(CNN) - A significant winter storm will take shape beginning Friday over the Southern Plains and Texas, then spread this weekend through the Southeast and Mid-Atlantic.
How far south freezing air will reach remains an open question, making it difficult to precisely forecast ice and snow accumulation.
Winter storm watches are already in place Thursday for 6 million people in northern Texas, Oklahoma and parts of Arkansas and Missouri.
As the storm develops, forecast models will more accurately predict the freezing line and thus the type of precipitation places will experience, CNN meteorologist Brandon Miller said.
Here is what CNN meteorologists expect, where and when:
Texas and Oklahoma
Timing: Friday morning through Saturday evening
- The primary precipitation will begin Friday, with freezing rain and drizzle in the panhandles of Texas and Oklahoma and possibly into central Oklahoma. The main winter weather impacts will occur late Friday, with a quick transition from a wintry mix to snow after midnight Friday for places including Oklahoma City.
- Precipitation in eastern Oklahoma won't turn to snow until later Saturday.
Impacts: Flooding rain, ice and snow
- Ice accumulations in Texas and Oklahoma could reach one-quarter to one-half inch, respectively.
- Snowfall in these areas could also top 6 inches. Expect dangerous travel conditions and power outages.
- Oklahoma City is expected to get one-10th to one-quarter of an inch of ice, plus 4 to 6 inches of snow.
- Tulsa, Oklahoma, will see less ice and snow, with ice accumulations near one-10th of an inch and snow of 2 to 3 inches.
- In Texas, the wintry weather will stay in the panhandle and the Red River Valley.
- Large amounts of rain will impact major Texas cities, including Dallas and Houston.
- More than 13 million people are under a flood or flash flood watch in eastern Texas. Cities like San Antonio and Houston could see flash flooding as rainfall total could reach 3 to 6 inches and isolated spots could see as much as 10 inches.
Arkansas and western Tennessee
Timing: Friday afternoon through Saturday
- Most of Arkansas won't feel the precipitation until later Friday, with some rain early in the south and central portions of the state.
- Winter weather won't begin until the overnight hours and will transition eastward Saturday morning.
Impacts: Flooding rains, wintry mix, mountain snow
- The southern half of Arkansas will get caught up in a similar situation as East Texas, with flash flooding possible. The Arkansas-Louisiana-Texas region could see 4 to 5 inches of rain.
- Winter precipitation will be likely on Saturday morning and through the day Saturday in Arkansas and western Tennessee, mostly north of Interstate 40.
- One forecast model shows colder air filtering a bit further south, potentially impacting Little Rock, Arkansas, and Memphis.
- Right now, Little Rock looks like it will pick up a maximum 1 inch, with little or no accumulation possible. Fayetteville and Bentonville, in Arkansas, meantime, will see 3 to 4 inches. In the higher elevations of the Ozark Mountains, 6 to 8 inches of snow is possible on top of a small glaze of ice to one-quarter inch.
- Snow totals will be lower in northeastern Arkansas and northwestern Tennessee, where 1 to 2 inches is possible.
Eastern Tennessee, northern Georgia and the Carolinas
Timing: Saturday morning through Monday morning
- Precipitation begins to creep into the region early Saturday morning, with rain in Georgia and some light, wintry precipitation in the mountains.
- From there, things get interesting. The situation will begin to pick up Saturday afternoon, with larger accumulations Sunday into early Monday morning.
Impacts: Record snow, blizzard conditions, wintry mix
A significant amount of snow across the mountains of North Carolina, South Carolina and northeast Georgia is expected but not certain.
If warmer air mixes in, it could lessen the amount of snow that accumulates. It all depends on a regional weather phenomenon called "cold-air damming" -- when cold air is forced from the northeast and pushes against the Appalachians. The mountain range acts as a dam, keeping the cold air pooled like a lake.
This shallow layer of cold air allows for freezing temperatures to remain near the surface as warmer, moist air moves over it. If the air column stays cold enough near the surface and extends high enough into the atmosphere, moisture will fall as snow. If air above freezing gets into the middle layer, there is a greater chance for ice pellets or even a wintry mix.
Right now, most forecast models show this lasting through the weekend and into early Monday.
- Early snow estimates range from 1 foot to almost 2 feet across the mountains and foothills west of Interstate 85.
- This means portions of North Carolina could see record snowfall. "This could be a once-in-a-generation event for areas that experience mostly snow and ice," the National Weather Service in Greenville, South Carolina, said Thursday.
- There is even the possibility on Sunday afternoon for blizzard conditions in the higher elevations of the Appalachians.
- North Carolina cities impacted by wintry precipitation will be Asheville, Greensboro, Raleigh and Charlotte.
The amount of snow in areas east of I-85 largely depends on how established the cold-air damming, also called "the wedge" in this region, becomes.
"(We) will hold of(f) on getting too cute with any snow amounts at this time given the degree of uncertainty and how the varying p(recipitation) types would impact accumulations," the National Weather Service in Raleigh said Thursday.
While most models keep the winter precipitation tucked into Carolinas, there is one model that shows the possibility of freezing rain reaching down the I-85 corridor to impact Atlanta on Sunday night into Monday.
The storm moves out to sea Monday, models showed Thursday. It also could turn and climb up the East Coast, bringing heavy snow to major metro areas, though this seems unlikely.
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