On November 11, 2015, a generally well-forecast (Figure 1) high-shear, low-to-moderate CAPE severe event affected portions of the Midwest. Convection initially developed during the late morning in the vicinity of the triple point of an intense surface cyclone across northeastern Kansas and southeastern Nebraska, with some discrete supercells noted early in the event. Over time, convection congealed into a QLCS with embedded supercellular elements as it raced eastward towards the Great Lakes, producing isolated tornadoes and damaging straight-line winds through much of Iowa, northern Missouri, and northwestern Illinois. After dark, convection gradually weakened below severe limits, with the last report of wind-related damage coming just after 8:30 PM in north central Illinois.
Aside from one unique feature (a dryline, which can be seen in the surface observations of Figure 2), this setup was very similar to HSLC events across the Southeast and Ohio Valley. Forcing was intense through the depth of the troposphere, with a potent surface cyclone, low-level jet, upstream mid-level vorticity maximum, and upper-level jet streak all present. Additionally, SHERBS3 values were elevated across most of the warm sector, extending from southeast Nebraska and southern Iowa southward through Texas (Figure 3).
This case could be dissected in several ways, but I wanted to address a few key points that came to mind while watching this event unfold and in the hours after.
First, the SHERBS3, by itself, showed a very large false alarm area across locations where convection struggled to get organized, particularly across the Ozarks. The convection ultimately attaining severe strength developed and persisted in an environment with both favorable SHERBS3 values and intense forcing. Differential divergence, the 300 hPa analysis, and 700-400 hPa differential vorticity advection from the SPC mesoanalysis are shown in Figures 4, 5, and 6, respectively, to illustrate my point.
Second, as forcing (particularly low-level convergence; see Fig. 7) began to wane after dark and the line began to out race the favorable environment (Figs. 8 and 9), convection quickly diminished in intensity. The line was likely sensitive to the fact that favorable forcing and thermodynamics were becoming displaced with time; I suspect this was ultimately the result of the system’s demise, rather than simply losing surface-based instability.
Finally, a sounding launched from DVN approximately one hour before the convective line arrived was not all that impressive (Fig. 10). This brings to mind a few considerations; either a) the environment was evolving extremely rapidly ahead of the line, b) given the intense system kinematics, only very weak convection was necessary to mix down significantly severe winds, or c) the sounding is unrepresentative of the local environment given its horizontal displacement after launch (as suggested by Jessica King this morning during a conversation about the event).
All of these are plausible. Jessica’s simulations have revealed rapid destabilization occurring prior to the arrival of HSLC convective lines. Further, areas upstream (e.g., eastern Kansas/northwestern Missouri) were reporting near-severe winds with showers early in the event. Additionally, given the strong winds throughout the profile, DVN’s 0000 UTC sounding was likely in central or eastern Wisconsin by the end of its ascent.
I will leave potential (idealistic) ways to address all of these possibilities in future events for further discussion. For now, please share your thoughts on this recent event!