00:48 Sioux Falls Tornado Rips Apart Building in Newly Released Footage Newly released footage from inside a health center in Sioux Falls shows the damage done by an EF-2 tornado that struck the city one year ago.

At a Glance Sunday was the deadliest U.S. tornado day in years.

Severe weather was in the forecast before the outbreak began.

It was uncertain how many tornadic supercells there would be. Sunday was the deadliest tornado day in the United States in nearly six years, and while severe weather was forecast for the South well in advance, there was uncertainty in how many tornadic thunderstorms would develop in the hours before it began.

There were three dozen reports of tornadoes in Alabama, Florida, Georgia and South Carolina Sunday, all within a few hours from early afternoon into early evening. The exact number of tornadoes that caused destruction in those states won't be known for several days after the National Weather Service conducts surveys and assigns ratings.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/storm-rports.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/storm-rports.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/storm-rports.jpg?v=ap&w=980&h=551 800w" > Each red circle is a report of a tornado from Sunday, March 3, 2019. Some are multiple reports of the same tornado.

The outbreak killed 23 people in Lee County, Alabama. That's more than double the 10 tornado-related deaths that occurred in 2018, which had the fewest for a calendar year on record .

The death toll from Sunday is also the largest from tornadoes in a single day since the Moore, Oklahoma, EF5 tornado killed 24 people on May 20, 2013.

About 24 hours before the outbreak began, NOAA's Storm Prediction Center (SPC) placed parts of southern Alabama and southwest Georgia in an enhanced risk for severe weather, which means numerous severe storms are possible . That is the third highest of five categories SPC can issue in its severe thunderstorm outlooks for a particular day.

That enhanced risk remained in place much of Sunday, but was increased in size to cover larger parts of southern Alabama as well as central and southern Georgia.

Conditions in the atmosphere were favorable for the formation of severe thunderstorms packing damaging winds and possible tornadoes.

The ingredients included a potent jet stream disturbance pivoting across the South which intercepted ample moisture lifted northward from the Gulf of Mexico by a warm front. That warm front in combination with a wave of low pressure and strong low-level winds helped add to the wind shear, or spin, in the atmosphere required for the development of tornadoes.

But there was still uncertainty Sunday morning how the event would unfold, specifically whether numerous tornadic supercells would develop.

(MORE: What Is a Supercell?)

"Tornado-event density, and risk of significant tornadoes, still is somewhat unclear – being strongly dependent on existence/number of preceding supercells that can develop in the weakly forced, but also weakly capped, warm sector," SPC said in its update about 7 a.m. CST Sunday .

What SPC was explaining is that it was uncertain whether numerous individual supercell thunderstorms would form in the enhanced risk area from southern Alabama into Georgia. Supercell thunderstorms are typically responsible for spawning more and sometimes stronger tornadoes.

This is in contrast to a severe weather event that features a squall line of thunderstorms where damaging wind gusts are the primary threat, though embedded isolated and typically briefer tornadoes can still occur. That scenario being the dominant threat was also a possibility in this setup.

It was later Sunday morning when it became clear that numerous supercells would develop ahead of a squall line and produce tornadoes, according to a tweet from SPC warning coordination meteorologist Patrick Marsh.

Once it became certain that supercells would pose a significant danger, SPC made a remarkable short-term forecast that included Lee County, Alabama. At 1 p.m. CST, SPC highlighted a few counties in southeast Alabama, including a part of Lee County, and said a strong tornado was possible in the area in next 30 to 60 minutes.

The first tornado warning for Lee County was issued at 1:58 p.m. CST Sunday.

Doppler Radar detected strong rotation and lofted debris shortly thereafter, indicating a tornado was causing considerable damage and prompting the National Weather Service (NWS) to declare a tornado emergency about 2:10 p.m. CST. A tornado emergency is "an exceedingly rare tornado warning issued when there is a severe threat to human life and catastrophic damage from an imminent or ongoing tornado," the NWS says.

Reports of destruction came in from the hard-hit Beuaregard area not long after that at 2:40 p.m. CST .

Sunday's outbreak provided an example of how a severe weather threat can be accurately forecast. It also illustrates why there can still be lingering uncertainty hours before the event begins and why you should always have a way to get warnings whether the threat level is low or high.