July 1st 2012 Derecho: Decision Support for a High Impact, Low Confidence Event

An organized line of thunderstorms crossed the National Weather Service (NWS) Newport/Morehead City (MHX) County Warning Area (CWA) on the afternoon of July 1st, 2012. The Mesoscale Convective System (MCS) swept across eastern North Carolina with very strong winds and claimed three lives along its path. The MCS, which was later categorized as a derecho, was fueled by several factors, including residual outflow boundaries, a Piedmont Trough, mid-level shortwaves, and extreme surface heating and moisture content. The latter two factors led to an extremely unstable environment with unprecedented convective available potential energy (CAPE) and lifted index (LI) values. Observed regional soundings indicated unusually cold air aloft, a pronounced elevated mixed layer (EML) present with weak capping in place, and steep 700-500 mb lapse rates. These factors combined to produce an environment capable of producing severe convection, which led to the derecho.

A forecast challenge for the weather forecast office and the Storm Prediction Center ahead of this event was 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 prior to the event. Area Emergency Managers, Broadcast Meteorologists, and Social Media outlets were not fully prepared for this potential event. During the event, limited interaction was made through NWSChat and social media, with much of the staffing focused on radar, verification, and telecommunication. Many lessons and best practices were gained from this high impact, low confidence event that will be used moving forward. More often than not, these types of events are within the range of possible outcomes, but may not come into fruition. Weather forecast offices need to be prepared for high impact, low confidence events in an effort to achieve optimal decision support for essential personnel and public.