"It's a nasty interplay that makes it very hard".
Flaherty said there is a small piece of good news, in that the "flow" in the southeastern quadrant of the storm has been reduced slightly.
It's now expected to continue weakening and dissipate in five days.
At her most fierce, wind speeds of up to 85mph were being recorded south west of the Azores in the Atlantic.
The centre of Isaac passed between Martinique and Dominica with maximum sustained winds of about 45 miles per hour.
Tropical storm warnings were in effect for Martinique, Dominica and Guadeloupe, while watches were in effect for Montserrat, St. Kitts and Nevis, Antigua, and Saba and St. Eustatius.
Some roads and bridges may be forced to close, while fallen trees could present an additional hazard.
Helene, which has been downgraded from a hurricane, is among a glut of tropical storms brewing in the region, with mass evacuations under way in southern USA states as Hurricane Florence barrels across Virginia and the Carolinas.
As of 11 a.m., Florence was located at 33.4 N, 75.5 W, about 145 miles (230 km) ESE of Wilmington, North Carolina, and about 195 miles (315 km) east of Myrtle Beach, South Carolina. Storm winds could intensify through the day and tonight, but weaken slightly Thursday as it nears the coast.More news: Woodward: Mattis, Kelly denials of his book quotes untrue
The big question is whether the high pressure ridges can push Florence substantially to the south or whether will Florence hold its ground as they clash.
"As long as it seems there is a threat to lives, we want to make sure people are safe", Shaw said.
Helene is now 2,065km south-southwest of the Azores in the Atlantic and Met Éireann said the storm would approach the south coast by Tuesday.
A system in the central Gulf of Mexico was a little less certain.
The agency is also monitoring a tropical disturbance that has a 60 percent chance of forming a tropical storm.
Current guidance suggests the potential for warning level winds and perhaps rain with the system.
Hurricane Florence is now generating waves up to 83 feet, almost double the height measured on Tuesday.
Here's all you need to know about Hurricane Helene which is set to arrive close to the United Kingdom on Tuesday.