Hurricane Maria became a Category 5 storm Monday as it moved over the Leeward Islands - battered by deadly Irma just over a week ago - according to the National Hurricane Center.
For the first time in 85 years, Puerto Rico is expected to suffer a direct landfall from such a strong hurricane.
Maria's sustained wind speeds of 125 miles per hour at 2 p.m. ET make it a strong Category 3 already.
The "potentially catastrophic" hurricane had top winds of 257kph as it swirled just 24km east-southeast of Dominica at 00:00 GMT on Tuesday, the US-based National Hurricane Center (NHC) said. And forecasters say a unsafe storm surge accompanied by large and destructive waves will raise water levels by as much as 4 to 6 feet above normal tide levels near where the center of Maria moves across the Leeward Islands.
Most of the forecast models suggest Maria will continue to track towards the WNW moving through the NE Caribbean Sea and approaching Puerto Rico later Wednesday into Thursday.
Soldiers from the 602nd Area Support Medical Company and other Army personnel board a Navy landing craft during their evacuation in advance of Hurricane Maria, in Charlotte Amalie, St. Thomas, U.S. Virgin Islands.
Others were evacuating to Puerto Rico.
If Maria strikes the island as forecast, it will be "more unsafe than Hugo and Georges", he said.
And as Maria is forecast to strengthen, what was Tropical Storm Lee has weakened.
The islands directly affected by the storm's core face the likelihood of destructive winds of 120 to 150 miles per hour and 6 to 12 inches of rain (with isolated totals of 20-25 inches, especially in high terrain), which will cause life-threatening flash floods and mudslides.
East Island Excursions, which takes people on snorkeling trips, has made about 40 trips in the past week, taking residents of St. John to Puerto Rico.
Trump issued an emergency declaration for the US Virgin Islands.
Some of these islands are still recovering after being hit by Irma - the category five hurricane which left at least 37 people dead and caused billions of dollars' worth of damage.
A British military reconnaissance team is on standby to go to Montserrat and assess needs, the office said.
The warning means that tropical storm conditions are expected within 36 hours.
After being stationary for a while Monday evening, Jose began moving northward again at 8 miles per hour and "dangerous surf and rip currents are expected to continue along the east coast of the United States", says the National Hurricane Center.
"Minor to moderate coastal flooding is possible from DE to southern New England", the center said. Trailing Maria is Tropical Depression Lee. "Swells generated by Jose are affecting Bermuda, the Bahamas, and much of the U.S. east coast".