May 4-5, 2017 High Shear Low Cape (HSLC) Severe Weather Outbreak

Throughout the overnight hours of May 4th into May 5th, 2017, a severe weather outbreak occurred across central North Carolina. As a result of these storms, the National Weather Service Forecast Offices in Raleigh and Wilmington, NC as well as Greensburg-Spartanburg, SC and Wakefield and Blacksburg, VA confirmed 95 wind damage reports and five tornadoes. This severe weather event occurred in a high shear, low-CAPE [convective available potential energy] environment characterized by a large magnitude of change in wind speed and wind direction with height and relatively low energy (or fuel) available to thunderstorms. These types of environments, often abbreviated as HSLC environments, are fairly common throughout the Southeastern and southern Mid-Atlantic regions of the United States and often present great forecasting difficulties.
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Severe thunderstorms are often thought to occur during the day with ample heating at the surface fueling thunderstorm development. However, in some cases, such as the May 4th-5th event, severe weather can occur overnight. North Carolina has a history of producing nighttime severe weather outbreaks in HSLC environments. With overnight outbreaks, alerting the public of oncoming severe weather becomes very difficult, since most people are asleep during this time, and visual observation of storms is limited or impossible. Adding to the difficulty of providing awareness of such a threat is that HSLC environments are relatively hard to predict. Even just a few hours prior to such an event, the true extent and severity of the threat is often still unknown. HSLC storms are also often smaller than classic storms in more unstable environments; this makes them more difficult to assess with radar.

During the evening and nighttime hours of May 4th, the NCSU Soundings Club collaborated with NCSU graduate student Keith Sherburn to aid in his research on HSLC environments. The club launched weather balloons with a radiosonde sensor attached to it from the roof of Jordan Hall in order to sample the atmospheric environment ahead of the severe weather event. Following the launch, data collected by the radiosondes were plotted and then sent to Raleigh’s National Weather Service (NWS). These data allowed meteorologists to better understand the current environment, compare observed conditions to those simulated by numerical weather models, update forecasts, and increase confidence in discussions regarding the issuance of any severe weather products. The Soundings Club launched three different weather balloons at two hour intervals between 8pm EDT and 12am EDT, with the resulting data concluding that the pre-existing environment during all three of these launches appeared to be less than favorable for the persistence and additional development of thunderstorms across the region. Consistent with this, the Storm Prediction Center had placed a marginal risk, entailing a 5% chance of severe conditions (tornadoes, hail larger than 1” in diameter, and/or damaging winds of 58mph or greater) within 25 miles of a given point across central and eastern NC [Figure 1]. With this information, the team decided to call off additional operations following the 12am EDT launch. However, just a few hours later, it became evident that the atmospheric environment rapidly changed as a severe weather outbreak began across the region in the predawn hours of May 5th as shown by the radar loop provided in Figure 2.

Figure 1: SPC Day 1 Convective Outlook for the overnight period between May 4th and May 5th, 2017, issued ahead of the HSLC severe weather outbreak

Figure 2: Animated loop of base reflectivity (left) and base velocity (right) from the KRAX radar site during the HSLC event with severe thunderstorm (yellow) and tornado (red) warnings overlaid using GR Analyst.

The severe weather outbreak, resulting in five confirmed tornadoes and 95 separate wind damage reports throughout North Carolina, began taking place shortly after 2am EDT on May 5th [Figure 3] and lasted into the mid-morning hours. Out of those tornado reports, the tornadoes near Eden and Oxford, NC, were given ratings of EF-1 on the Enhanced Fujita Scale with winds estimated between 86 - 110mph. The remaining three tornadoes, located near Rockyhock, Mount Gold, and Camden, NC, were classified as EF-0s with estimated winds between 65 - 85mph. Amazingly, there were no major injuries or fatalities reported as a result of these storms in NC. This likely owes to the broadcast of the potential threat by local meteorologists during the course of the preceding days.

Figure 3: SPC Severe Weather Reports following the HSLC event on May 4-5th, 2017 overlaid on the Day 1 SPC Convective Outlook valid for that period.

Keith Sherburn's collaboration with the Soundings Club at NCSU has been ongoing since early 2016, while his investigation into HSLC events began in 2011. Since early 2016, data have been collected for many HSLC severe weather events throughout central NC. This research has been used by operational meteorologists, including those at NWS Raleigh, to aid in real time analysis of the atmospheric environment and is dedicated to improving severe weather forecasts in North Carolina associated with HSLC environments. Many late nights and early mornings have been focused on atmospheric data collection prior to these events. Keith’s research was highlighted in a recent CIMMSE article written by Jonathan Blaes, the Science and Operations Officer at NWS Raleigh. This article detailed the collaborative efforts of Keith and the Soundings Club over the past year and for the May 4th-5th HSLC event, which was one of North Carolina’s largest severe weather outbreaks of 2017. Due to the unpredictability of HSLC events, Keith’s research has been instrumental in understanding the events and how meteorologists can better predict them in the future.

Despite operational advancements associated with recent research of Keith and other NCSU students, prediction of these events even a few hours before the event itself, remains a challenge and further research is necessary to better understand and predict these HSLC scenarios in the future. For his dedication to this difficult research, Keith was also awarded with the John S. Irwin Award for Scientific Excellence at the Central North Carolina Chapter of the American Meteorological Society (CNCC AMS) awards banquet in May 2017.

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Contributors:
Authors/Editors: Michael Mugrage, Shaelyn McLamb
Faculty Review Board: Dr. Matthew Parker, Keith Sherburn