The event was well anticipated, with the 12Z NAM model from the previous day (15 April 2011) forecasting CAPE values and 0-6 km shear vector magnitudes that would easily support supercells, as well as long, strongly curved hodographs with very large values of 0-1 km and 0-3 km storm-relative helicity. The mid-day 1600 UTC supplemental sounding from Greensboro, NC, revealed observed wind profiles that were even more favorable for supercells and tornadoes than what models had forecast, although thermodynamic instability was somewhat limited owing to widespread overcast and stratiform precipitation during the morning hours. A squall line had formed by 1600 UTC along a cold front that was crossing western North Carolina; this squall line accounted for 4 reports of weak tornadoes in southwestern North Carolina. Apparently due to the exceptional vertical wind shear and hodograph curvature, over the subsequent 2-3 hours, the squall line broke up into discrete supercells which then moved off of the primary cold front into the warm sector. During that time span, breaks in the overcast, accompanied by continuing warm advection, also led to surface temperature increases of roughly 5-6 degrees C. As a result, by 1800 UTC mesoanalyses indicated a broad region of CAPE values > 1000 J/kg over central North Carolina. By that time, a group of long-track supercells were moving rapidly northeastward and had begun to produce the stronger, longer-lived tornadoes that constituted the bulk of the outbreak in central North Carolina.
Our presentation will address both the unique meteorological aspects that caused 16 April 2011 to be a record-breaker in North Carolina, as well as some of the service and societal aspects of the warning delivery system during the outbreak.