Tens of millions of Americans, from the South through the Midwest, will face the threat of dangerous weather on Friday and Saturday.

A dynamic, springlike storm system will unleash a line of thunderstorms with damaging winds and risk of tornadoes from Texas through parts of the Deep South.

“Severe thunderstorms are expected from eastern OK/TX to western GA/FL Panhandle tomorrow into Saturday,” the National Weather Service warned on Twitter. “Intense, damaging wind gusts and strong tornadoes will be possible. Don’t be caught off-guard by severe storms in January; share this info with family and friends!”

Meanwhile, the cold front associated with the storm will help to focus heavy rainfall along a broad swath of the nation’s midsection through the Midwest and the Ohio Valley.

“[R]ainfall totals are forecast to be 3 to 5 inches on Friday and Friday night in a swath from far northeastern Texas to eastern Oklahoma, northwestern Arkansas, and southwestern Missouri,” the National Weather Service wrote. “Thus, there is a Moderate Risk of flash flooding in those areas for Friday. Flood and Flash Flood Watches are in effect across much of Missouri, Illinois, and Indiana ahead of the rainfall.”

In the cold air on the storm’s backside, ice may glaze sections of the Plains and Midwest, while a shot of heavy snow is even possible in the Great Lakes late Saturday into Sunday.

With nasty weather in the offing for many, we break down the timing, hazards and impacts:

The setup


The European ECMWF depicts surface low pressure moving up the Mississippi Valley on Saturday after forming over the Red River along the Texas/Oklahoma border Friday. (ECMWF/WeatherBell.com)

The slurry of chaotic weather is thanks to a low-pressure zone that will ride along an area of steep temperature contrasts draped southwest to northeast across the eastern Lower 48.

The low-pressure zone will begin to take shape Friday morning along the Texas/Oklahoma border near the Red River, strengthening throughout the day and moving northeast. Ahead of it, anomalously warm temperatures, in some places characteristic of early springtime, will overspread much of the Deep South and Gulf Coast. As cold air streaming in from high altitudes encounters the warm, moist tropical air, showers and thunderstorms will erupt.


The North American Mesoscale model (NAM) depicts a roaring low-level jet stream, with 70 mph winds barely a mile above the ground Saturday. (NOAA/WeatherBell.com)

The storms will be intensified by an exceptionally vigorous low-level jet steam carrying a river of warm, moisture-rich air north at highway speeds barely a mile above the ground. In addition to nudging the balmy, humid air mass farther northward, it will also help to increase wind shear — or a change of wind speed/direction with height — across areas ahead of the low and its associated front. That will help storms to rotate, setting up a tornado risk from eastern Texas into Georgia on Friday and Saturday. These roaring winds just above the surface may also be drawn to the ground in thunderstorm downbursts.

Friday


The Storm Prediction Center has issued a moderate risk of severe weather for a small portion of extreme eastern Texas, northwestern Louisiana and southwest Arkansas for Friday. (NOAA/SPC)

Thunderstorms should initiate just east of Interstate 35 corridor in Texas and Oklahoma around lunchtime Friday.

As the afternoon wears on, storms will fill in, forming a semi-continuous line along the cold front as far south as Interstate 10 in Texas. This includes areas east of Austin, as well as Houston.

Overall, the greatest threat Friday exists in eastern Texas and Oklahoma, southwest Arkansas and western Louisiana, including Dallas.

The Storm Prediction Center has declared a moderate risk of severe weather for a small portion of extreme eastern Texas, northwestern Louisiana and southwest Arkansas.


An unstable air mass overspreads much of the South on Friday and Saturday, measured by this European model simulation of CAPE. CAPE, or Convective Available Potential Energy, is a measure of how primed the atmosphere is for storms. Values over 1,000 indicate an unstable atmosphere. (WeatherBell.com)

The band of storms that develops will be known as a “quasi-linear convective system,” or QLCS. It is not important to remember the name, but it is key to understand this bendy line of storms will likely produce widespread strong to damaging winds as its primary concern.

