The Derecho: NWS Warning Challenges, the Associated Societal Response, and a Proposal for the Development of a Derecho Classification Scale

On July 8, 2014, an organized and long-lived line of severe thunderstorms formed into a mesoscale convective system (MCS) that intensified and produced widespread wind damage over an ~ 800 km path from northeast Kentucky to southeastern New York, affecting millions of people across its swath of damage. This storm system caused two fatalities, eight injuries, and left hundreds of thousands without power – some outages lasted several days. In the days after, the National Weather Service (NWS) Baltimore/Washington Weather Forecast Office (WFO) was inundated by media inquiring if the MCS was officially classified as a “derecho.”

Societal interest in derechos in the greater Baltimore/Washington region has heightened greatly since the intense media exposure following the devastating derecho of June 29, 2012 that impacted the Ohio Valley and Mid-Atlantic regions. That derecho killed nearly two dozen people and adversely impacted millions due to long-lived power outages from falling trees and power lines.

Prior to the June 2012 derecho, many in the eastern U.S. were unaware that the term “derecho” even existed. Since then, people living in the eastern U.S. are more sensitive to any potential thunderstorm system that could be classified as a derecho. In addition, media interest is also keen anytime linear-appearing MCSs are either forecast by models or are depicted by radar. Although derechos are a relatively rare phenomena in the eastern U.S. (once every ~4 yrs), a great deal of public anxiety concerning derechos exists whenever any severe thunderstorms are forecast or occur.

The classic definition for a derecho Johns and Hirt (1987) (JH87) sets forth a few objective criteria. More recently, other researchers have set forth similar criteria to that of JH87 for defining a derecho. The NWS Storm Prediction Center (SPC) maintains some information on past MCSs that have been classified by researchers as derechos, but currently there is no requirement for any entity of the NWS to consistently review and classify organized MCSs for possible classification as derechos. With no central, definitive source of derecho classification in the U.S., local NWS WFOs are challenged to respond to media interest on these significant MCSs. The 8 July 2014 storms appeared to meet the JH87 criteria, and possibly some of the other aforementioned criteria in the more recent studies, but no NWS official classification was provided.

Additionally, derechos that meet the basic meteorological criteria can have varying levels of societal impacts, owing largely to their maximum wind gusts, overall size and their path relative to population centers; all criteria not included in existing definitions. For example, the June 2012 derecho was clearly recognized as having significant societal impacts. By contrast, other derechos, while technically meeting the meteorological criteria, produce less societal impact. For example, both the July 2014 derecho as well as the derecho of June 4, 2008 (Zubrick et. al 2009) took a similar path as the June 2012 derecho, but both caused far fewer fatalities/injuries and damage.

Overall, for the three aforementioned derecho cases, NWS watch and warning services generally met or exceeded goals for advance notice and detection of severe thunderstorms. Still, many people were not expecting the intensity and widespread nature of the damage caused by these derechos. The NWS Severe Thunderstorm Warning product is largely the same, whether it's for an isolated severe thunderstorm with a targeted damaging wind gust over a few hundred meters, an organized squall line, or a significant derecho.

Not only is the type and content of message similar in all NWS Severe Thunderstorm Warnings, but the distribution methods are as well. In contrast, NWS Tornado Warnings take advantage of alerting people through the FCC/FEMA Wireless Emergency Alerts (WEA) System over nationwide cellular networks; whereas Severe Thunderstorm Warnings are not communicated over the WEA System - largely due to their frequency of occurrence.

Therefore, this paper contains three recommendations for consideration: 1) have the scientific meteorological community develop a consistent derecho definition and classification system that is analogous to the classification systems of tornadoes, hurricanes, and drought; 2) train forecasters to identify in real-time the existence of a derecho, and 3) to have the WEA System include NWS Severe Thunderstorm Warnings for derechos once they are identified, to assist in increasing the response of the public to their enhanced threat to life and property.

By better defining these deadly windstorms, recognizing their existence in real-time, and then using enhanced techniques to warn the public before they strike, the outcome – though damaging – can be less tragic. This three-step approach would further society's progress toward becoming a Weather-Ready Nation.