(CNN) - Hurricane Sally roared into the Alabama and Florida Panhandle coasts early Wednesday, making landfall with damaging Category 2 winds and torrential rain that -- because of its agonizingly slow forward pace -- threatens catastrophic flooding for miles.
Sally crossed land near Gulf Shores, Alabama, around 4:45 a.m. CT with sustained winds of 105 mph and higher gusts.
With Sally's crawling pace -- generally 3 mph -- some areas already have gotten 15 inches of rain and could collect up to 35 inches by storm's end.
Damaging winds already have taken a toll in both states. Pieces of hazardous debris "have become too numerous to list," police in Pensacola, Florida, warned, urging drivers to stay off roads. The National Weather Service office in Mobile, Alabama, warned people to "hunker down.
"Nothing is going to go away anytime soon," National Hurricane Center Director Ken Graham told CNN. "The winds, the torrential rainfall, the slow movement and the storm surge -- this is a dangerous situation all around."
On Florida's Pensacola Beach, sounds of transformers exploding and metal scraping along the ground -- debris from torn roofs -- could be heard early Wednesday.
Power has been knocked out for at least 459,000 customers in Alabama and Florida alone, utility tracker PowerOutage.us reported.
The National Weather Service office in Mobile declared a flash flood emergency for "severe threat to human life & catastrophic damage from a flash flood."
"This is a LIFE-THREATENING SITUATION. SEEK HIGHER GROUND NOW!!" the NWS Mobile office tweeted.
Rainfall totals of 10 to 35 inches are possible across parts of Alabama and the Florida Panhandle, from Mobile Bay to Tallahassee, forecasters say.
The storm's slow forward speed is expected to continue through Wednesday as it turns to the north and then northeast, taking with it strong winds and more flooding potential.
Central Alabama and central Georgia could eventually see 4 to 12 inches of rain, with significant flash flooding possible. Parts of the Carolinas could receive 4 to 9 inches of rain by later in the week.
Mandatory evacuations were ordered for much of the coast and low-lying areas from Mississippi to Florida, and shelters opened to accommodate evacuees.
Florida's National Guard has activated 175 members to prepare for search-and-rescue operations, according to the Division of Emergency Management.
Water repairs delayed
Those in the path of Sally are also experiencing water outages as the storm makes conditions unsafe to repair busted utilities.
Escambia County Utilities Authority issued an alert Tuesday that it's unable to respond to a significant water main break in Pensacola Beach due to storm conditions and bridge closures ahead of Sally.
The water system had to be shut off at approximately 11 p.m. as storm surge and tide levels were expected to also inundate the sewer collection system, the authority stated.
"We urge residents who are still on Pensacola Beach to store water if possible. ECUA will dispatch crews to locate the break and make repairs as soon as possible post-storm," the notice said.
Businesses close and military bases restrict access
Businesses shut down ahead of the storm, with Walmart announcing 54 closures due to Sally, company spokesman Scott Pope told CNN on Tuesday.
"We are tracking the storm in real time and have activated our Emergency Operations Center in order to support our associates in the impacted areas," Pope said.
Across the Gulf Coast, three military installations announced that only mission-essential personnel should report to work Wednesday.
The installations are the Naval Air Station Pensacola and Eglin Air Force Base in Pensacola, along with Keesler Air Force Base in Biloxi, Mississippi, home to the 53rd Weather Reconnaissance Squadron, often called the "Hurricane Hunters."
Residents prepared for a serious storm
People began preparing for Sally over the weekend, filling sandbags, grabbing supplies and prepping their homes.
Merrill Warren of Summerdale, Alabama, which sits about 16 miles inland from the Gulf, told CNN he brought in furniture, purchased gas and other supplies, and got his generator ready for the storm.
On Tuesday night, he reported that heavy rains and winds of up to 39 mph had already hit inland. Warren was more concerned about the potential for increased rainfall and surges than anything else, he said.
"This isn't the first Category 1 Hurricane that I have been through. I have been there through Hurricane Nate and Tropical Storm Gordon," Warren said. "I'm more worried about the rain for this one ... The rain and storms surge are definitely going to be the bigger issue with a storm moving at 2 mph."
CNN's Gary Tuchman, Ed Lavandera, Devon Sayers, Joe Sutton, Sharif Paget, Micahel Guy, Dave Hennen, Rebekah Riess, Kay Jones and Amanda Jackson contributed to this report.