Progressive Derechos in the Presence of Closed Upper-level Subtropical Anticyclones

Progressive derechos are a type of long-lived mesoscale convective system that produces large swaths of wind damage. In contrast to their serial derecho counterparts, which form in association with extratropical cyclones, progressive derechos often occur in the presence of benign synoptic conditions on the poleward side of closed upper-level subtropical anticyclones. Forecasters have been known to struggle predicting progressive derechos with any certainty due to the common lack of large-scale support for severe weather in regimes dominated by anticyclonic conditions. This study will classify a group of days on which there was a closed upper-level anticyclone over the United States and a progressive derecho did not occur and a group of days on which there was a closed upper-level anticyclone over the United States and a progressive derecho did occur, examine the synoptic environments of the two groups, and identify derecho null cases. By analyzing the null cases, derecho failure modes will be determined, which will help forecaster situational awareness and reveal the science behind the environmental conditions necessary for, and detrimental to, derecho development.

This presentation will include climatologies of both derechos and closed upper-level anticyclones over the United States for June, July, and August of the years 1994-2013 (the modern radar era). The presentation will also include closed anticyclone-relative composites of both derecho cases and derecho null cases. The composites will elucidate which conditions are necessary for, and which are detrimental to, derecho development. The hypothesis is that derecho failure days occur due either to the lack of a triggering mechanism or other phenomena working against convective development such as a strong capping inversion or transverse ageostrophic circulations around an upper-level jet creating subsidence in an otherwise favorable environment. A representative case study will be included to highlight a common derecho failure mode.