The 2011 Super Outbreak, the largest tornado outbreak in recorded history, was caused by a volatile set of meteorological conditions that led to the outbreak.
Overview
On April 25, NOAA's Storm Prediction Center (SPC) noted the potential for a multi-day severe weather outbreak across most of the central and eastern United States. A vigorous negative-tilt upper-level trough had moved into the Southern Plains states early in the day, with an northeastward-moving extratropical cyclone forming between northeastern Oklahoma and western Missouri. Similar conditions were noted on April 26, with a high likelihood of a severe thunderstorm outbreak. This included the potential for all hazards - damaging winds of up to 80-90 mph, large hail of roughly 2-3" in diameter, and especially the potential for strong to violent, long-tracked tornadoes. As the afternoon and evening of April 26 progressed, mixed-layer CAPE values across the ArkLaTex region ranged from around 3000-4000 J/kg, and combined with moderate wind shear and steep lapse rates, created an extremely volatile and unstable environment, producing initially discrete supercells capable of strong to violent, long-track tornadoes during the afternoon and early evening, shifting over to a wind and hail-driven mesoscale convective complex in the nighttime.
This vigorous storm system moved into the Ohio, Mississippi and Tennessee Valleys on April 27. During this time, a vigorous 80-100 kt jet stream overspread the Ohio and Tennessee Valleys behind the trough, creating very strong wind shear, along with a strong low pressure center moving quickly northeastward that same day. That afternoon, temperatures ranged from the 70s °F (mid-20s °C) to the lower 90s °F (near 35 °C); this helped contribute to CAPE values of between 2000 and 3000 J/kg, with the moderate instability moving northeastward across the southern Tennessee Valley. Combined with 0–1 km storm-relative helicity values in the 450–600 m2/s² range, this created an atmosphere extremely conducive to significant tornadic activity and strong to violent, long-track tornadoes.
In total, the SPC issued 56 severe weather watches during the entire four-day outbreak - 41 tornado watches, 10 of which were deemed 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 accidentally skipped over 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 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 also struck the Hot Springs Village area earlier that evening; the 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 I-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 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
Overnight severe weather persisted into the early morning hours of April 27, and would continue virtually unbroken for the remainder of the day. At 06:10 UTC, the SPC issued a second-consecutive, high risk day-one outlook, featuring a 30% hatched risk of tornadoes centered over northern Alabama. At 16:30 UTC (11:30 AM CDT) this region was expanded to include portions of eastern Mississippi, southern Tennessee, and northwest Georgia. Additionally, a rare, 45% hatched risk of tornadoes was introduced along a corridor from Meridian, Mississippi, to Huntsville, Alabama. Environmental conditions continued to be extremely favorable for discrete tornadic supercells throughout the day.
Morning squall lines During the early morning, 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, and 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 tornado 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 tornado 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
After around an hour-long lull in activity, the most intense supercells of the outbreak began to fire off in Mississippi and progress eastward through the afternoon and early evening. Benefiting from the extreme instability and wind shear, these supercells near-instantly became tornadic and minutes later began producing strong to violent tornadoes. The Storm Prediction Center, seeing that this already unstable atmosphere was projected to become even more so throughout the afternoon and early evening, issued yet another PDS tornado watch at around 1:45 PM CDT (1845 GMT) for most of Alabama and into portions of Mississippi, Tennessee, and extreme northwestern Georgia. A widespread complex of supercellular storms sprawled over Mississippi and Alabama, producing tornadoes ranging from weak and short-lived to violent and long-tracked throughout the afternoon.
At around 2:30 PM CDT (1930 GMT), as instability, wind shear and already ample low-level moisture continued to increase, a tornado emergency was declared for Neshoba, Kemper, Noxubee and Winston counties in Mississippi, as a large and extremely destructive tornado was reported on the ground by emergency spotters and storm chasers, as well as a camera atop a television tower from ABC affiliate WTOK-TV in nearby Meridian, Mississippi. While it just barely missed the town of Philadelphia, Mississippi proper to the northeast, this EF5 tornado (the first of four to touch down that day) caused incredible damage to the northeastern portion of Neshoba County, as pavement was torn off 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.