At the same time, the aforementioned “wind shear” will foster areas of spin within the line, presenting the threat of a few erratic, quick-hitting tornadoes anywhere along the line. Hail does not appear to be of significant concern.

That line of storms is feeding off the approaching cold frontal boundary. Ahead of it, individual supercell thunderstorms may develop in the “warm sector” of the sprawling storm system. Any rotating supercells that form could produce tornadoes — one or two perhaps being on the strong side — as well as large hail and damaging winds.

Whether supercells materialize in this region ahead of the squall-like QLCS is a question of temperature. If cloud cover from the approaching QLCS limits daytime heating, then the energy available to fuel early supercells would be less.

But, as many models indicate, if temperatures can warm sufficiently, supercells will form.

The QLCS will continue marching east overnight, presenting a damaging wind and widely scattered tornado risk throughout the night. This is a danger, particularly considering the populated areas potentially impacted.

The line should make it to near the Mississippi River near midnight.

Saturday


The Storm Prediction Center's forecast for severe weather depicts an “enhanced” risk for portions of Mississippi and Alabama on Saturday. (NOAA/SPC)

On Saturday, the area to watch will primarily include eastern Louisiana, Mississippi and Alabama — although conditions marginally supportive of severe thunderstorms could reach as far north as Nashville and as far east as Atlanta.

The QLCS will suffer a bit of weakening overnight Friday into Saturday morning in response to the loss of solar heating, but it will surge in intensity again Saturday by noon.

Similar threats will exist, with wind and occasional tornadoes along the line, and potential supercells ahead of it.

There are some indications that tornado potential may be a greater concern, particularly for eastern Mississippi and western/central Alabama.

A few strong to locally severe thunderstorms may make it to the Georgia border or even the Appalachians by midnight Saturday night or very early Sunday morning.

Flooding potential


The American GFS model depicts a band of very heavy rain coming down through Sunday night. (WeatherBell.com)

As if that was not enough, we are also closely monitoring the potential for some very heavy rain and localized flood concerns, particularly through Saturday.

Friday will begin with drenching rainfall in eastern Oklahoma and parts of Arkansas as storms get going. The Weather Prediction Center has a “moderate risk” of excessive rainfall.

Moisture being conveyor-belted northward will reach a “brick wall” — the frontal system stretching up the Mississippi Valley and into the Midwest.


A plume of exceptional moisture will ride northward as shown in this ECMWF model simulation. It depicts “precipitable water,” a measure of how much moisture exists in a column of atmosphere at any given time. Values surpassing 1.5 inches favor very heavy downpours. (ECMWF/WeatherBell.com)

That will focus repeated rounds of heavy rainfall in a band from eastern Texas and Louisiana north toward pockets of Illinois, Indiana and the Ohio Valley.

A widespread 2 to 4 inches can be expected, with isolated 5 or 6 inch amounts possible.

Heavy precipitation may even make it into interior northern New England on Sunday. Atmospheric moisture values may near record territory for January in some areas.

Ice and snow risk


The European ECMWF model indicates potential heavy snow falling west of Lake Michigan through Sunday. However, our forecast calls for totals about 30 to 40 percent less than those depicted here based on forecaster intuition. Regardless, the geographic extent of snow is evident. (ECMWF/WeatherBell.com)

There is even the risk of some wintry weather in parts of the Central Plains, Midwest and Great Lakes. This will occur as moisture overruns the cold air behind the storm front.

Freezing rain is possible in parts of northeast Missouri and western Illinois, perhaps with more than a tenth of an inch of glaze accreting late Friday and Saturday. Late Saturday could also feature a hazardous glaze of freezing rain in central Michigan.

At the same time, a coating of snow is possible from northern Oklahoma up through Kansas City and northwestern Missouri, southeastern Iowa and northwestern Illinois. A few spots may see up to several inches.

More substantial snows may develop during Saturday night into Sunday in the eastern half of Wisconsin and northern Michigan, where some areas could see double-digit totals.