April 27th, 2011 has become the one day we all think about when severe weather threatens Alabama. In that one day alone, over 290 tornadoes were reported in the U.S., with over 60 confirmed in Alabama. In addition, most tornadoes in our state were violent, with long tracks. Many people ask me, what made April 27th so different? Why were there so many tornadoes? Just like winter weather events, severe weather forecasting can be very challenging. Thankfully, all the severe weather ingredients don’t often line-up to create major outbreaks. First off, April 27th consisted of two events, a morning round of severe storms with tornadoes. Then the second round, with more classic supercell-type storms and violent long track tornadoes.
I’ve included the atmospheric sounding (NWS Birmingham weather balloon data) for April 27th at 7 p.m. This is about as classic and volatile as it gets when it comes to parameters for severe weather in Alabama. Notice the CAPE (Convective Available Potential Energy) over 3,000 k/kg. This is the buoyancy for storms, aided by the surplus of warmth and moisture at the time. This energy was trapped for a portion of the day and was released violently as the environment conquered a capping inversion. Also, notice the helicity over 400 m2/s2, even from the surface to 1km. The helicity is driven by the increased wind shear, speed and directional, with height. These severe parameters evolved within a warm sector of a 992 MB low that passed over western Tennessee. In addition, conditions aloft lined-up, with a negative tilted trough and strong difluence associated with a jet streak.
Now let’s compare the April 27th, 2011 sounding with the sounding from 7 p.m. on April 28th, 2014. You may recall on the night of April 28th, 2014 we watched as an EF-2 tornado touched down in the Bessemer area. That was a very noteworthy severe weather night but you can see parameters on that night paled in comparison to what happened in 2011. Notice the SRH (Storm Relative Helicity) at 0-1km and you can see a value less than 200 m2/s2. While still conducive to possible tornado, the event in April 2014 helps put into perspective the kind of setup we faced back in 2011. It was a once in a generation type of event.
WBRC First Alert Meteorologist Wes Wyatt