Thursday, April 1, 2010

Another small CAPE/large shear tornado setting: 3/28/10 in North Carolina

Last Sunday afternoon and evening (3/28/10) in central North Carolina was a great example of a small CAPE setting (300-500 J/kg) that had very large low-level and deep layer shear and was able to support a strong tornadic supercell (see photos above), including an EF3 tornado at High Point, just southwest of Greensboro.

Lowest elevation base reflectivity from radar and a surface map just before 2300 UTC (7 p.m. EDT, see 2nd graphic above) showed this supercell moving northeast near a southwest-northeast stationary front with a well-defined moisture axis (dew points low to mid 50s F) impinging on it from the south. The SPC mesoanalysis at 2200 UTC (3rd graphic above) showed very large storm-relative helicity (SRH) over central NC but only weak/marginal CAPE, with larger CAPE indicated back to the southwest. But as suggested by the moisture axis on the surface map, strong moist advection was definitely taking place across North Carolina.

This case highlights how important it is to update and modify model soundings with actual surface data when estimating environments. The raw RUC sounding at Asheboro (HBI, about 20 miles southeast of the tornadic supercell track, see 4th graphic above) showed only 61/55 F as temperature and dew point at 2300 UTC, but observed surface data indicated at least 63/57 F, which can make a considerable difference in CAPE computation. Modifying this sounding with the observed data (see last graphic above) and using a surface-based lifted parcel (the low-levels were nearly saturated) changed the CAPE from barely over 100 J/kg to well over 400 J/kg, a nearly four-fold increase! Combined with the very large 0-1 km SRH (> 500 m2/s2) and deep layer shear (near 70 kts), this was more than enough to support a significant tornado that caused injuries.

When using model soundings to estimate a severe weather environment, it's vitally important to watch how well the model is performing by checking actual observations, and to make adjustments when necessary... no April fooling!

- Jon Davies 4/1/10

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