Thoughts on 11 November 2015 HSLC severe event in Midwest


Figure 1. Verification of SPC’s Day 1 outlooks for 11 November 2015, from earliest (bottom) to latest (top). Credit Dr. James Correia.

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.


Figure 2. Surface chart from 1900 UTC 11 November 2015. Note a surface cyclone center in southeast Nebraska, a warm front extending into Iowa, a dryline from eastern Kansas through central Oklahoma, and a cold front from central Kansas through the Texas and Oklahoma panhandles. Image courtesy of UCAR.

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).


Figure 3. SPC mesoanalysis SHERBS3 values valid at 2000 UTC, approximately the time of first tornado occurrence in western Iowa. Click image for better resolution.

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.


Figure 4. SPC mesoanalysis differential divergence valid at 2000 UTC. Click image for better resolution.


Figure 5. SPC mesoanalysis 300 hPa analysis valid at 2000 UTC. Click image for better resolution.


Figure 6. SPC mesoanalysis 700-400 hPa differential vorticity advection valid at 2000 UTC. Click image for better resolution.

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.


Figure 7. SPC mesoanalysis differential divergence valid at 0300 UTC, approximately half an hour after the final severe wind report. Click image for better resolution.


Figure 8. SPC mesoanalysis SHERBS3 valid at 0300 UTC. Note that the convective line is largely east of the SHERBS3 bullseye (cf. Fig. 8) and approaching values of SHERBS3 < 1. Click image for better resolution.


Figure 9. SPC mesoanalysis 0-3 km CAPE (red contours) and surface vorticity (blue contours) valid at 0300 UTC. Note that the convective line is largely east of the analyzed 0-3 km CAPE bullseye.

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).


Figure 10. 0000 UTC DVN sounding from the SPC. Click image for better resolution.

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!

This entry was posted in Convection, CSTAR, High Shear Low Cape Severe Wx. Bookmark the permalink.

5 Responses to Thoughts on 11 November 2015 HSLC severe event in Midwest

  1. Grant Wise says:

    Great post Kieth. I really appreciate how you all regularly update this blog. I have only recently heard about the SHERB in forecasting HSLC events. Could you point me towards some more info about the science behind this parameter/how it is calculated? Thanks!

  2. Greg Carbin says:

    Please credit figure 1 to Dr. James Correia (@jimmyc42), who is the one producing those forecast comparison map plots at SPC. Thank you.

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