The National Hurricane Center said in its 11 a.m. ET advisory that up to 5 inches of rain could fall in Puerto Rico. "The recent flare-up in activity is associated with an enhanced west African monsoon system, which has been helping to fuel the development of storms over the far eastern Atlantic". It was located about 345 miles east of Guadaloupe.
Hurricane and tropical storm watches have been posted for several Caribbean islands as a weakening Tropical Storm Isaac continues its trek toward the Leeward and Windward Islands. "It's just not anywhere near a level of Maria", forecaster Eric Blake said.
Computer models are in general agreement that Subtropical Storm Joyce will move northeasterly across the Atlantic towards Europe.
On its current track, the storm will pass over or near the Azores late Saturday and Sunday and is likely to bring gusty winds and heavy rains.
"It's just extremely busy and it's even busier than past year and all at once", Blake said. One system, moving toward Texas in the Gulf Coast, was forecast at a 70 percent chance of cyclone formation over the next 48 hours. "It's a nasty interplay that makes it very hard".
Storm Helene is heading towards the UK from the Atlantic and is separate from Hurricaine Florence, which is starting to hit the eastern United States. The forecast was complicated because the storm was still small, allowing it to quickly change. Isaac is expected to dissipate within the next few days.
The latest guidance suggests that the strongest winds will occur along the south and west coasts, where gusts could exceed 130-140 kph.
The centre of Isaac passed between Martinique and Dominica with maximum sustained winds of about 45 miles per hour.
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Hurricane Warning: An announcement that sustained winds of 74 miles per hour or higher are expected somewhere within a specified area in association with a tropical cyclone.
The NHC forecast track shows the storm decreasing in forward speed as it heads west towards Central America.
With predicted rainfall measured in feet not inches, forecasters say people living along creeks and rivers in the Carolinas should move to higher ground well ahead of the storm's arrival. The biggest threats from Hurricane Florence are life-threatening storm surge, devastating flash-flooding, and hurricane-force winds.
Some slight strengthening is possible during the next couple of days.
With the ground already saturated from recent rain, another drenching from Florence combined with winds could easily lead to trees being uprooted and widespread power outages, he said.
"I don't care if this goes down to a Category 1", CNN meteorologist Chad Myers said on Thursday.
Behind Isaac, Hurricane Helene also continued to deliver good news.
Helene is now one of a glut of tropical storms in the Atlantic, with Florence sparking mass evacuations as it heads towards landfall in the USA states of North Carolina, South Carolina and Virginia.