Jamaica is bracing for unprecedented devastation as Hurricane Melissa intensified into a Category 5 storm on Monday, the strongest hurricane ever recorded to strike the island since record-keeping began in 1851.

Forecasters warned that the storm will unleash catastrophic flooding, landslides and severe damage as it makes landfall on Tuesday.

With sustained winds of 165 mph (270 kph), Melissa has already claimed six lives across the northern Caribbean, including three in Haiti, two in Jamaica and one in the Dominican Republic.

The U.S. National Hurricane Center said the hurricane was centered 145 miles southwest of Kingston on Monday morning, moving slowly westward at 3 mph. Eastern Jamaica could see rainfall exceeding 40 inches, raising fears of “catastrophic flash flooding and numerous landslides.”

Mandatory evacuations have been ordered in several flood-prone communities, but some residents have refused to leave. “I hear what they say, but I am not leaving,” said 64-year-old fisherman Noel Francis from Old Harbor Bay. Others, like 23-year-old Kingston resident Hanna Mcleod, are preparing for the worst. “I just told them to keep the door closed,” she said.

“I’m definitely worried. This is the first time I’ll be experiencing this type of hurricane.”

The slow-moving storm threatens to become a humanitarian disaster, with experts warning of prolonged flooding and power outages. “This can become a true humanitarian crisis very quickly,” said Jonathan Porter, chief meteorologist at AccuWeather.

In Cuba, hurricane warnings were issued for the eastern provinces of Granma, Santiago de Cuba, Guantánamo and Holguin, with up to 20 inches of rain expected. Jamaica’s disaster authorities have urged citizens to evacuate low-lying areas, warning that “Melissa is not a safe bet.”