They want to be in place when the monster storm hits, possibly as a Category 4, to assist local emergency response teams.
Hurricane to tropical-storm-force winds could extend inland depending on the storm's exact track.
The South Carolina governor ordered the state's entire coastline to be evacuated by today and predicted that 1 million people would flee.
While there remains some uncertainty around Hurricane Florence's exact path, one thing does seem certain: it will dump potentially historic rainfall amounts. She said it was built in 2009 in Buxton, North Carolina, up on a ridge, and made to withstand a hurricane.
"All the rainfall we're going to have, plus you talk about the wind - that means a lot of trees coming down, it's a lot of power outages, a lot of impacts with this system", National Hurricane Center director Ken Graham said on Tuesday. Many parts of the Appalachians have been saturated with rain over recent weeks, adding to concerns for flash flooding and mudslides inland. All models are now pointing to a direct hit from Florence in the Carolinas.
On Monday, McMaster ordered residents of eight coastal counties to evacuate.
John Johnson, a hardware store manager in Charleston, South Carolina, told AFP news agency there had been a "nonstop" rush on batteries, flashlights, plastic tarpaulins and sandbags. Those states plus Maryland and Washington DC have all declared a state of emergency. Troops have received ready-to-deploy orders for Virginia, South Carolina, and North Carolina.
Earlier, President Donald Trump said that the federal government is "absolutely, totally prepared" for Hurricane Florence as it heads toward the Eastern Seaboard.
"To the incredible citizens of North Carolina, South Carolina and the entire East Coast - the storm looks very bad!" wrote the president, who canceled a planned rally Friday in MS because of the impending storm. The current forecast track suggests the hurricane will most likely approach North Carolina or SC on Thursday as it gathers speed.
The National Oceanic and Atmospheric Administration's Goes-East weather-monitoring satellite used its lightning mapping technology for a video showing Florence, Tropical Storm Isaac and Hurricane Helene as they traveled across the Atlantic on Tuesday.