The April 25-28, 2011, "Super Outbreak" was one of the worst tornado outbreaks in U.S. history. It claimed more than 300 lives, injured 2,775 and was responsible for $10.8 billion in total damage, making it the costliest tornado outbreak in U.S. history.  

We've compiled a list of the most jaw-dropping facets of this outbreak that we'll never forget.

A Swarm of Tornadoes

An incredible 349 tornadoes were spawned over just over 72 hours from April 25-28, 2011, according to The Weather Channel severe weather expert, Dr. Greg Forbes. While the Deep South - Alabama, Mississippi, Georgia and Tennessee - bore the brunt of it, tornadoes were reported as far north as central New York.
 

Tornado Plows a Field

The first of four EF5 tornadoes on April 27, 2011, raked through east-central Mississippi, leaving some incredible damage in its wake.  

Northeast of Philadelphia, Mississippi, a swath of ground was scoured out to a depth of two feet, in some places. The tornado even ripped sections of asphalt from a road.

(MORE: NWS summary with photos | Most Bizarre Tornado Damage Photos)
 

Off the Chart Parameters

The combination of instability (warm, humid air near the surface overlaid by cold, dry air aloft) and wind shear in the atmosphere, particularly at low levels crucial for the spawning of tornadoes was quite simply in another realm on April 27, 2011.

Meteorologist and researcher Jon Davies blogged about the rarity of seeing such extreme values of shear and unstable air. Typically, any high values of one are at least somewhat tempered by lower values of another. Not on April 27.

In May 2011, Davies wrote, "I've looked through my own databases of tornado cases over the past decade, and can't find any tornado environment that even approaches this one."

The "significant tornado parameter" or STP, uses a combination of both wind shear and instability in low levels to also diagnose the tornado risk. A value of 1 is considered an environment capable of at least F/EF2 tornadoes. 

On April 27, STP values reached values from 10-12 in eastern Mississippi and Alabama in the afternoon (see Fig. 2 in the definitive April 27 outbreak BAMS paper from Knupp, et al.)

No wonder there were so many long-track, violent tornadoes. 
 

Watching the Tuscaloosa Tornado Live

Perhaps the most indelible live coverage of this Super Outbreak was the split screen shown on ABC 33/40 in Birmingham, Alabama, showing both a live cam of a wedge tornado roaring through Tuscaloosa, Alabama, as meteorologist James Spann was showing the classic hook echo and debris ball (indicating of tornado-lofted debris) on radar. 

(MORE: A Meteorologist's Marathon Broadcast of the Superoutbreak)

In roughly 90 minutes, the EF4 tornado claimed 65 lives along an 80-mile path from Greene County, Alabama, through Tuscaloosa into the northern suburbs of Birmingham. 

Despite the deadly toll, we'll never truly know how many lives were saved by timely National Weather Service warnings, and the considerable effort of the media to communicate those warnings. 
 

A Supercell's Long Journey

The term supercell refers to a usually rotating thunderstorm that is persistent, on a timescale much longer than a single, ordinary thunderstorm you may see, for instance, in the summer over the Southeast. 

The parent supercell spawning the Tuscaloosa and west Birmingham EF4 tornadoes on April 7, 2011, was tracked on radar for over seven hours from its formation in Newton County, Mississippi, to Macon County, North Carolina, after which it finally fizzled. One supercell, 380 miles, four states.

(MORE:  Tuscaloosa, Alabama...one of the worst U.S. tornadoes)
 

Mobile Home Thrown 300 Yards, Obliterated; Cars "Unrecognizable"

The same EF5 tornado that scoured the ground near Philadelphia, Mississippi, claimed its three lives in an unfathomable way.

The National Weather Service - Jackson, Mississippi, storm survey report tells the story.

The three fatalities occurred in northwest Kemper County when a strapped down doublewide mobile home was thrown a distance of approximately 300 yards into a treeline, and then obliterated with the debris and framing scattered many hundreds of yards down the path. There was no indication of ground impacts between the original site of the mobile home and where it ended up to indicate that the mobile home bounced extensively as it traveled. 

Also seen almost one month later in Joplin, Missouri, this tornado also left cars crushed into almost unrecognizable balls, similar to what you can do with aluminum foil.
 

One City: Twice in One Day

Another sinister aspect to the 2011 Super Outbreak was the repeated rounds of severe thunderstorms and tornadoes. The city of Cordova, Alabama, northwest of Birmingham, was an unfortunate example on April 27, 2011.  

Around 5:20 a.m. CDT, a tornado intensified quickly as it tracked into the city of just over 2,000 residents. Damage there was rated EF3 on the Enhanced Fujita scale.  

Later that afternoon, another long-track tornado approached the city. Sadly, after weakening quickly west of town, the tornado strengthened quickly as it moved into Cordova, carving a one-half-mile wide path of damage rated EF3 in the downtown area.

Incredibly, Marshall County, Alabama, was affected 15 separate times by a tornado on April 27, 2011.
 

Three Separate Rounds of Tornadoes in One Day

It not uncommon for a morning cluster of severe t-storms to be followed by a more dangerous round of afternoon supercells. The morning t-storm cluster typically produces damaging winds, flooding rain, perhaps a few tornadoes.

An early-morning squall line in the Tennessee Valley on April 27, 2011, produced an incredible 76 tornadoes in Mississippi, Alabama, east Tennessee, and north Georgia, according to the Knupp et al. 2014 paper. This included some long-track EF3 tornadoes, an intensity quite rare for early-morning squall lines.

That would be an impressive outbreak alone.

Just hours later, a second line of thunderstorms in the late morning hours spawned another seven mainly weak EF0 or EF1 tornadoes in northern Alabama. 

