A powerful storm system is unleashing a severe thunderstorm outbreak across the southern Plains and portions of the Mississippi Valley, expected to last from Friday into Saturday, with widespread damaging winds and the potential for tornadoes, a few of which could be strong, along with heavy rainfall that could cause flooding. Accompanying threats include a full-on ice storm and heavy snow.

The Storm Prediction Center (SPC) has issued a “moderate risk” outlook of severe weather for the areas most likely to see damaging straight-line winds as the thunderstorms barrel through Friday afternoon into the overnight hours.

As storms approach southern Arkansas and northern Louisiana tonight, “a couple long-track tornadoes are possible,” according to the SPC. With vicious storms continuing well after nightfall, the stage is set for a notably dangerous event.


The Storm Prediction Center has issued a moderate risk outlook of severe weather for Friday. (NOAA/SPC)

Tornadoes that occur at night are especially dangerous because of the challenge of warning people who may be asleep.

Hail nearing the size of golf balls has already accompanied strong storms Friday morning in Oklahoma. A severe thunderstorm watch was issued early in the day, and by early afternoon, the first tornado watch of the day had also been issued for parts of the state.

The potential exists for a long-lived squall line capable of producing significant, damaging winds over multiple states. This may even qualify as a derecho event, depending how widespread the wind damage is.

In addition, along the squall line itself, quick-hitting tornadoes may form sporadically, with perhaps a few supercell thunderstorms popping ahead of the main line and lending themselves to an increased tornado risk. One or two of these tornadoes could be on the strong side.


Weather alerts issued by the National Weather Service early Friday. (Pivotal Weather)

Wind gusts greater than 70 mph will probably be common with this event. Power outages are possible, especially near the western border of southern Arkansas and northwestern Louisiana. If you live in this area and your house does not have a place in which to shelter during storms, consider spending the night with friends or relatives whose house does. Charge portable electronic devices and cellphones before the onset of hazardous weather.

Storms will continue Saturday across Mississippi and Alabama. The National Weather Service office in Birmingham advised residents to consider sheltering for severe thunderstorms as if they were tornadoes given the potentially destructive straight-line winds.

Meanwhile, heavy rainfall caused by this system will lead to potential flooding farther north, up the Mississippi Valley and into the Corn Belt region, affecting Illinois, Indiana and parts of the Ohio Valley, as well.

On the storm’s backside, encroaching cold air will flip rain to snow. In fact, some locales are expected to face both strong thunderstorms and snow in the same day. Alva, Okla., found itself under a severe thunderstorm warning and a winter storm watch at the same time Friday morning.


The HRRR model's projection for surface radar imager for Friday afternoon. Notice strong storms forming a line, while precipitation on the backside falls as snow. (WeatherBell)

Ahead of this line, the development of supercells hinges on whether cloud cover from the main complex limits daytime heating and destabilization Friday.

Saturday


The Storm Prediction Center has issued an enhanced risk of severe weather for Saturday. (NOAA/SPC)

Overnight, the low level jet stream will strengthen as it races north out of the Gulf of Mexico. Winds exceeding hurricane force will be present barely a mile above the ground. This will further fuel storm strength as the line redevelops Saturday morning and eyes portions of Dixie Alley.


The NAM model's depiction of the low-level jet ramping up in intensity early Saturday. This map shows forecast wind speeds about a half-mile above the ground. (WeatherBell)

Alabama and Mississippi are most at risk for Saturday’s storms, which may feature a continued tornado hazard.

One uncertainty concerns exactly how far north the low-pressure system’s “warm sector” gets. That’s the zone ahead of the system’s center where strong southerly winds gather heat and moisture, and it marks where the most unstable air mass will be located.

By Sunday, the severe risk is modeled to wane significantly east of the Appalachians. However, a subtle, marginal risk of severe storms may materialize in the lee of the mountains and in a few spots across the Southeast.

Flood risk


Heavy rainfall as simulated by the European model through early Sunday. (WeatherBell)

In addition to the severe storms, flooding will be a problem as the powerful low-level jet stream will transport copious amounts of moisture northward.

Flash-flood watches stretch from Oklahoma and north-central Texas up the Mississippi Valley and near the Great Lakes, including Chicago and approaching Cleveland. A widespread 2 to 4 inches of rain is possible late Friday through Saturday, with localized 5-inch amounts.

Heavy rain is possible in the Northeast on Sunday.

Trouble on the storm’s wintry side

Behind the system, precipitation will fall in the form of heavy snow. Along the boundary between chilly air to the north and unseasonably mild air to the south, an ice storm is possible.

In northern Oklahoma, a coating to an inch of snow is likely, beginning Friday night and wrapping up Saturday. A general 1 to 2 inches appears likely in much of northwestern Missouri and eastern Kansas. Kansas City will find itself beneath a blanket of white.

A plowable snowfall, perhaps approaching half a foot, is possible in northeastern Iowa, extreme northwest Illinois, and much of southern/central Wisconsin. Additional heavy snow will target northern Michigan.

Meanwhile, a narrow zone of ice accretion is possible somewhere along the Mississippi River region that divides Iowa from Illinois. That strip will extend northeastward; about a quarter-inch of glaze, give or take, is possible, which will cause slick travel and possible power outages.

More formidable icing can be anticipated in central Michigan, including Lansing and Flint. A full-fledged ice storm is possible there. In fact, temperatures aloft may approach 50 degrees, while surface temperatures will hover in the 20s. It’s the classic setup for rain falling and freezing, clinging to trees and power lines, potentially bringing both down.