Unsure of where Hurricane Dorian might strike, Florida residents watched the increasingly dire forecast with a sense of helplessness Friday and braced for what could be the most powerful storm to hit the state's east coast in nearly 30 years.

President Donald Trump - whose Mar-a-Lago resort in Palm Beach was in the crosshairs - warned it could be an "absolute monster."

"All indications are it's going to hit very hard and it's going to be very big," Trump said in a tweeted video, comparing Dorian to Hurricane Andrew, which obliterated thousands of homes south of Miami with winds topping 165 mph (266 kph) in 1992.

The National Hurricane Center said the Category 2 storm is expected to strengthen into a potentially catastrophic Category 4 with winds of almost 140 mph (225 kph) and slam into the U.S. on Tuesday - nearly 10 mph faster and a day later than previously forecast.

The hurricane center's projected track showed the storm hitting around Palm Beach County, the site of Mar-a-Lago. But predicting its course with any confidence this far out is so difficult that the "cone of uncertainty" on the map covered nearly all of Florida's 500-mile (800-kilometer) coastline, with Miami, Fort Lauderdale and Orlando all within the danger zone.