The National Weather Service says more than 5.4 million people live in areas now under hurricane warnings or watches on the U.S. East Coast.
Another 4 million people are under a tropical storm watch. Assorted bad weather advisories stretched from Florida to Maine on Tuesday evening.
Those facing the most serious threat are in the Carolinas, as Category 4 Hurricane Florence barrels toward the coast, with an expected landfall Friday.
The National Hurricane Center in Miami said Tuesday that a hurricane warning had been issued from South Santee River, South Carolina, to Duck, North Carolina, and the Albemarle and Pamlico Sounds.
A Tropical Storm Watch has been issued from north of the North Carolina-Virginia border to Cape Charles Light, Virginia, and for the Chesapeake Bay south of New Point Comfort.
Forecasters at the University of Michigan predict that 2.4 million people will lose power from Hurricane Florence and some outages could be prolonged.
Nearly a hundred Louisiana emergency personnel are headed north to the Carolinas as Hurricane Florence bears down on the coastline.
That's about one-fourth the number who suffered outages from Hurricane Sandy, which hit a more populated area around New Jersey in 2012.
Seth Guikema is an associate professor of engineering at Michigan. He says outages could be more widespread if Florence veers north or stalls, leading to flooding.
The estimate is based on the National Hurricane Center's forecast for Florence's path and wind speeds.
Duke Energy spokeswoman Grace Rountree says the utility doesn't forecast outages, but is "anticipating significant widespread outages from a storm of this magnitude."
She says the company is bringing in up to 2,000 workers from Florida and the Midwest to augment its 4,600 workers in North Carolina and South Carolina.
Duke has 4 million customers in the Carolinas.
The second tropical system in a matter of days is expected to form in the Gulf of Mexico in the coming days, forecasters said.