The system currently has maximum sustained winds of 40 mph, with strengthening expected over the next 48 hours.
The official forecast from the NHC has Isaias reach tropical-storm strength by Tuesday night and then moving through the Leeward Islands on Wednesday.
According to the NHC, tropical-storm-force winds could extend outward up to 230 mins from the center of the storm, primarily northeast of the center.
"It is important to not focus on the exact track of the system," the NHC tweeted Tuesday afternoon. "The disturbance is large and heavy rain and gusty winds extend far from the center."
Tropical storm conditions are expected to spread across the U.S. and the British Virgin Islands and Puerto Rico Wednesday through Thursday morning.
Total rain amounts can be expected to 3 to 6 inches, with up to 10 inches possible.
"This rainfall may produce life-threatening flash flooding and mudslides, as well as potential riverine flooding," the NHC said.
The longer-range forecast for this future tropical storm is highly uncertain, as environmental conditions and land interaction could very well keep this system from further organizing late this week and over the weekend.
"The good news here is that the upper-level conditions in the atmosphere are not allowing the storm to strengthen, but we're still going to have to watch it over the next couple of days as it comes very close to Florida and the East Coast next week," Fox News senior meteorologist Janice Dean said on "Fox & Friends."
Beyond Puerto Rico, the Dominican Republic, Bahamas and Florida are advised to closely monitor changes with the forecast in the next days.
Colorado State University hurricane research scientist Phil Klotzbach said on Twitter the record for the earliest "I" storm in the Atlantic basin is Irene, which formed on Aug. 7, 2005.
The 2020 Atlantic Hurricane Season will include the names: Arthur, Bertha, Cristobal, Dolly, Edouard, Fay, Gonzalo, Hanna, Isaias, Josephine, Kyle, Laura, Marco, Nana, Omar, Paulette, Rene, Sally, Teddy, Vicky and Wilfred.