ATLANTA -- The snow and sleet have stopped falling and traffic was moving again around Atlanta following a crippling storm - but transportation and rescue officials said that didn't mean it was safe to drive yet, especially after the sun goes down.

Officials from the Georgia Department of Transportation said Wednesday night they were concerned with sub-freezing overnight lows potentially leading to layers of black ice coating roads that might appear to be safe.

Temperatures were expected to drop to about 15 degrees overnight in the Atlanta area, according to National Weather Service forecasters. Although it was supposed to be in the high 30s Thursday, it is forecast to dip below freezing again before rising into the 50s on Friday.

Whatever little leeway that we've gained with melting and slushiness is gonna refreeze, Georgia Department of Transportation spokeswoman Karlene Barron said of local road conditions. She added that the department was especially concerned with semi-trucks jackknifing and leading to additional traffic jams.

Heeding the warnings, school districts and state and local governments stretching from northwest to coastal Georgia announced that offices and classrooms would remain closed Thursday.

For a third straight day, Atlanta's airport was leading all others in the number of cancelled flights.

A total of more than 400 of Thursday's scheduled flights in and out of the airport had been cancelled by 6 a.m. EST Thursday, out of 653 nationwide, data from the flight tracking service Flight Aware showed. Many of those flights were cancelled before the day began.

Hartsfield-Jackson Atlanta International Airport is the world's busiest and a major hub for Delta Air Lines.

A storm that dropped just inches of snow Tuesday wreaked havoc across much of the South, closing highways, grounding flights and contributing to at least a dozen deaths from traffic accidents and a mobile home fire. Yet it was Atlanta, home to major corporations and the world's busiest airport, that was Exhibit A for how a Southern city could be sent reeling by winter weather that, in the North, might be no more than an inconvenience.

The Georgia State Patrol responded to more than 1,460 crashes between Tuesday morning and Wednesday evening, including two fatal crashes, and reported more than 175 injuries. About 1,000 arrivals and departures were canceled at Hartsfield-Jackson Atlanta International Airport, and thousands of schoolchildren either slept on the buses that tried and failed to get them home, or on cots in school gymnasiums. All were back home by Wednesday evening, officials said.

State transportation crews spent much of Wednesday rescuing stranded drivers and moving disabled and abandoned vehicles that littered the interstates, medians and shoulders. Gov. Nathan Deal said emergency workers, police, and the National Guard would help drivers Thursday to recover their cars and would provide them with fuel if necessary.

Crews planned to use four-wheel-drive vehicles to take motorists to vehicles they abandoned to reclaim them Thursday. State officials also said they were creating a database to help motorists locate vehicles that were towed to impound lots.

Atlanta was under a Civil Emergency until noon Thursday; only emergency vehicles were allowed on roads.

Many of the cars were marked with yellow tape to show they'd been abandoned, reports CBS News correspondent Mark Strassmann.

Elsewhere, weather conditions were improving in South Carolina after snow and ice fell in the state earlier this week.

The National Weather Service warned motorists of possible icy roads, especially in the central and northeastern parts of the state. Forecasters said black ice was a possibility as standing water freezes on roads.

Temperatures were expected to rise above freezing by noon.

North Carolina was still facing icy conditions, with dangerous roads in much of the state.

The National Weather Service issued a winter weather advisory for most of the state. The weather service said ice and snow-covered roads would be a problem at least through noon Thursday.

Nearly 300 Marines returning to North Carolina from Afghanistan were stuck at Raleigh-Durham International Airport Wednesday night due to slick roads, reports CBS affiliate WRAL-TV. The troops were on their way to Cherry Point Marine Corp Air Station.

Parts of Alabama were reopening after the winter storm, but it will be at least another day before things get back to anything like normal.

Forecasters say temperatures should rise into the upper 40s in central Alabama on Thursday. That should melt much of the remaining snow and ice from roads.

But many school systems will remain closed until Friday. Teachers and students need some rest because more than 11,000 children spent Tuesday night in school buildings, and officials say some remained there Wednesday night.

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