A tale of two outbreaks.
April 27, 2011 featured classic supercells that were extremely-fast moving (forward speeds of 60+ mph) in a northeast direction. James Spann noted this while covering the EF4 tear through Tuscaloosa on the skycam: "Looks like something you'd see in Kansas. It's surreal." Because the storms moved so fast, heavy rain and hail were not as much of a threat, despite the multiple mile-wide EF3+ grinders that killed so many. There was a little over 1 inch of rain in Birmingham, most of it caused by the early morning QLCS. Shelby County Airport only recorded 0.60".
Now compare that to May 31, 2013, where you had HP supercells (that looked like massive blobs on radar) meandering, sometimes erratic, moving at 20-30 mph. You had entire mesocyclones falling to the ground, completely wrapped in torrential rain and giant hail. And the 2.6 mile wide El Reno EF5 that killed three esteemed tornado researchers was no exception. Epic, historic and catastrophic flash flooding along with hail larger than softball size devastated the OKC metro: El Reno recorded 6 inch diameter hail and Will Rogers Airport received 6.76" of rain, with local amounts reaching 8 inches.
How is it determined if a tornado outbreak will produce fast-moving classic supercells or slow, meandering with catastrophic flash flooding and giant hail?
A couple of footnotes:
- Some parts of central AL did get HP supercells and crippling hail. The supercell that spawned the Cordova EF4 produced baseball-sized hail over downtown Cordova. And the Hackleburg-Phil Campbell EF5 was rain-wrapped in an HP supercell. However, precipitation amounts in general from the discrete suprecells weren't extremely high, and hail mostly topped out at golf ball size.
- The tornado emergency for downtown OKC mentioned softball-sized hail, but the largest hail report I was able to find was in Yukon at 6:55 pm, and it was 1.75" (golf ball size). Bethany got half-dollar-sized hail and Moore got only quarter-sized hail.
- Perhaps the rainfall amounts on May 31 were an outlier because of central OK's unique geography and climate. The OKC metro sits in a flat plain along the Canadian River, and the soil is mostly red clay that can absorb a lot of moisture. All of these factors exacerbate flooding. Central AL on the other hand is more hilly, and few larger cities sit on a major river (only Pell City and Gadsden, which are on the Coosa River).