U.S. forecasters issued a hurricane warning for Jamaica Saturday as Storm Melissa reached hurricane strength, threatening catastrophic flooding in the northern Caribbean.

Tropical Storm Melissa has strengthened into a hurricane and is expected to become a major hurricane by the end of the weekend, the U.S. National Hurricane Center said on Saturday.

“Millions of people are at risk of catastrophic impacts,” DaSilva said. “We are increasingly concerned about the threat of a humanitarian disaster unfolding, especially if this storm stalls.”