Largest tornado outbreak in United States history
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The 2011 Super Outbreak was the largest, costliest, and one of the deadliest tornado outbreaks ever recorded, taking place in the Southern, Midwestern, and Northeastern United States from April 25 to 28, 2011, leaving catastrophic destruction in its wake. Over 175 tornadoes struck Alabama, Mississippi, and Tennessee, which were the most severely damaged states. Other destructive tornadoes occurred in Arkansas, Georgia, Kentucky, Louisiana, New York, and Virginia, with storms also affecting other states in the Southern and Eastern United States. In total, 367 tornadoes were confirmed by NOAA's National Weather Service (NWS) and Government of Canada's Environment Canada in 21 states from Texas to New York to southern Canada. Widespread and destructive tornadoes occurred on each day of the outbreak. April 27 was the most active day, with a record 223 tornadoes touching down that day from midnight to midnight CDT (05:00–05:00 UTC). Four of the tornadoes were rated EF5, which is the highest ranking on the Enhanced Fujita scale; typically these tornadoes are recorded no more than once a year.
In total, 348 people were killed as a result of the outbreak, including 324 tornado-related deaths across six states and 24 fatalities caused by other thunderstorm-related events such as straight-line winds, hail, flash flooding or lightning. In Alabama alone, 238 tornado-related deaths were confirmed by the Storm Prediction Center (SPC) and the state's Emergency Management Agency. April 27's 316 fatalities were the most tornado-related fatalities in the United States in a single day since the "Tri-State" outbreak on March 18, 1925 (when at least 751 people were killed). Nearly 500 preliminary local storm reports were received for tornadoes over four days, including 292 in 16 states on April 27 alone. This event was the costliest tornado outbreak in United States history, with total damage reaching $10.2 billion (equivalent to $ billion in 2023). In 2023, tornado expert Thomas P. Grazulis created the outbreak intensity score (OIS) as a way to rank various tornado outbreaks. The 2011 Super Outbreak received an OIS of 378, making it the second most intense tornado outbreak in recorded history behind only the 1974 Super Outbreak.
Meteorological synopsis.
The outbreak was caused by a vigorous upper-level trough that moved into the Southern Plains states on April 25. An extratropical cyclone developed ahead of this upper-level trough between northeastern Oklahoma and western Missouri, and moved northeastward. Conditions were similar on April 26, with a predicted likelihood of severe thunderstorms, including an extended threat of strong to violent long-track tornadoes throughout the afternoon and evening hours; mixed-layer CAPE values were forecast to be around 3000–4000 J/kg, around east Texas, Louisiana, and Arkansas. The storm mode on April 26 was predicted to include mostly discrete tornadic supercells during both the afternoon and the early evening, shifting over to a mesoscale convective complex, with more of a threat of damaging winds and hail during the nighttime hours.
As the storm system moved eastward toward the Ohio, Mississippi, and Tennessee Valleys on April 27, a very powerful 80–100 knot mid-level jet stream moved into the Ohio and Tennessee Valleys behind the trough and created strong wind shear, along with a low pressure center moving quickly northeastward across those areas on April 27. During the afternoon of April 27, CAPE values were estimated to be in the range of 2000–3000 J/kg across Louisiana and southern Mississippi, with the moderate instability moving northeastward across the southern Tennessee Valley; additionally, temperatures across the southeastern United States ranged from the 70s °F (mid-20s °C) to the lower 90s °F (near 35 °C). Helicity levels ranged from 450 to 600 m2/s2, which supported some significant tornadic activity and strong to violent long-track tornadoes.
A total of 56 severe weather watches were issued by the Storm Prediction Center (SPC) over those four days in the outbreak area. This included 41 tornado watches—10 of which were particularly dangerous situation (PDS) watches—and 15 severe thunderstorm watches. The SPC assigns numbers to each severe weather watch issued starting at the beginning of each year; the organization unsuccessfully used two of their allocated watch numbers during this outbreak (numbers 208 and 209).
