DERECHO HAZARDS IN THE UNITED STATES BY WALKER S. ASHLEY AND THOMAS L. MOTE Derechos are shown to be as hazardous as many tornadoes and hurricanes that affect the United States. uring the summer, hikers and canoeists often straight for them. The group was canoeing on Knife flock to Minnesota's Boundary Waters Canoe Lake in the BWCAW around noon when threatening DArea Wilderness (BWCAW) to observe the skies brought the group to shore. The group quickly beauty and tranquility of this remote land that is beached their canoes and took cover. filled with picturesque lakes and forests. However, As the storm hit, Emily and Maddy quickly pulled on the July 4th weekend in 1999, the serenity so often the girls together and huddled over them. Watching identified with the region briefly gave way to a terrify- trees crash around them, the two leaders hurried the ing display of one of nature's most violent storms—a group to an area that appeared safer near the water's derecho. edge. Although the storm seemed to last an eternity, On this holiday weekend, camp counselors Emily blue sky appeared on the western horizon a mere 30 Boyd and Maddy Bennett were guiding a group of min later. six teenage girls on a weeklong canoe exploration After the storm, the group returned back to the out of Moose Lake near Ely, Minnesota. Unknown location where they had huddled to gather their life to the group, a large windstorm producing gusts in vests. To their shock, felled trees had crushed the excess of 40 m s_1 had already cut a swath of dam- vests. One can only imagine in horror what would age across northern Minnesota and was heading have happened to the girls if Emily and Maddy had not moved them to the safer location. Emily later recalled that it was a "true miracle that no one [from AFFILIATIONS: ASHLEY—Meteorology Program, Department of their group] died in the storm that day." Shocked, Geography, Northern Illinois University, DeKalb, Illinois; MOTE— yet uninjured, the group spent the next two days Climatology Research Laboratory, Department of Geography, paddling over 20 miles and making six portages for The University of Georgia, Athens, Georgia home. Regrettably, 25 other hikers and canoeists CORRESPONDING AUTHOR: Walker Ashley, Meteorology in the area were not as lucky and suffered injuries, Program, Department of Geography, Northern Illinois University, including broken backs and necks. During the next DeKalb, IL 60II5. several days, rescue crews searched 2,200 camp- E-mail: [email protected] DOI: 10.1175/BAMS-86-11-1577 sites in the BWCAW and Superior National Forest, evacuating about 20 campers by air (Breining 2000). In final form 20 July 2005 ©2005 American Meteorological Society Unfortunately, the storm killed two campers farther downstream—one in Quebec, Canada (Mainville AMERICAN METEOROLOGICAL SOCIETY NOVEMBER 2005 BAFft | 1577 Unauthenticated | Downloaded 10/08/21 03:33 PM UTC 1999), and another in Vermont (NCDC 1999). The axis between 40 and 400 km) or for larger events, a Vermont death occurred as a person camping in a tent family of downburst clusters (major damage axis of on a raft was blown into the water, became entangled 400 km or more). Broadly, Johns and Hirt (1987) de- in the tent, and subsequently drowned. fined the derecho to include any family of downburst Convectively generated windstorms occur over clusters (Fujita and Wakimoto 1981) produced by an a wide array of temporal and spatial scales (Fujita extratropical convective system, but also developed 1978; Fujita and Wakimoto 1981; Golden and Snow specific criteria to define derechos utilizing con- 1999); however, the more widespread and longer temporary terms and datasets (cf. their section 2). lived of these windstorms are termed "derechos" Later studies by Bentley and Mote (1998), Evans (Hinrichs 1888; Johns and Hirt 1987). Though an and Doswell (2001), Bentley and Sparks (2003), and extreme derecho case, the 4 July 1999 windstorm Coniglio and Stensrud (2004) have utilized differing that swept through the BWCAW is an unforgettable criteria to define derechos. example of the ferocity that can be associated with United States derechos have been documented in derechos. the literature since the early 1980s (e.g., Johns and Although, Johns and Hirt (1987), Johns (1993), Hirt 1983); only recently have enough events been and Wakimoto (2001) have claimed that derechos documented over a sufficient period to begin to ana- account for much of the structural damage and lyze the climatology of these windstorms. Johns and casualties resulting from convectively induced non- Hirt (1987) were the first to present a climatology of tornadic winds, no study has examined the damage these events for the warm seasons from 1980 to 1983. and casualty statistics from derechos. Changnon et al. Bentley and Mote (1998) and Bentley and Sparks (2001) suggest the threat from weather disasters is (2003) recently analyzed the years 1986-2000, provid- likely to grow because of a number of societal factors, ing the basis for a 15-yr derecho climatology, includ- including rapid population growth and expansion, ing both cool- and