Finally, the most destructive and deadly supercells ripped across Mississippi, Alabama, north Georgia, east Tennessee and far southwest Virginia that afternoon and evening.  
 

Tornado Paths Total:  Thousands of Miles

The individual tornado paths from the April 25-28, 2011, Super Outbreak added up to an almost unfathomable 3,200 path miles, by far the greatest of any U.S. outbreak. This total path was over 600 miles longer than the April 3-4, 1974, Super Outbreak.

If one could place these tornado paths end to end, it would be roughly equal to the driving distance from Portland, Maine, to San Francisco.

In 2012, Engineering Analysis Incorporated released a study saying 939 square miles of Alabama land was disturbed by tornadoes in 2011, the greatest amount for any state, for any year dating to 1950.

This is 1.85 percent of the entire land area of the state, and is greater than the average size of an entire Alabma county (757 square miles).
 

One "Powerful" Outbreak

The Tennessee Valley Authority's electrical generation and distribution took a massive hit from the 2011 Super Outbreak.

Over 300 transmission towers, such as the one pictured above, were mangled on April 27, 2011. Given this, two power plants were forced to shut down, including the TVA's Browns Ferry Nuclear Power Plant near Decatur, Alabama.

Power outages affected Huntsville, Alabama, for several days, affecting everything from gas stations to traffic signals.

The Knupp et al. study concluded power outages caused by the morning pair of squall lines, including non-functioning outdoor sirens and NOAA weather radio transmitter outages, reduced the ability to get warnings out to the public during the most deadly afternoon and evening supercells in central and northern Alabama.
 

A Swarm of Tornado Warnings

A total of 303 tornado warnings were issued by the six most-affected National Weather Service Forecast offices on April 27, 2011, alone.  

The NWS-Huntsville, Alabama office issued 90 tornado warnings, alone that day. A pair of additional tornado warnings for the NWS-Huntsville's area of responsibility were issued by the NWS office in Jackson, Mississippi, while Huntsville employees themselves took shelter.

The average lead time for tornado warnings April 27 among those six NWS offices was found to be from 20-22 minutes.  
 

Appliances "Shredded or Missing"

One of the four EF5 tornadoes on April 27, 2011, raked through the town of Smithville, Mississippi. These NWS survey results speak for themselves:

• Appliances and plumbing fixtures shredded or missing
• A home's concrete slab was pulled out of the ground, with large part missing in the center
• An SUV tossed one-half mile into the town's water tower, then blown another one-quarter mile. (NWS photos)

(PHOTOS: Strangest Tornado Damage)

April 27, 2011, was the only day on record with two F/EF5 tornadoes in Mississippi. They were the first 5-rated tornadoes there since March 3, 1966.
 

Over 200 Tornadoes in One Day

Officially, 207 tornadoes in 15 states were counted on April 27, 2011, a record for any calendar day in the U.S. This topped the number of tornadoes witnessed on April 3, 1974 (148), the other Super Outbreak.

The 20-year average number of U.S. tornadoes in the entire month of April is 188, according to The Weather Channel severe weather expert Dr. Greg Forbes.

In the four-day period from April 25-28, 2011, 349 tornadoes touched down in 21 states.  
 

Radar "Samples" a Tornado

Doppler radar lead the way to improving severe weather nowcasting, including increasing lead times for tornadoes. On April 27, 2011, a Doppler radar succumbed to a massive tornado.

NBC affiliate WAFF-TV in Huntsville, Alabama, was showing live Doppler radar and cam shots of the massive EF5 tornado that earlier devastated the towns Phil Campbell and Hackleburg. The radar was directly in the path of this wedge tornado as it tracked into Limestone County. Both the radar and radome encasing the radar were blown off and destroyed, leaving just the tower standing.

The radar was replaced later in 2011 and the church has reopened in a new location.
 

Horrific Death Toll

Sadly,  the siege of tornadoes claimed 321 lives, including 240 in Alabama.  

Of the 207 tornadoes April 27, 15 were rated "violent tornadoes", either EF4 or EF5 on the Enhanced Fujita scale. A study by the Centers for Disease Control and the Red Cross found 89.5 percent of the deaths in this outbreak were from these violent tornadoes.

(MORE: The 10 Deadliest U.S. Tornadoes)

According to the National Weather Service assessment, the average path length of the 15 violent tornadoes was 66 miles. The Hackleburg/Phil Campbell EF5 tornado was on the ground for 132 miles. 

Storm motions were fast, generally in the 45-70 mph range, according to the NWS assessment, which left much less time to take action.   

Two of the violent tornadoes April 27 made Forbes' list of the Top 10 Worst U.S. Tornadoes.
 

A Monthly Tornado Record

The Super Outbreak and several other tornado outbreaks pushed April 2011's U.S. tornado count to a monthly record of 758, smashing the previous month record from May 2003 (542) by over 200 tornadoes.

This is not only just over four times the April average (188), but also exceeded the combined average of the three most active months, April through June (681).
 

Debris 220 Miles Away

A comprehensive study of lofted tornado debris in 2012 from the April 27, 2011, Super Outbreak was the first such study to utilize social media.

A team led by Dr. John Knox, associate professor at the University of Georgia, pored through hundreds of pictures of debris transported downstream by the outbreak's massive tornadoes to map out start and end points of individual pieces of debris.

A photograph from Phil Campbell, Alabama, leveled by an EF5 tornado landed in Lenoir City, Tennessee, some 220 miles away.

It's quite common for violent (EF4 or EF5) tornadoes to transport debris tens of miles downstream, mainly paper such as canceled checks and photos. 

A jacket from Hackleburg, Alabama, however, was found 68 miles away.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

MORE ON WEATHER.COM: The April 2011 Super Outbreak (PHOTOS)