Millions brace for tornado outbreak as highest severe weather risk level issued on Saturday

Fast Facts:

• An outbreak of severe storms is likely Friday afternoon and potentially into the overnight hours as a powerful storm system intensifies across the central U.S.
• Highest risk level issued for severe weather as tornado outbreak expected on Saturday.
• The final day of storms will slam the East Coast on Sunday from Florida to the Northeast.

A widespread and dangerous severe weather outbreak is expected to develop Friday and into the weekend as a powerful storm system unleashes its fury across the central and eastern U.S., impacting more than 150 million people. The Storm Prediction Center upgraded Saturday's risk to its highest level as a tornado outbreak is now expected. 

"Today may end up as one of the more prolific severe weather outbreaks in recent memory in the bi-state region," the National Weather Service in St. Louis wrote in a discussion Friday morning.

The FOX Forecast Center said that multiple days of potent thunderstorms could bring destructive straight-line winds up to 100 mph, hail up to the size of baseballs and significant long-tracked tornado activity (EF-3 or higher).

The main action Friday will come in the form of a powerful squall line of storms which is expected to develop during the afternoon across Missouri and Iowa and charge east through the Mississippi Valley, the FOX Forecast Center said. 

"This storm has it all," Bill Bunting, operations branch chief for NOAA and the National Weather Service Storm Prediction Center (SPC), told FOX Weather. "The moisture is plentiful. And so our fear is all of these ingredients – the wind shear, the moisture, the lift – are going to combine to produce a very explosive and potentially deadly attack beginning this afternoon, continuing overnight and also on Saturday as you look farther south and east."

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The highest risk for tornadoes is across 750 miles from Davenport, Iowa, to Jackson, Mississippi. The overall tornado threat is more than 1,000 miles long. The states with the most concern look to be centered in the mid-Mississippi Valley, including cities such as St. Louis, Missouri; Memphis, Tennessee; Paducah, Kentucky; and Little Rock, Arkansas.

Anticipating widespread severe weather, Alabama Gov. Kay Ivey declared a state of emergency Friday for all 67 counties and put the Alabama National Guard on standby.

Multiple intense, long-tracked tornadoes possible Saturday

On Saturday, the powerful storm system will continue tracking east as a tornado outbreak is possible across the central Gulf Coast states and Deep South into the Tennessee Valley.

A Level 5 out of 5 on the severe storm threat level is impacting 2.7 million people in major cities like Birmingham, Alabama, Jackson, Mississippi, Tuscaloosa and Hoover in Alabama and Hattiesburg, Mississippi.

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The storms will develop along the Mississippi River and quickly move east from midday to the afternoon, according to the FOX Forecast Center. The line of supercells is then expected to swing through central and southern Mississippi into northern Alabama, central and east Tennessee and north Georgia. 

Cities such as New Orleans, Louisiana, and Birmingham, Alabama, are under a Level 4 out of 5 risk, indicating a high likelihood of supercell thunderstorms capable of producing tornadoes and damaging winds. 

Final day of storms likely to slam East Coast on Sunday

By Sunday, the storm will have traversed the entire U.S. with its eye now on the East Coast, including the Interstate 95 corridor. The threat of tornadoes will be confined to the Virginia coast and south into the Carolinas. Damaging wind gusts and large hail will be the main threats.

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Like on Friday and Saturday, plenty of wind shear will be present, allowing any singular storm to rotate and produce a tornado, according to the FOX Forecast Center.

North of the Delmarva into the Northeast, severe storms will also be possible.