A broad and dangerous severe weather outbreak was affecting a large swath of the eastern U.S. from Ohio to Alabama over Tuesday night.

Why it matters: This is the most expansive and significant severe weather threat of the spring season to date, with high winds, hail and tornadoes threatening the affected areas. There were reports of widespread storm damage across the Ohio and Tennessee valleys.

The big picture: An intensifying, low-pressure area moving into the Midwest was intensifying. Associated warm and cold fronts are the focus areas for severe thunderstorms, some of which have already packed a punch.

• A line of severe thunderstorms swept across Kentucky and into West Virginia Tuesday — bringing 92 mph winds and prompting West Virginia Gov. Jim Justice to declare an emergency for several counties. An estimated 103,500 customers were without power in the state early Wednesday. Some 77,000 others in Wisconsin were also without electricity.

I've declared a State of Emergency for Fayette, Kanawha, Lincoln, and Nicholas Counties because of storms, which caused flooding, downed trees, power outages, and road blockages. The NWS predicts another storm event this evening in the same region.https://t.co/A0ykAvmDOZ pic.twitter.com/E1Q7sbv11n

— Governor Jim Justice (@WVGovernor) April 2, 2024

• Overall, the storm system is bringing a panoply of hazards, from heavy snow in Wisconsin and northern Michigan, where a blizzard warning is in effect, to "A few significant/long-track tornadoes" in Ohio and portions of Kentucky.
• The NWS' Storm Prediction Center was also highlighting the potential for hurricane-force (74 mph) straight-line winds in some thunderstorms, along with large hail.

Threat level: There is a "moderate risk" of severe thunderstorms in much of Ohio and portions of northern Kentucky, including the cities of Columbus, Cincinnati, Louisville, Dayton and Lexington.

• A moderate risk is a Level 4 out of 5 on the center's threat scale. An enhanced risk, which is a Level 3 out of 5 on the threat scale, covers a far larger area, and 32 million people are included in these two zones.
• The enhanced risk area includes Nashville and Knoxville, Tennessee, along with Birmingham and Huntsville, Alabama, as well as Atlanta, home to the nation's busiest airport.
• So far, tornadoes have been reported in Alabama, Illinois, Tennessee, Kentucky and Ohio.

Zoom in: The set up for this event involves moisture from the Gulf of Mexico flowing north into the Ohio Valley.

• At the same time, ahead of the low-pressure area across the Midwest, wind shear, which refers to winds that change direction and/or speed with height, is becoming increasingly abundant across the threatened areas.
• Wind shear is a key ingredient in tornado formation; it can create long-lasting, rotating thunderstorms known as supercells.
• These thunderstorms can spawn tornadoes, along with high winds and large hail.

What they're saying: "Tornadoes, damaging winds, and large hail are expected with supercells that evolve," the Storm Prediction Center said in a Tuesday morning online forecast discussion.

• "Some tornadoes may be long-lived and strong," forecasters wrote, noting the possibility of tornadoes that would be of greater than EF-2 intensity, particularly in Ohio and Kentucky.
• Meanwhile, flood watches stretch from central Illinois across large portions of Indiana, Ohio, northeast Kentucky, much of West Virginia, far southwest Pennsylvania, parts of Virginia and Maryland.
• About 24 million people live within these flood watch zones.

Context: Climate change is increasing the frequency and magnitude of precipitation extremes, including heavy downpours.

• Studies also show that climate change is altering the environment in which tornadoes form.
• A warmer, more humid atmosphere increases atmospheric instability, which gives rise to thunderstorms.
• Climate change is also suspected to be playing a role in shifting traditional "Tornado Alley" southeastward, toward more heavily populated areas in the South, Southeast and Ohio Valley.
• However, wind shear may decrease over the long-term, potentially leading to fewer tornado days.

Yes, but: When both ingredients are present, it may mean larger tornado outbreaks will occur.

The intrigue: A network outage occurred Monday night into early Tuesday morning that affected numerous NWS offices, preventing forecasters from accessing certain weather data. That has since been resolved, the NWS stated.

• The NWS has known infrastructure issues, with network failures occurring multiple times per year, including during severe weather events.

What's next: Tuesday's storm is likely to take a retro turn on Wednesday, morphing into an unusually intense and heavy elevation-dependent snowstorm in New England late Wednesday that could last into the weekend.

• This spring nor'easter is likely to bring damaging winds, and heavy, wet snow inland — likely measured in feet.

Go deeper:

• Climate change is spurring a longer spring — with consequences
• Extreme precipitation risks currently underestimated
• What we know about how climate change affects tornado outbreaks

Editor's note: This a breaking news story. Please check back for updates.