July 1st 2012 Derecho: Overview of a High Impact, Low Confidence Event across Eastern North Carolina

A violent line of severe thunderstorms traversed the National Weather Service (NWS) Newport/Morehead City (MHX) County Warning Area (CWA) on the afternoon/evening of July 1st, 2012. This Mesoscale Convective System (MCS) quickly swept across eastern North Carolina with wind gusts reaching as high as 84 miles per hour, claiming three lives along its path. The MCS, which was later classified a derecho, was fueled by several factors, including residual outflow boundaries, a Piedmont Trough, mid-level shortwaves, and extremely high surface heating and moisture content. The latter two factors led to an extremely unstable environment with unprecedented convective available potential energy (CAPE) values of around 8000 J/kg, Wind INDEX (WINDEX) values above 60 kt, and lifted index (LI) values near -18 ⁰C. Observed regional soundings indicated unusually cold air aloft, a pronounced elevated mixed layer (EML) present with weak capping in place, and 700-500 mb lapse rates on the order of 9-9.5 ⁰C/km. A broad trough was situated across New England, with high pressure anchored across the southeast. Mid-level impulses were riding the base of the upper trough under mean west-northwest flow aloft, leading to previous MCSs traversing portions of the Ohio River Valley and Mid-Atlantic region the day before. This resulted in residual outflow boundaries across the Piedmont and Coastal Plain regions of North Carolina early on July 1st. These factors combined to produce an environment capable of producing severe convection.

Even though the environment was very conducive for severe convection, there were a few forecast challenges. Ahead of this event, the weather forecast office and the Storm Prediction Center struggled to forecast the potential placement and spatial coverage of the severe weather. Given such uncertainty, broad wording was placed within the area forecast discussions and convective outlooks, and as a result little decision support was provided to Emergency Management, media, and social media outlets. This event review reflects the overall implications of a high impact, low confidence event that if not communicated properly can have significant consequences. Many best practices were developed from this event with the main recommendations made toward better communication, radar analysis, polygon maintenance, and social media interaction.