The makings of a 'catastrophic, crippling' storm for Atlanta
"THIS IS A STORM OF HISTORICAL PROPORTIONS WITH POTENTIALLY CATASTROPHIC... CRIPPLING IMPACTS."
In mentioning as much as 1 inch of ice accumulations are possible:
"IN CASE IT HASNT BEEN MADE CLEAR ALREADY...THESE ARE CATASTROPHIC AND CRIPPLING TOTALS WHICH COULD RESULT IN WIDESPREAD POWER OUTAGES THAT MAY LAST FOR DAYS. IF RESIDENTS HAVE NOT COMPLETED THEIR PREPARATIONS IT MAY BE TOO LATE."
And after what the forecaster calls "Big Geeky Words" he finished up with:
"PRETTY SURE THAT THERE ARE SOME THINGS I HAVENT COVERED HERE BUT HOPEFULLY THIS LIGHT READING MATERIAL IS GOOD ENOUGH TO GET EVERYONES DAY STARTED. THE BOTTOM LINE...AS THE ENTIRE WEATHER ENTERPRISE HAS BEEN ADVERTISING FOR DAYS NOW...
"THIS-IS-AN-EVENT-OF-HISTORICAL-PROPORTIONS!! CATASTROPHIC...CRIPPLING... PARALYZING... CHOOSE YOUR ADJECTIVE. THIS IS A VERY VERY BIG DEAL ESPECIALLY FROM METRO ATLANTA EAST ALONG THE I-20 CORRIDOR...GENERALLY ANYWHERE SOUTH OF I-85 AND NORTH OF I-16 WHERE THE MOST SIGNIFICANT ICE TOTALS ARE FORECAST. STAY SAFE OUT THERE!"
(You can read the entire discussion this link)
And of course for Atlanta, this is coming just two weeks after their major storm that trapped people on the freeways hours to days.
How does this happen?
An ice storm occurs when you have cold air aloft, then a wedge of warm air in the middle altitudes, followed by freezing air at the surface. Snow falls into the warmer air and melts to rain, but then that rain encounters freezing temperatures just before reaching the ground, super-cooling the raindrop to where it freezes on impact. (Portland is quite familiar with freezing rain events when chilly outflow from the Columbia River Gorge keeps temperatures near the ground below freezing while warmer marine air moves in at the mid-levels.)
In Atlanta Wednesday morning, weather balloons showed the temperature was 22 degrees at 10,000 feet, but warmed to freezing at 6,300 feet, peaking at 39 degrees at 4,500 feet, melting the snow into rain. But Atlanta was reporting just 28 degrees at the surface, so that rain cooled to just below freezing, and turning into ice on impact -- making for a major ice storm as this process keeps accumulating ice on everything -- roads, power lines, trees, etc.
If the pool of cold air at the surface is thicker and the rain drop has a chance to freeze solid before reaching the ground, it becomes sleet. Atlanta reported sub-freezing temperatures again at 2,700 feet, not giving the rain much time to freeze before hitting the ground.