Among the dead were four children who were buried alive when a mudslide crushed their home in San Jose Pinula, 17km east of the capital, the National Disaster Agency (Conred) said.
Elsewhere in the capital, four people died and 11 were missing after floods swept through their homes as the storm front's outer fringes lashed western Guatemala with rainfall.
Twelve Austrian tourists were rescued from a resort area that was cut off.
"We have 3,377 people affected by the storm, with 574 homes damaged by rains brought by storm Agatha," David de Leon, a Conred spokesman, said.
|It is feared that the rains will turn the volcanic ash from Pacaya into cement-like mud [AFP]
He said the eye of the storm was expected to make landfall overnight along Guatemala's western coast, bringing even heavier rains and stronger winds on Sunday.
"We believe Agatha could wreak more damage in the country than tropical storm Mitch (1998) and Hurricane Stan (2005)," Ronaldo Robles, the spokesman for Guatemala's President Alvaro Colom, said.
Colom earlier said the government was considering putting the entire country under a state of emergency, extending one he declared on Friday in an area around the Pacaya volcano.
Hundreds of people are in shelters after the Pacaya erupted on Thursday, billowing clouds of ash that has blocked drainage systems.
Officials fear the rains will turn the black volcanic ash into cement-like mud.
The ash fallout has closed the country's biggest airport in Guatemala City for five days.