Gordon made landfall on Tuesday night near the Alabama-Mississippi border with near hurricane-force winds, the National Hurricane Center said.
The storm reached land about 10 p.m. and brought heavy rains and strong winds.
As of early Wednesday, Gordon was about 20 miles northwest of Hattiesburg, Mississippi, with sustained winds of 40 mph.
With hurricane season in full swing, Gordon may not be the last tropical depression to strike the USA this year.
The center predicted total rain amounts of 4-8 inches (10-20 centimeters) in the Florida panhandle and parts of Alabama, Mississippi, Louisiana, Arkansas, Missouri, Iowa and IL.
A hurricane warning is in effect from the mouth of Mississippi's Pearl River to the border between Florida and Alabama.
Louisiana Governor John Bel Edwards declared a state of emergency, as did New Orleans Mayor LaToya Cantrell. Schools have been closed along the coast and governors in MS and Louisiana have already declared a state of emergency as the storm advances.
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The Gulf of Mexico is home to 17 percent of U.S. crude oil and 5 percent of natural gas output daily, according to the U.S. Energy Information Administration.
Storm surge - sea levels driven higher - of as much as 5 feet (1.5 m) could hit a stretch of coast from Shell Beach, Louisiana, to Dauphin Island, Alabama, forecasters said. As of now, there is nothing to worry about in regards to Florence for Alabama, but the East coast will have to watch it as we head into the weekend and early next week.
Boaters are evacuating to safe harbors, and motorists are fleeing barrier islands as the Gulf Coast hustles to get ready for the storm. Dauphin Island, Alabama measured a sustained wind of 57mph with a gust to 72mph.
Northwest Florida remains at risk for heavy rain and flash flooding almost 12 hours after Tropical Storm Gordon made landfall near Pascagoula, Mississippi.
"It was just very bad", she said, adding that the sounds were nearly indistinguishable from the storm's howling winds.
Hurricane season in the U.S. started on June 1 and will last until November 30.
Pat Landry, who owns the Landry House bed & breakfast on Louisiana's Grand Isle, said late Monday night that he was trying to pick up everything low in the yard in preparation for the storm surge expected with Gordon's arrival. "Now is the time to get your plans all set", he said.