Ernesto is strengthening as it plows through Caribbean islands. Here's where it could go next and how strong it could get.

An area of showers and thunderstorms is organizing into the summer's next tropical storm a few hundred kilometres east of the Caribbean Monday morning. (CNN Weather via CNN Newsource)

Tropical Storm Ernesto is strengthening as it slams the northern Caribbean with heavy rain, gusty winds and rough seas Tuesday and approaches Puerto Rico.

Hurricane watches were issued late Tuesday morning for the U.S. and British Virgin Islands, Vieques and Culebra, as forecasters grew more concerned Ernesto could rapidly intensify into a dangerous hurricane.

“Models indicate Ernesto could be near or at hurricane strength in about 24 hours,” the National Hurricane Center cautioned late Tuesday morning, noting that several indicators were high for the possibility of rapid intensification.

The storm’s sustained winds increased by 20 mph to 60 mph over a nine hour period from Tuesday morning to early Tuesday afternoon. In order to meet the definition of rapid intensification, Ernesto will have to gain at least 35 mph of strength over a 24-hour period.

Ernesto could become a hurricane while it tracks just north of Puerto Rico early Wednesday. Previously, it was forecast to reach hurricane-strength later in the day Wednesday.

Ernesto’s center was moving through the northeastern Caribbean’s Leeward Islands late Tuesday morning after tracking near or over Guadeloupe early Tuesday. It’s been on a collision course with the region since forming early Monday evening.

Tropical storm-force winds extended well away from Ernesto’s center and are delivering gusty winds to much of the region. A 65 mph gust at Saint Barthélemy – known commonly as St. Bart’s – occurred when Ernesto was about 100 miles away.

Ernesto’s wind and rain will spread over more of the region overnight Tuesday and Wednesday.

Tropical storm warnings are in effect for Puerto Rico and the Leeward Islands, including the U.S. and British Virgin Islands. Ernesto’s strong wind gusts are capable of damaging some structures and taking down trees and power lines.

Drenching, potentially flooding rainfall looks to be the most significant threat over parts of the Caribbean this week. Heavy rain will persist for much of the Leeward Islands through Tuesday night but wet weather will linger into Wednesday.

A deluge of rain will begin for the US and British Virgin Islands Tuesday evening and reach Puerto Rico Tuesday night. The heaviest rain should cease over these areas late Wednesday.

Rainfall totals of 4 to 6 inches will be widespread, with up to 10 inches possible in parts of Puerto Rico. Flash flooding and mudslides are possible as a result, especially in the higher terrain areas of eastern and southern Puerto Rico.

Tropical storm-force winds will also pound areas within Ernesto’s path through at least Wednesday night. These winds will continue to create dangerous seas and up to 3 feet of storm surge for many islands in the region.

The combination of rain and wind could cause issues for Puerto Rico’s vulnerable electrical infrastructure.

Puerto Rico officials activated the National Guard, canceled classes in public schools and warned Ernesto would cause widespread power outages given the fragile state of Puerto Rico’s power grid, the Associated Press reported, which crews are still rebuilding after Hurricane Maria struck the island in September 2017 as a Category 4 storm.

“That’s a reality,” Juan Saca, president of Luma Energy, a private company that operates the transmission and distribution of power in Puerto Rico, told the AP.

A gradual turn to the north is expected to begin Wednesday and pull Ernesto away from the Caribbean into the open Atlantic. Once over open water, Ernesto will strengthen even further.

How strong Ernesto gets will depend heavily on very warm ocean water and how potent storm-disrupting upper level winds become over the region. It’s possible Ernesto becomes a major hurricane – Category 3 strength or greater – late this week.

But Ernesto’s track could shift depending on a number of factors, including when it is pulled northward. A later turn would mean the storm would impact areas farther west like Hispaniola or the southern Bahamas.

Ernesto could be a powerful hurricane by the weekend as it approaches Bermuda. It’s too early to know exactly how close Ernesto will come to Bermuda and how much rain and wind it’ll bring.

But the storm will have wide-reaching impacts later this week and this weekend despite a track somewhere over the open Atlantic.

The storm will churn up seas hundreds of miles away and could create dangerous rip currents for the U.S. East Coast, the Bahamas and parts of the Caribbean into early next week.