At least 19 people were reported dead as a direct or indirect result of the storm.
Guatemala's disaster prevention agency said five people died after a hillside collapsed on their house in Alta Verapaz province, burying them.
Authorities in El Salvador said five Salvadoran army soldiers died after a wall collapsed at a house where they sought refuge in the town of Comasagua, where hundreds of police and soldiers have been conducting anti-gang raids.
Another soldier was injured.
Two other people died in the eastern El Salvador town of Guatajiagua after heavy rains caused a wall of their home to collapse.
Another man in El Salvador died when he was swept away by a current, and another died when a tree fell on him.
Rivers overflowed their banks and El Salvador declared a state of emergency and opened 80 storm shelters.
In neighbouring Honduras, a 22-year-old woman died when she was swept away by currents and three people died when their boat swamped or capsized in northern Honduras.
A man in Nicaragua was killed by a falling tree.
Julia hit Nicaragua's central Caribbean coast early on Sunday as a hurricane with maximum sustained winds of 140 km/h and survived the passage over the country's mountainous terrain, entering the Pacific late in the day as a tropical storm..
By Monday, Julia had moved inland over Guatemala and its winds were down to 60 km/h.
The US National Hurricane Center said Julia was centred about 60km east-northeast of Puerto San Jose, Guatemala, and was moving west-northwest at 24 km/h.
The centre said life-threatening flash floods and mudslides were possible across Central America and southern Mexico through Tuesday, with the storm expected to bring as much as 38cm of rain in isolated areas.
In Guatemala, two people were listed as missing and two were hospitalised, and about 1300 people had to leave their homes because of flooding and rising streams.
Julia was expected to dissipate later on Monday as it passes along the Guatemalan coast.
Heavy rains and evacuations were also reported in Panama, Honduras and Costa Rica, where some highways were closed due to the downpours.