Ernesto getting stronger as it plows through Caribbean islands. Here’s where it could go next and how fast it could intensify.

(CNN) — Tropical Storm Ernesto is steadily growing stronger as it slams the northern Caribbean with heavy rain, gusty winds and rough seas Tuesday, lashing the Virgin Islands and heading toward Puerto Rico.

Hurricane watches were in effect for the US and British Virgin Islands, Vieques and Culebra, as forecasters grew more concerned Ernesto could rapidly intensify into a dangerous hurricane thanks to very warm ocean water.

“Ernesto is expected to become a hurricane later tonight or early Wednesday to the north of the Virgin Islands and Puerto Rico,” the National Hurricane Center said Tuesday evening.

Previously, it was forecast to reach hurricane-strength later during the day on Wednesday.

The storm’s maximum sustained winds increased by 25 mph to 65 mph over a 12-hour period from Tuesday morning to Tuesday evening. In order to meet the definition of rapid intensification, storms have to gain at least 35 mph of strength over a 24-hour period.

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

US National Weather Service meteorologists issued a flash flood warning for St. Croix in the US Virgin Islands until 4 a.m.

“Doppler radar indicated southern rainbands from Tropical Storm Ernesto producing heavy rain across the warned area,” the weather service said late Tuesday. “Between 1 and 2 inches of rain have fallen. Additional rainfall amounts of 2 to 3 inches are possible in the warned area. Flash flooding is ongoing or expected to begin shortly.”

Tropical storm-force winds extended up to 115 miles 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 US 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. Crews are still rebuilding the grid 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.

Preparations need to be finalized before the storm arrives, Jaclyn Rothenberg, FEMA’s Public Affairs Director, urged Tuesday.

“The people of Puerto Rico and the US Virgin Islands are no strangers to storms,” Rothenburg told CNN. “They know how to prepare, but I know that there also are a lot of travelers – people that are visiting the islands during the summer for fun.”

With power outages a possibility, it’s important to have non-perishable food on hand, charge up any electronic devices and make a plan to keep medications that need refrigeration cool.

“It’s really important that people aren’t complacent, that they are taking the storm seriously and that they’re preparing for the impact,” Rothenberg said.

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 will become 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.

The island is expecting to see heavy rain from another weather system in the next few days. “Residents need to prepare now before conditions worsen. Time is of the essence,” Minister of National Security Michael Weeks said Tuesday.

Ernesto 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 US East Coast, the Bahamas and parts of the Caribbean into early next week.

FEMA only funding immediate storm response

With Ernesto bearing down on US territories, the Federal Emergency Management Agency’s disaster fund is in a deficit. It’s the second year in a row the fund has been depleted before the peak of hurricane season.

“This is, without a doubt, because of the increase in extreme weather events caused by climate change,” Rothenberg, the agency’s spokesperson, told CNN.

FEMA Administrator Deanne Criswell shifted the agency into what’s known as immediate needs funding on August 7, Rothenberg told CNN. The agency is waiting for Congress to pass a $9 billion supplemental funding request to replenish the fund, but the House and Senate are in recess until September. It’s unclear when lawmakers will address the fund after they return.

Rothenberg stressed FEMA has enough resources to respond to impending disasters like Ernesto.

FEMA will continue to fund “search and rescue operations, being able to bring in equipment, generators, things like that,” Rothenberg told CNN, adding, “I don’t want the people of Puerto Rico and USVI to be worried about this at all.”

But on pause will be responses like long-term recovery from previous disasters, of which there’s no shortage. Through July, the National Oceanic and Atmospheric Administration reported the US experienced 19 separate weather and climate disasters, with damage exceeding $1 billion – the second-highest amount for the first seven months of the year. It falls just behind a record set in 2023.

The NOAA tally doesn’t include devastating flooding from Hurricane Debby, which made landfall earlier this month.

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CNN Meteorologists Taylor Ward and Robert Shackelford contributed to this report.