Two Similar Soundings, Two Wildly Different Outcomes

Figure A

Summer break has just started for most schools across the United States and coupled with that is the awakening of Tornado Alley. Last year, the number of tornadoes that occurred was “severely lacking” according to many avid tornado chasers that frequent this area over the months of April, May, and June. This year is a completely different story. Tornado Alley has had quite an awakening this week (May 20-24) with the Storm Prediction Center issuing a slight to moderate risk every day since May 17 and will be continuing to issue them into the beginning of next week (May 27) as well. This has been an incredible issue when it comes to flooding and communities being able to recover and rebuild from the tragedies that have struck. Tornadoes are a dime a dozen during the day the week of May 20. However, there was one day that stuck out to meteorologists alike, and that was Monday, May 20.

The sounding shown above is from a notable day in history, April 27, 2011, in Alabama. Note that in this image, there are winds backing with height, a large dry pocket, the atmosphere is saturated, but the surface also contains another large dry pocket with a low LCL (lifted condensation level). The LCL matters because that is the level where an air parcel reaches maximum saturation as it is lifted through the atmosphere. Low LCL’s are ideal for tornadic development. Something also to note on this sounding are the CAPE and lapse rate values. CAPE stands for Convective Available Potential Energy and describes the instability of the atmosphere (the larger the CAPE, the more unstable the parcel, the more likely it is to produce severe weather, thus why it is also a big factor in tornado genesis). Lapse rate values depict how fast the parcel temperature is falling with height (the steeper the lapse rate, the more instability added due to an even larger temperature gradient, more severe weather, etc). CAPE values for that day reached 3014 J/kg, which is a significant number as the normal CAPE on any given day in the South is anywhere from 900-1400 J/kg. The lapse rates recorded this day were 8°C/km, which is about a 46.4°F decrease per kilometer! So, in this sounding, there is an atmosphere that is incredibly unstable, with temperatures rapidly decreasing with height, and a lot of shear to back it up. With wind shear, a favorable atmosphere with tornadoes would have both directional and speed shear. What this means is that tornadoes are favorable when winds are “backing” with height (this means that winds are going in a counter-clockwise directional pattern with height) and with increasing wind speeds as the column increases with height. This day had significant shear values with an inflow shear of 56 knots (~64 mph). This day also was denoted as a “High Risk” day from the SPC (Storm Prediction Center) and that day delivered and will forever go down in history as one of the deadliest outbreak days of the Super Outbreak of 2011.

Figure B

This sounding (Fig. B) was taken in Oklahoma after SPC issued a “High Risk” for almost the entire state (excluding the panhandle). The main difference between May 20, 2019 and April 27, 2011 is that this forecasted outlook ended up being a “bust”, sparing Oklahoma a day of tragedy.. The environment in this sounding would have made many meteorologists agree with the issue of “High Risk” quality. To begin with, the LCL was half that of the 2011 sounding, with the cloud deck appearing to just about scrape the ground and CAPE values topped out at 4,143 J/kg. This is an incredibly unstable environment already, but the lapse rates weren’t as high, with rates topping out at 5.3°C (41.54°F) per kilometer and wind shear was also lacking due to the dry pocket not being as large and the saturation of the environment being closer to the surface. What ultimately led to the demise of this spooky sounding was a cold pool which came in and cut off inflow to the supercells too quick to the majority of the area in the high risk. There were still 19 tornadoes that day. While it may not be 2011 worthy, it is still an incredible number of tornadoes in a short span of time.

When looking at similar soundings, anything that can come close to a sounding of the Super Outbreak of 2011 will (and should) raise flags and be approached with extreme caution. The SPC was being cautious and wanted to ensure the public was weather-aware and prepared to do whatever they had to if that day unfolded as it did in the past.

© 2019 Meteorologist Ashley Lennard