Earlier this month, NC State student Keith Sherburn presented a poster documenting HSLC convection at the 12th Annual AMS Student Conference during the AMS Annual Meeting in Austin, TX. The poster was composed of two primary themes. The first half of the poster focused on a general nationwide climatology of HSLC significant severe weather, while the second half discussed ongoing research to improve the forecasting of HSLC significant severe events. The poster garnered a lot of attention, both from students—including Ashley Athey from Virginia Tech, who had helped identify some of the HSLC events for our development dataset—and non-students, such as Russ Schneider from SPC and Jeff Waldstreicher from Eastern Region Headquarters.
A PDF of the poster is attached to this blog entry for those interested. The poster does include multiple figures previously undocumented in CSTAR material, as it focuses on the national climatology of HSLC events and skill of composite parameters rather than just investigating our CSTAR CWAs.
Notably, in the climatology, some of the CSTAR CWAs saw very few HSLC significant severe reports between 2006 and 2011, with MHX and RAH, for example, averaging only one per year. The maximum in HSLC significant severe reports was JAN, with a whopping 181 significant severe reports meeting our HSLC criteria. However, there is a pretty clear non-meteorological signal in some locations, such as in JAN, PAH, and ILX, where there are prominent “hot” or “cold” spots for HSLC significant severe reports that are likely attributable to different warning verification methods. On the other hand, the transition from HSLC tornadoes and winds in the Southeast, Mid-Atlantic, and Mississippi Valley to primarily a wind and hail threat in the Plains likely is meteorological.
Through the annual cycles, it becomes apparent that many regions contribute only a small fraction of the total U.S. HSLC significant severe reports. In the cool months, the majority of HSLC significant severe reports occur in the Southeast, Mid-Atlantic, and Mississippi Valley. This maximum shifts to the Plains and Midwest in the summer. An interesting topic discussed during the poster session was how many of these summertime events could be nocturnal MCSs, which would likely have low to nonexistent SBCAPE but plentiful MUCAPE and sufficient deep-layer shear to meet our criteria. The annual cycle including nulls also indicates that the relative frequency of nulls increases in the winter, suggesting a decrease in warning skill during that season. Diurnally, the main message is that these events, as documented previously, can occur at any time of the day, though they are relatively more common during the afternoon and evening. I plan to make an additional plot of the diurnal cycle using local hour, rather than UTC, since the national reports encompass four time zones.
In the last panel, the plot on the upper right indicates the maximum skill for each of the given composite parameters at discriminating between HSLC significant severe reports and nulls. Clearly, there is much regional variability when it comes to the skill of all parameters, including the SHERB and SHERBE. Regardless, in the regions encompassing our CSTAR CWAs (7 and 8), SHERB and SHERBE clearly outperform the other composite parameters. This indicates two things: 1) The HSLC environment in the Southeast and Mid-Atlantic appears to be unique compared to the rest of the country, and 2) More work must be done to improve the skill of our parameters for use in other regions if they are to be accepted nationally.
One potential failure point of the SHERB/SHERBE in other areas (or even in parts of our CSTAR region) is the use of 0-3 km and 700-500 mb lapse rates, as these will overlap as elevation increases. A possible solution is to use 0-3 km and 3-6 km lapse rates; unfortunately, 3-6 km lapse rates are not included in our SPC relational databases, so we were not able to test their utility.
Also, through recent analysis, it appears that many of the SHERB false alarms in the Plains occur when LCLs and 0-3 km lapse rates are both high (i.e., the old dry boundary layer problem that has been discussed previously). The attached figure shows cumulative distribution functions (CDFs) of 0-3 km lapse rate (LLLR) and surface LCL height (SLCH) for SHERB or SHERBE false alarms (FA), correct nulls (CN), hits, and misses for region 9 (see map in poster, or more generally, the Southern Plains). For the SHERB FA, note that a considerable fraction of the total distribution had high LLLRs and high SLCHs (e.g., over 60% of the FA had SLCHs above 1500 m, while over half had LLLRs above 8 K/km). Thus, we may be able to put a limit on the contribution of the 0-3 km LR term or alternatively add an LCL fade out in order to improve the skill in that region.
If anyone has any comments, suggestions, or questions, please share.