with forecasters predicting it could strengthen into a hurricane as it heads for Mexico and the United States. The state of Louisiana and the city of New Orleans are preparing for a weekend impact from the storm, which could be the third hurricane to hit the southern US in less than two months. Costa Rica declared a national emergency, closing schools and government offices. It and neighboring Nicaragua struggled with mudslides, washed out roads, overflowing rivers and cut-off communities. Nicaragua's vice president, Rosario Murillo, sharply revised upwards a previous toll to say 11 people had died in her country. Seven more were missing, she said. Officials in Costa Rica said at least six people died there: four Costa Ricans -- including a three-year-old girl -- hit by falling trees and mudslides, and two young Nicaraguan farm workers. At least another 15 people were missing, including the crew of two swamped fishing boats, police said. In Honduras, officials said three people had died. - Forming into a hurricane - More than 5,000 people were being put up in shelters in Costa Rica after having to abandon their homes because of flooding and the risk of unstable ground giving way, the director of the country's National Emergency Commission, Ivan Brenes, said. More than a dozen main roads were closed while others were only partially open. In Costa Rica's northwest Guanacaste region, many roads were impassable to all but four-wheel-drive vehicles and horses. The rain was caused by Tropical Storm Nate, which was located over eastern Honduras late Thursday. The US National Hurricane Center said it expected the storm to keep tracking north, weakening a bit over land and regaining force as it went back over water. It said Tropical Storm Nate would be "near hurricane intensity" by the time it hit Mexico's southern Yucatan Peninsula late Friday. It would then then strengthen over the weekend into a hurricane as it crossed the Gulf of Mexico to hit the southern United States somewhere between the states of Louisiana and Florida. "We do anticipate a direct hit," Louisiana Governor John Bel Edwards told a news conference. The mayor of flood-prone New Orleans declared a state of emergency, as did the governor, who said he would ask President Donald Trump for a federal disaster declaration ahead of the storm's arrival. Officials called for a voluntary evacuation of some low-lying areas northwest of New Orleans. Some offshore oil and gas rigs in the Gulf of Mexico were also being evacuated ahead of the storm, the US government Bureau of Safety and Environmental Enforcement said in a statement. - Crocodile alert - The United States is recovering from two major hurricanes: Hurricane Harvey that tore through Texas in August, and Hurricane Irma that hit Florida in September. Another powerful storm, Hurricane Maria, ripped through the Caribbean in late September, wreaking destruction on several islands, including Dominica and Puerto Rico -- the latter a US territory. In Costa Rica, an alert was issued for people to be wary of crocodiles that might be roaming after rivers and estuaries flooded. A World Cup qualifying match between Costa Rica and Honduras scheduled for late Friday in the capital San Jose was postponed to Saturday. The country's main international airport was open, but with multiple flight cancellations and delays. The annual rainy season is currently underway in Central America, a five-month period typically ending in November in which the risk of flooding and mudslides rise. This year, some areas in the region have gotten up to 50 percent more rain than average for September and October.