April 25.
A large area of possible severe storms for April 25–27 was forecast as the Storm Prediction Center (SPC) issued a moderate risk of severe weather for three consecutive days, centered over Arkansas through Tennessee. At 3:25 p.m. CDT (20:25 UTC), the SPC issued a particularly dangerous situation (PDS) tornado watch for much of Arkansas and parts of Missouri, Oklahoma, Texas, and Louisiana. By the evening hours of April 25, tornadoes had been reported across a few states, some of which caused significant damage in Arkansas. An intense supercell thunderstorm tracked near the Little Rock area and a tornado emergency was declared for the city of Vilonia. A large EF2 wedge tornado struck the town, subsequently causing severe damage and killing four people. A strong EF3 tornado had also struck the Hot Springs Village area earlier that evening; that tornado caused severe damage and resulted in one death. Later that evening, another EF2 tornado caused extensive damage to both a school building and Little Rock Air Force Base as well. Severe flooding continued across a large area from the Red River valley to the Great Lakes. A total of 42 tornadoes and five tornado-related deaths were confirmed on the 25th.
April 26.
A high risk of severe weather was issued for April 26 for portions of Louisiana, Arkansas, Oklahoma, and Texas along and near the Interstate 30 corridor as conditions became even more favorable for extreme weather. A large PDS tornado watch with very high possibilities for tornadoes was issued for that same area that afternoon. Widespread tornado warnings were then issued in that area later that evening.
An upper-level negatively-tilted trough with two embedded shortwaves generated two surface lows that propagated generally east. One of the surface lows tracked northeast along the Mississippi River into Wisconsin as it occluded. Tornado watches were issued for the Lower Great Lakes during the afternoon hours as supercell thunderstorms developed along the warm front lifting north across central Michigan. Two tornadoes touched down in Michigan and caused damage to farm structures. Further east, severe thunderstorms caused scattered wind damage and large hail across Pennsylvania and New York. Two-inch-diameter hail was reported in Lock Haven, Pennsylvania. An isolated supercell moved across Central New York throughout much of the afternoon, producing golf ball-sized hail in Syracuse and spawning a very brief EF1 tornado in Verona Mills, which primarily caused damage to trees. Another tornado — this one being in Gilbertsville — caused significant damage to a school's athletic field.
The second surface low corresponded to an area of strong upper level divergence ahead of the downstream shortwave. As the low formed across Texas and deepened while moving east, a tightening pressure gradient force further strengthened the low-level jet, therefore creating a broad warm sector across the southeastern states. This also generated stronger wind shear, providing better organization for the supercell storms as a result. Numerous tornadoes touched down across several states, including Texas, Louisiana, and Arkansas. Most of those tornadoes were weak, but a few of them caused considerable damage. A long-tracked wedge tornado caused EF2 damage in rural portions of Texas and Louisiana. An EF3 tornado destroyed structures and caused severe damage at Fort Campbell, Kentucky, as well. A total of 55 tornadoes were confirmed on the 26th, although no fatalities occurred.
April 27.
Significant severe weather was ongoing early on April 27 (in the overnight hours) and continued for the entire calendar day virtually unbroken. For the second day in a row, the SPC issued a high risk of severe weather for the Southern United States. Later that morning, the SPC even increased the probability for tornadoes to 45 percent along a corridor from Meridian, Mississippi, to Huntsville, Alabama, an extremely rare issuance exceeding the high risk standards. Conditions became increasingly favorable for tornadoes during such an extreme tornado outbreak.
Morning squall lines.
