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Upper-air sounding from Fort Worth, Texas, modified to reflect noon-hour conditions at Waco.
The combination of incredible moisture content (a dewpoint of 79 degrees), and the previously mentioned very steep mid-level lapse rates resulted in surface-based Convective Available Potential Energy (CAPE) in excess of 5000 J/kg on this day! In order to put this in perspective, values of 3000-4000 J/kg are typically categorized as high, so it’s fair to say that extreme levels of instability were present south and east of the cold front on the afternoon of the 27th. An additional important value to point out is the 3CAPE reading in the bottom left of the figure. Like SBCAPE, this is a measure of potential energy available to thunderstorms, but this parameter limits the focus area to the lowest 3 kilometers (roughly up to 700 mb). As a result, we can think of this as a “low-level CAPE” parameter. Research has shown this to be an important parameter in tornadic thunderstorms as higher values can support more substantial stretching of low-level vorticity (this is analogous to a figure skater’s increasing rotation as the arms are brought in towards the body). The value of 249 J/kg is extraordinarily high (typical high-end springtime values range from 75-150 J/kg), and it’s possible these high values allowed for exceptionally strong stretching of low-level vorticity (spin) near the cold front, which ultimately aided in the production and development of the Jarrell tornado.
