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 2000-3000 J/kg, with the moderate instability moving northeastward across the southern Tennessee Valley. Combined with 0-1km 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
… excerpt ends here. Continue reading the full article.
