Tropical Storm Agatha, the first of the season, slammed into Guatemala's Pacific coast on Saturday, its torrential rains triggering landslides and floods that left at least 12 people dead.
Another 11 people were reported missing in Guatemala, according to David de Leon, spokesman the National Disaster Relief Coordinator.
The poor Central American nation was already contending with heavy eruptions from its Pacaya volcano that have blanketed the capital in ash and destroyed 800 homes.
Officials worried that heavy rains from Agatha, which formed in the East Pacific early Saturday, could exacerbate the damage by turning black volcanic ash into cement-like mud.
Agatha's center reached the coast near the Guatemala-Mexico border on Saturday evening, said the U.S. National Hurricane Center in Miami. The storm had maximum sustained winds of 45 mph (75 kph) and was centered just southeast of Tapachula, Mexico, moving northeast at about 10 mph (16 kph).
It was expected to dump from 10 to 20 inches (25 to 50 centimeters) of rain and as much as 30 inches (75 centimeters) in isolated areas of Guatemala.
De Leon said four children were buried in a rain-triggered landslide in the town of Santa Catarina Pinula outside Guatemala City, and four adults were killed in a poor part of the capital.
Earlier Saturday, he said two children and two adults were killed when rains generated by Agatha dislodged a boulder that crushed a house they were in. The deaths happened in the department of Quetzaltenango, 125 miles (200 kilometers) west of Guatemala City.
More than 850 people have been evacuated from their homes because of flooding affecting much of central and southern Guatemala.
The storm is expected to lose force as it comes ashore overnight.
"The storm will start to weaken and we hope that on Sunday it will be just a tropical depression," said Romero Garcia of Guatemala's Meteorological Institute. "That is not to say that there won't be heavy rains."
The Pacaya volcano, which is just south of the capital, started spewing lava and rocks Thursday afternoon, forcing the closure of Guatemala City's international airport. A TV reporter was killed by a shower of burning rocks.
Airport official Felipe Castaneda told reporters Saturday that the airport would be closed for the next five days while ash is removed.
"The work to remove the ash was going forward, but the rain has complicated it," Castaneda said.
In El Salvador, authorities began evacuating hundreds of families in areas at risk for landslides and flooding, suspending fishing and tourism along the Pacific coast.
Five days of steady rainfall has already swollen a major river flowing through the capital San Salvador.