Cyclone Freddy is expected to hit Mozambique again this week, after causing 21 deaths, with four occurring during its recent impact on Madagascar.
The storm has displaced thousands of people in both countries since it first caused destruction in February.
It has been active over the Indian Ocean for 32 days, making it a "remarkable" event according to the WMO.
Cyclone Freddy has broken records for strength and distance travelled across the Indian Ocean.
It may become the longest-lasting storm on record, but confirmation may take several months.
Over 160,000 people have been impacted by the storm, according to the UN World Food Programme.
Madagascar has received three times the average monthly rainfall in the past week.