Taking a look at the 2011 Super Outbreak - 10 years on

10 Years ago today, an extremely rare combination of atmospheric ingedients lead to what has been called an environment 'the most conducive for violent tornadoes ever documented'. On the afternoon/evening of the 27th April, this environment became utilised, and produced a massive, destructive and tragic outbreak of tornadoes, leaving scars that would never be forgotten. 

In total the event produced 360 tornadoes in a continues 4 day period, and in the 24 hours of the 27th, over 200 tornadoes were confirmed, with 15 of those being rated as violent (EF4/5), and 4 of those being rated EF5 (The highest on the scale)

Meteorological Synopsis of the event

April 2011 was a record month, and was characterised by numerous large troughs moving through the continental United States.

This particular system was a large longwave trough, and a smaller shortwave trough, which was becoming neutrally to negatively tilted as it ejected into the Eastern U.S. through late April. Highly diffluent flow around the Mississippi  River (on the right, can be seen near Illinois) prompted the development of a surface low with a large, unstable warm sector, as well as a powerful low level jet at the 850mb level moving due north right from the Gulf of Mexico, transporting large amounts of moisture into the Deep South. The morning of the event had 3 Mesoscale Convective Systems, 2 of which were highly tornadic, and produced numerous strong tornadoes due to the extreme shear in place (fairly similar to last years Easter Outbreak). 

These lines also exhibited interesting mesoscale features, such as a comma head (right) and small scale hook echoes in the line. These systems produced 76 tornadoes, 5 of which were EF3. At their peak, 16 tornadoes were produced over a 30 minute time span. These systems also played an important role in tornadogenesis in the afternoon storms. Some of the tornadoes in this line were also very long track (40+km). Towns such as Coaling AL, Cordova AL, and Eupora MS were hit particularly hard by these morning lines.

These storms had cleared by the early afternoon, and from then on the afternoon recovered into an extremely rare parameter space conducive for violent tornadoes. The very strong low-level jet aided in 2 main things in my opinion; 1 the sufficient moisture transport needed to allow the warm sector to recover from morning storms and 2, the extreme shear that was present throughout the whole day, especially the afternoon. 

The extreme shear was probably what made the outbreak. It is unbelievably rare to have discrete supercell storms moving through the amounts of shear and helicity that were in place, and a look at the observed sounding from Jackson, MS at only 7AM CDT shows the extreme curvature in the lowest level (red line). 

0-1km Storm Relative Helicity (SRH) was widely on the order of 400-1000, reaching the higher ends as winds became backed near the surface low. For context, even 200 0-1km SRH can be enough for strong-violent tornadoes. As well as this, 2000-3500 j/kg of Surface Based (SB) CAPE, an uncapped warm sector, subtle forcing, and shear vectors parallel to the boundary meant that a discrete storm mode was likely, and so it was; numerous discrete supercell thunderstorms traversing one of the most favourable environments ever. That's when the real horrors started.

Mesoscale Influences

As well as the favourable overall environment, mesoscale details came together perfectly to aid in the development of tornadoes. In particular the outflow boundary from the morning storms in place across N MS and AL generated baroclinic vorticity across the thermal boundary that helped the tornadogenesis process occur. 

In white is the rough location of the thermal boundary, and as you can see, numerous supercells rode along it. Two of the three storms noted produced EF5s, and the one on the right produced a very destructive EF4, as well as numerous other violent tornadoes (in fact all of them did!). On the cooler side of the boundary, storms struggled a bit more to produced tornadoes, but eventually managed it as that boundary pushed north during the day, with some very violent tornadoes into Georgia and Tennessee. Another thing, perhaps more on the storm scale, was the small reflectivity segments that many storms ingested, whether they were weak showers or developing storms, but got sucked into the storms inflow and aided the tornadogenesis process, and many of these storms produced tornadoes a short time later.

The Tornadoes

There were so, so many tornadoes that it would be impossible for me to talk about all of them. I will talk about the most notable ones, however there are still too many of these as well!

The radar images just there are mos of the violent rated tornadoes in the outbreak. Notice how all of them are just textbook images, and it's like somebody got a 'perfect' supercell and pressed copy & paste all over the place in the Southern US. Each one has in intense velocity couplet, inflow. eagle notch, debris ball etc. Each of these tornadoes also caused immense destruction and tragedy, and it's hard to realise that it happened on such a large scale. Here are the paths of the tornadoes on April 27th, and the paths of Supercells on April 27th, via Tony Lyza (@tlyzawx) on Twitter.

The first notable tornado was an extremely powerful EF5 near Philadelphia, Mississippi with winds of over 200mph. It scoured out the ground to an incredible depth of 60cm. The tornado also ripped large strips of asphalt from roads, and famous Storm Chaser (and personal inspiration!) Reed Timmer captured this beast on tape, and caught an incredible roar from the tornado, as well as very violent motion. 

More violent tornadoes occured, including powerfuls EF4s in Cullman Alabama, and Enterprise Mississippi. Another incredibly violent EF5 tornado occured in Smithville, Mississippi, which levelled well built homes (including ripping plumbing fixtures from the foundation), as well as tossing a truck into the towns water tower, then carrying it to the other side of town and mangling it into a ball. Some floors had tiles scoured from the foundation, and large trees were debarked, denuded and thrown like toys.

Further violent tornadoes continued through the afternoon. One particular observation of many tornadoes was the extremely rapid/violent motion, as well as horizontal vorticies spinning around the tornado like tentacles. This was showing massive amounts of horizontal vorticity ingested into the storms. An EF4 with winds of 190mph (possibly higher) went right through downtown Tuscaloosa and Birmingham, Alabama, and levelled many buildings, as well as tragically killing many. 

Two more EF5s occured in Alabama. Perhaps the most exceptional tornado of the outbreak, an EF5 tracked through Hackleburg and Phil Campbell in Alabama, and even into Tennessee along its 132 mile path. This tornado had speeds of 210mph (maybe more) and maintained EF5 intensity for an unusally long duration of its path. The tornado levelled homes, even destroying the actual concrete foundation in cases, and carried debris over 40 miles away. Grass was scoured from hillsides, trees were debarked and debris was shredded and rowed long distances. 

The last tornado I will be talking about was a high end EF4 in Ringgold Georgia, and SE Tennessee. 12 homes were obliterated, debris was streaked into long rows, and trees were denuded. This tornado had winds of 190mph. More strong/violent tornadoes continued into the night.

It is so crazy that has been 10 years since this event. The extreme number of violent tornadoes meant that many significant/destructive tornadoes got lost along the other more 'famous' ones, and even many weather enthusiasts don't know a fair amount of the violent tornadoes that day. My thoughts go out to those affected, the homes that were destroyed, the belongings that were lost, and the 324 people that tragically died from tornadoes. No matter how long ago it was, people will always remember the terrible events of April 27th, 2011.

Alex