On a Modified Definition of a Derecho. Part II: An Updated Spatial Climatology of Derechos across the Contiguous United States

Derechos are known as widespread severe windstorms that accompany extratropical MCSs. However, questions remain whether the term derecho should apply to widespread windstorms from all linear storm structures, or just MCSs that evolve from a progressive cold pool, which support concentrated swaths of damaging winds. Part I of this manuscript focused on the derecho definition to windstorms with hurricane-force gusts that originate from cold-pool-driven MCSs. While Part I modified the derecho definition and formulated quantitative criteria for identifying future events, a derecho spatial climatology across the contiguous United States was derived for 2000-22 using this more focused definition in Part II. The derecho climatology was created from cases identified with the same algorithm employed in Part I of this manuscript, which ingested a combination of datasets to objectively detect derechos. For the 2000-22 climatology, derechos were found to be primarily warm-season events, with corridors of derecho occurrence noted across the Midwest and southern plains, in agreement with earlier studies, as well as the northern plains. Seventy derechos were identified over a period of 23 years. Derechos occur, on average, three times per year nationally, and once every 1-2 years at given points within the plains and Midwest corridors, with greater frequencies noted in earlier studies, likely due to stricter criteria requiring hurricane-force gusts and supporting cold-pool-driven MCS structures in the present study. Derechos frequently form in the afternoon and dissipate later at night but peak in intensity just a few hours after development. SIGNIFICANCE STATEMENT: Part I of this study modified the definition of the word "derecho" to associate the term only with hurricane-force-bearing thunderstorm wind swaths originating from cold-pool-driven MCSs. Part II focuses on showing a 23-yr climatology of derechos consisting only of events meeting this new definition and accompanying quantitative criteria, likely being a first of its kind. This derecho climatology is also unique in that it encompasses a longer sampling period compared to all known earlier studies yet shares little to no overlap with these studies, which were performed in wind-gust-reporting regimes that were less reliable compared to more recent decades.