Despite its falling strength, forecasters said the storm's size means its impact is being felt over a wide area. She had no further details.
Jacksonville - the largest city geographically in the country - is grappling with a record storm surge and will get slammed with severe flash flooding before Irma continues her destructive northward march to Georgia and beyond. The storm, which is expected to remain at Category 3 strength, is extremely risky, packing 130 mile-per-hour winds and enormous amounts rainfall.
A high tide of 6.2 feet was forecast to hit Charleston Harbor at 12:23 p.m., the Weather Service said.
Many streets were flooded in downtown Miami and other cities.
"There's a lot of homes that have water in the them right now", Gillen said.
A state of emergency has been declared in Georgia, South Carolina, Virginia and Florida. The light green is tropical storm force winds.
South Carolina Gov. Henry McMaster issued a mandatory evacuation for some barrier islands.
Hartsfield-Jackson Atlanta International Airport, the world's busiest for passenger traffic, remained open Monday.
Irma was at one time the most powerful hurricane ever recorded in the open Atlantic, with a peak wind speed of 185 miles per hour (300 kph) last week.
Almost 4.5 million homes and businesses across Florida lost power, and utility officials said it will take weeks to restore electricity to everyone. "Life-threatening storm surge" is a danger along portions of the coasts of Florida, Georgia and SC, the NHC said.
Florida Power & Light, the state's largest electric utility, said there were almost 1 million customers without power in Miami-Dade County alone.
"The message has been clear - the Keys are going to be impacted, there is no safe area within the Keys, and you put your life in your own hands by not evacuating".
Hurricane IrmaMIAMI, FL - SEPTEMBER 10: A construction crane is damaged from the wind caused by Hurricane Irma, in Miami, Fla., on Sept. 10, 2017.
The National Weather Service said flooding rains were a major concern Monday, with 8 to 15 inches (20 to 38 centimeters) of rainfall predicted in southeast Georgia. But he had to move down to the lobby after his hotel room windows took a beating from the strong winds.
A tornado spun off by Irma was reported on the Georgia coast, and firefighters inland had to rescue several people after trees fell on their homes.
Lowndes County officials called for voluntary evacuations of their 112,000 residents, according to a news release Sunday.
Emera's Tampa Electric utility said the storm could affect up to 500,000 of the 730,000 homes and businesses it serves, and over 180,000 had already lost power. As of 6:30 p.m. Monday, almost 70 percent of Gainesville stations were empty, up four percent since that morning, while 56 percent were dry in Jacksonville.