Hurricane Melissa strengthened into a major Category 4 hurricane, with the possibility of intensifying to a Category 5 storm on Sunday night, unleashing torrential rain and threatening to cause catastrophic flooding in the northern Caribbean, including Haiti and Jamaica, the US National Hurricane Centre said.

The weather agency said Melissa is likely to reach the southern coast of Jamaica as a major hurricane late on Monday or Tuesday morning and urged people on the island to seek shelter immediately.

“Conditions [in Jamaica] are going to go down rapidly today,” Jamie Rhome, the centre’s deputy director, said on Sunday. “Be ready to ride this out for several days.”

“I urge Jamaicans to take this weather threat seriously,” said Jamaican Prime Minister Andrew Holness. “Take all measures to protect yourself.”

Melissa was centred about 180 km (110 miles) south-southeast of Kingston, Jamaica, and about 445 km (280 miles) south-southwest of Guantanamo, Cuba, on Sunday morning. It had maximum sustained winds of 220km/h (140mph) and was moving west at 5km/h (3mph), the hurricane centre said.

Melissa was expected to drop torrential rains of up to 760mm (30 inches) on Jamaica and southern Hispaniola – Haiti and the Dominican Republic – according to the hurricane centre. Some areas may see as much as 1,010mm (40 inches) of rain.