A Category 1 Hurricane Sally is pummeling southern Alabama and the Florida Panhandle after it crossed land Wednesday morning, prompting water rescues, sapping power, dropping trees and threatening catastrophic flooding as it crawls at an agonizingly slow pace.
High-water rescues were underway Wednesday morning as homes flooded and trees toppled onto roofs in Gulf Shores, Alabama, city spokesman Grant Brown said.
Sally made landfall as a Category 2 storm near Gulf Shores around 4:45 a.m. CT with sustained winds of 105 mph. By 8 a.m. CT it was downgraded, 20 miles from Gulf Shores, with winds at 90 mph.
With Sally's slow pace -- generally 3 mph -- some areas already have collected more than 15 inches of rain and could receive up to 35 inches by storm's end.
Floodwaters have turned streets into rivers in Pensacola, Florida, images from the Associated Press show. Pieces of hazardous debris "have become too numerous to list," police there warned.
"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 more than 500,000 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.
Damage in the Mobile area
In Alabama, the floor and walls on the 16th floor of a hotel on the northern rim of Mobile Bay groaned as Sally made its way ashore.
The building shook as if in the throes an extended, low-grade earthquake, and very sturdy windows seemed poised to pop out, a CNN team there said.
As wind and rain whipped before midnight, enormous trees already had been felled west of Mobile.
Workers in raincoats endured Sally's bands as they worked alongside a digger truck to move thick piles of branches at Campfire and Ponderosa drives, CNN affiliate WALA reported.
Similar scenes unfolded around the same time -- still about six hours before Sally came ashore -- in midtown Mobile and across Mobile Bay in Fairhope, Alabama.
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, Gabe Ramirez, Michelle Krupa, Devon Sayers, Joe Sutton, Sharif Paget and Amanda Jackson contributed to this report.