During the early morning hours, a cold front with several embedded low pressure areas extended from east Texas northeastward into the Ohio River Valley. An upper-level disturbance that had moved across the frontal boundary the previous evening sparked an area of thunderstorms that morphed into a squall line. This line of severe thunderstorms would produce tornadic activity from the evening on April 26 into the late morning of April 27. Early in the morning the squall line, packing straight-line winds and numerous embedded tornadoes, moved through Louisiana and Mississippi before proceeding to affect North and Central Alabama and parts of Middle and East Tennessee. The line strengthened as it moved through Alabama, partially due to a high amount of low-level moisture from the Gulf of Mexico and increasing wind shear.
A majority of the tornadoes embedded in this initial squall line were weak, though several were strong and as such caused significant damage. An EF3 tornado caused major damage to homes in Coaling, Alabama, an EF2 and an EF3 tornado produced severe damage and a fatality near Eupora, Mississippi. Another EF3 tornado resulted in heavy damage in downtown Cordova, Alabama, which was struck by a violent EF4 tornado later that afternoon. An EF2 also struck Cahaba Heights near Birmingham. One embedded cell began producing tornadoes just after 5:00 a.m. CDT (10:00 UTC), starting with an EF1 tornado striking Berry, Alabama. This was soon followed by the Cordova EF3 tornado. As the line entered Cullman County, Alabama, the cell, a mesoscale convective vortex (MCV), began exhibiting a comma head appearance and produced another long-tracked EF2 tornado that struck the town of Hanceville, killing one person. The MCV would then produce 13 tornadoes (most rated EF1) to the northeast in Marshall County, many of which occurred simultaneously. Two more tornadoes were produced as the comma head-shaped embedded cell continued northeast along the Jackson/DeKalb county line, including a long-track EF1 that resulted in a fatality near Pisgah. The same area was impacted later that day by a high-end EF4 tornado. Another EF1 tornado occurred in Dade County, Georgia, with the cell, and six more tornadoes struck Hamilton County, Tennessee, five of which were in the Chattanooga area. The 26th and final tornado produced by the MCV was an EF2 tornado in northern Bradley County, Tennessee, which hit at 9:45 a.m. EDT (13:45 UTC). The initial storms caused widespread power and telephone line outages across Alabama and Tennessee. This line of storms also caused some NOAA weather radio transmitter sites to stop functioning for the remainder of the outbreak. Because of this, more than one million customers were without power and had no warning of any approaching tornadoes later that day.
From the late morning to the early afternoon, another squall line moved through northern parts of Mississippi and Alabama as high wind shear and low-level moisture persisted. However, this time several discrete supercells developed along and in front of the line, spawning seven weak tornadoes across Morgan, Limestone, and Madison Counties in northern Alabama around noon that day.
Afternoon supercells.
The most intense supercells of the outbreak developed around midday in central Mississippi and began tracking eastward. With the extreme instability and wind shear, they quickly intensified and eventually became tornadic, shortly thereafter resulting in strong to violent tornadoes. Responding to the high risk issued by the SPC and the already unstable atmosphere expected to become even more unstable throughout the afternoon hours, a PDS tornado watch was issued at 1:45 p.m. CDT (18:45 UTC) for much of Alabama and portions of Mississippi, Tennessee, and Georgia. A widespread complex of supercell storms overspread the states of Mississippi and Alabama and violent tornadoes began rapidly touching down as the afternoon progressed.
By mid-afternoon, as wind shear and low-level moisture continued to dramatically increase, a tornado emergency was declared for Neshoba County, Mississippi, as a large tornado was reported on the ground by both storm spotters and a camera atop a television tower from ABC affiliate WTOK-TV (channel 11) in Meridian, Mississippi. This powerful EF5 tornado caused incredible damage northeast of Philadelphia, Mississippi, where pavement was torn off from roads, vehicles were thrown, and the ground was scoured out to a depth of by the tornado. Three people died when a mobile home was thrown into a wooded area, obliterating it in the process. Another very long-tracked EF4 tornado passed near the town of Enterprise, Mississippi, killing seven people before crossing into Alabama and eventually dissipating. Four tornadoes were officially