The Evolving Potential for Severe Hazards in the Carolinas: 5/5/2020 - Operations Proving Ground

All severe hazards were possible on the afternoon and evening of May 5th in the Carolinas. The Storm Prediction Center (SPC) highlighted the area with a categorical Slight Risk based on a 15% probability of severe hail and wind and a 2-5% probability of tornadoes (Figure 1). The focus for convection was ahead of a cold front that was advancing eastward across the southeastern U.S. A weak east-west oriented boundary located in northern South Carolina (SC) would also be a feature of interest as the day unfolded.

As convection developed early in the afternoon along the southern portions of the Appalachians, it pushed eastward toward areas which remained stable as indicated by the continuation of stable billow clouds (Figure 3) and corroborated by SPC mesoanalysis (not shown). This stable air mass would, at least in the near-term, bound the northern extent of the severe threat to locations close to the NC/SC border. However, breaks in the clouds were allowing for some surface-heating to occur. Furthermore, in south-central NC, winds had started to veer, becoming more southerly by 20Z. So there were some signs that pointed toward destabilization with time, particularly in areas further downstream from the developing convection.

Figure 2: GOES-16 mesosector Vis/IR Sandwich satellite imagery valid 17:00Z - 18:58Z 5/5/2020 with annotation (18:58Z).

As convection developed early in the afternoon along the southern portions of the Appalachians, it pushed eastward toward areas which remained stable as indicated by the continuation of stable billow clouds (Figure 3) and corroborated by SPC mesoanalysis (not shown). This stable air mass would, at least in the near-term, bound the northern extent of the severe threat to locations close to the NC/SC border. However, breaks in the clouds were allowing for some surface-heating to occur. Furthermore, in south-central NC, winds had started to veer, becoming more southerly by 20Z. So there were some signs that pointed toward destabilization with time, particularly in areas further downstream from the developing convection.

Figure 3: GOES-16 mesosector Vis/IR Sandwich satellite imagery valid 19:00Z - 20:59Z 5/5/2020 with annotation (20:59Z).

Meanwhile, the environment along and just south of the front was characterized by 1000-1500 J/kg of MLCAPE and 50 kts of effective shear. This parameter space was more than sufficient to support supercell structures. SPC mesoanalysis at 20Z indicated a corridor of maximized effective SRH (Figure 4) in the vicinity of the warm front indicating a non-zero threat for tornadoes. Given the environment near the front and the understanding that the boundary may lift into south-central NC as time progressed, NWS Raleigh made the following statement highlighting the conditional threats for a localized region in a near-term area forecast discussion (AFD) issued at 1927Z:

“Given strongly curved hodographs across the area, this will set the stage for some embedded supercells to move through the area, potentially transitioning into a bow echo before exiting the area. If the warm sector does indeed become established across the far southern zones (this includes the areas from Wadesboro to Fayetteville to Clinton), damaging winds, large hail and an isolated tornado will be possible.”

Figure 5: GOES-16 mesosector Vis/IR satellite valid 21:05Z - 22:44Z 5/5/2020 with annotation (21:44Z & 22:44Z).

Mesoanalsysis would prove particularly valuable on a day such as this. As the afternoon progressed, it became clear that the air mass to the north and northeast of Charlotte was not destabilizing sufficiently to support a threat of organized severe weather hazards. Diagnosing this trend permitted forecasters to trim their area of focus down to a few counties where they could concentrate on precise, targeted IDSS messaging. While there was a possibility that the warm front would begin to advance northward in response to surface low development to the northwest, this became less likely with time. On this day in particular, tracking the surface front was a key component of understanding the localized severe threats, and therefore providing IDSS. One NWS Raleigh lead forecaster said, “If we vigorously monitor hourly data, combined with radar and satellite data - we can really hone in on these important boundaries - and their potential movement (if any). This way, for severe weather or any IDSS related events - we can better forecast where the worst of the storms will be.”

Meanwhile, on the opposite end of the spectrum, the likelihood of severe hazards in northern SC along the warm front increased as the boundary remained slow to move north. NWS Greenville-Spartanburg issued a near-term AFD at 2036Z which focused on the I-85 corridor as a region with higher potential to see severe thunderstorms. The severe threat became increasingly evident once convection strengthened along the boundary around 2130Z. Moreover, the enhanced SRH along the front kept a tornado potential within the spectrum of possibilities on the Probability Density Function (PDF) of potential severe outcomes. These potential severe outcomes, including a tornado threat, were communicated via the aforementioned near-term AFD. It stated, “Anticipate large hail and occasionally damaging wind gusts as the primary threat, but brief tornadic spinups near the stalled boundary are quite possible through 8 or 9 pm.” This communication serves as a good example of providing targeted IDSS by noting the severe hazards over a relatively short time-scale, and furthermore indicates the value of a thorough understanding of the mesoscale environment.

Special thanks to Dan Leins, Brandon Locklear, and Phil Badgett, all of WFO RAH, and to Bill Martin, Trisha Palmer, Andrew Kimball, and Harry Gerapetritis, all of WFO GSP, for their review and feedback on this analysis.