Credit: SPC Violent Tornado Page
DISCUSSION: Although it has been over 7 years since the Super Outbreak of 2011, this event still has me in disbelief, and in awe, that such an event transpired. This article will feature some details and explanations on this incredibly memorable weather event focused on Mississippi and Alabama and nearby locations as shown above to have been impacted by dozens of tornadoes.
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This article will focus on the four main features needed for severe weather: moisture, a lifting mechanism, instability, and wind shear. Let’s first assess the wind shear present in the atmosphere on the 27th of April, 2011.
When analyzing severe weather, wind shear is a common variable assessed to determine the type of convection that may develop in the atmosphere. The 0 – 6 km bulk shear is the most typical quantity with values greater than 40 knots indicating most of the convection likely to be supercellular. Above is the analyzed 0 – 6 km wind shear during the mid-afternoon (21Z) on April 27th 2011 overlaid with the tornado tracks and intensities developing at around the same time. Values where most of the tornadic activity is developing ranges anywhere from 50 – 80 knots of 0 – 6 km wind shear, easily eclipsing the threshold leading to essentially solely supercell storms at this time.
Next, let’s assess the instability. The same time that is analyzed in the previous section shows substantial instability present across Mississippi and Alabama where most of the violent tornadoes are developing during this time. 2000 – 3500 J/kg of surface based convective available potential energy (CAPE) shows plenty of potential for strong rising air and predominantly supercell convection, as we concluded in the previous section. However, just because there is a lot of CAPE does not necessarily mean severe weather, there needs to be a lifting mechanism to get the ball rolling, to allow for this air to rise on its own.
Shown above is the surface analysis for the 27th at 21Z which shows a sub 1000 mb low pressure system along the Tennessee and Arkansas border with the surface winds out of the south in head of one of the lifting mechanisms, being the cold front. At this time the front is situated somewhere in Mississippi indicated by the shift in the surface winds. This strong cold front is providing a lifting mechanism for these storms to fire across Mississippi and soon after Alabama, throughout the overnight, and on the 28th across much of the rest of the Southeast and Mid-Atlantic. A surface trough out in head of the cold front was analyzed by the WPC at this time increasing the convergence across the area of interest providing another source of lift and initiating storms.
The strong surface winds play another role is this setup, which is advecting that warm and moist Gulf of Mexico air into the Southeast enhancing the lift out in head of the cold front and destabilizing the atmosphere around the surface trough. The moisture is the last ingredient needed for severe weather and will be discussed below.
And here we have the surface dewpoint temperatures in degrees F. Dewpoints in the upper 60s to lower 70s throughout the region, especially across Mississippi and Alabama, is a strong indicator of severe convection with tornadoes possible. It should be noted that 850 mb dewpoint temperatures were also quite impressive in the upper 50s and even a few lower 60s which is indicative of an incredibly moist lower troposphere which enhanced the potential for a widespread severe weather outbreak.
Taking a quick look at a sounding from Birmingham, Alabama at 00Z on the 28th, 3 hours after the analysis from above, shows everything that has been pointed out thus far. The large amounts of instability in the form of CAPE, wind shear numbers off the charts, the moisture at the surface and at 850 mb, as well as a good indicator of the overall spin in the atmosphere. This can be assessed by looking at the hodograph which shows a nice “fish hook” meaning lots of storm relative helicity present. All of these ingredients developed in a spectacular, yet deadly fashion. Take a look below at this incredible radar loop of the storms developing. The amount of rotation in the atmosphere is quite notable just by looking at the reflectivity and noticing all the hook echo’s across the region (northern Alabama and Mississippi).
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An incredible meteorological event, but also incredibly deadly. Every so often mother nature reminds us that “she” can produce in a spectacular way when just the right condition are in place.
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©2018 Meteorologist Joe DeLizio
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