warm-season events. These stud- together with increases in wealth, development, and ies identified the tendency for derechos to occur in urbanization. In order to illustrate the future risk specific seasonal corridors in the eastern two-thirds and potential vulnerability of the U.S. population of the United States. A separate study by Coniglio to these extreme windstorms, the following study and Stensrud (2004) attempted to further interpret reveals the hazards1 associated with derechos by the climatology of derechos by classifying systemati- examining the climatology, casualty statistics, and cally, based on specific intensity classifications, 244 damage estimates of events that occurred during the derechos from 1986 to 2001 (cf. their Table 1). These 18-yr period 1986-2003. It is initially hypothesized climatologies do contain minor differences but, in that derechos can be as hazardous, due to relatively general, substantiate each other in regard to the rela- high frequencies and large spatial extents, as some tive spatial and temporal distributions. Nevertheless, hurricanes and tornadoes. none of the studies could be considered a definitive climatology because of issues regarding the severe BACKGROUND. In 1888, Iowa physical scientist storm wind-event database (e.g., Doswell and Burgess Gustavus Hinrichs termed any convectively induced 1988; Weiss et al. 2002; and Schaefer et al. 2003), straight-line windstorm, or "straight blow of the the varying criteria utilized (Coniglio and Stensrud prairie," a derecho (a Spanish derivative meaning 2004), and the relatively short time period examined "straight-ahead" or "direct") in correspondence (Bentley and Sparks 2003). Furthermore, no study with the Spanish derivative tornado (based on the has examined thoroughly the hazards associated Spanish word tornar—meaning to "to turn"). Fujita with derechos. Therefore, this investigation examines and Wakimoto (1981) illustrated many of the same the casualties and damage produced by these events characteristics of Hinrichs' definition, labeling these in order to provide researchers, policy makers, and events as either a downburst cluster (major damage emergency managers with valuable information re- garding the often-overlooked extreme straight-line 1 Derecho hazard encompasses some aspect of derecho cli- windstorms known as derechos. matology, but also includes the effect these storms have on people and the built environment. In terms of this study, a RESEARCH METHODOLOGY. Derecho dataset. derecho hazard is defined as any derecho that results in a The contiguous U.S. derecho dataset utilized in this human casualty or any amount of reported economic dam- study was compiled through several sources including age. This is analogous to the "tornado hazard" description two long-term climatological studies—Bentley and provided by Boruff et al. (2003). Sparks (2003) and Coniglio and Stensrud (2004). First, 1578 I BAF15- NOVEMBER 2005 Unauthenticated | Downloaded 10/08/21 03:33 PM UTC a dataset containing 230 derechos identified by Bentley Every effort was made to remove from the dataset and Mote (1998) and Bentley and Sparks (2003) for the cases in which swaths of damage were caused solely period 1986-2000 was obtained (M. L. Bentley 2003, by supercells rather than quasi-linear convective sys- personal communication). Data utilized in the Bentley tems. However, the limitations of the composite radar and Mote (1998) and Bentley and Sparks (2003) stud- data made it difficult in some cases to distinguish ies were derived from the Storm Prediction Center's storm types, especially prior to 1995. Therefore, it (SPC's) online database of several convective wind is possible that some supercell, particularly high- gusts and the SPC's SeverePlot software (Hart 1993). precipitation supercell, cases remain in the dataset Bentley and Mote and Bentley and Sparks (2003) modi- prior to 1995. fied existing derecho identification methods proposed Additionally, no attempt was made to include the by Johns and Hirt (1987) in order to facilitate analysis requirement of a minimum of three 33 m s_1 wind of the large dataset (cf. their Table 1 or Coniglio and gust reports as originally suggested by Johns and Hirt Stensrud 2004, their Table 1). (1987). There appear to be a number of unresolved Second, the derecho database employed in stud- issues that suggest that the inclusion of the 33 m s_1 ies by Coniglio and Stensrud (2004) and Coniglio wind gust criteria may promote irregularities and et al. (2004), consisting of 244 events from 1986 to biases in the dataset. These unresolved issues include 2001, was acquired (available online at www.nssl. population biases (Johns and Evans 2000), changes noaa.gov/users/mcon/public__html/derlist.htm). in the verification practices (Schaefer et al. 2003), Coniglio compiled the database (hereafter Coniglio significant irregularities in extreme wind gust data and Stensrud 2004 dataset) utilizing the SPC's severe when examined by National Weather Service (NWS) convective wind database, SeverePlot software, and county warning areas of responsibility (Weiss et al.