A derecho is a wide-spread, long-lived straight-line windstorm associated with a fast-moving mesoscale convective system (MCS). Persistent destructive gusts associated with derechos can cause severe damage to infrastructure and property and threaten human lives. Understanding the environments for derecho initiation is critical for derecho prediction and modeling, and hence for risk assessment and infrastructure planning.

This study investigates the large-scale and near-storm environmental conditions associated with derecho initiation using a newly developed high-resolution observational derecho dataset containing the detailed tracking of derechos in the United States from 2004 to 2021 and the ERA5 reanalysis. We applied the self-organizing map analysis to classify six types of derechos with distinct large-scale environments associated with their initiations. These include four derecho types primarily occurring in the warm season, one type dominating the cold season occurrences, and one type mainly initiated during the transitional seasons (i.e., spring and fall). The six types of derechos are concentrated in different regions (Fig. 1).

The warm-season derechos primarily occur in the northern Great Plains and Midwest along a west-east axis; the transitional-season derechos mainly occur in the central Great Plains along a south-north axis; the cold-season derechos are most frequent in the Lower Mississippi Valley. Compared to non-derecho MCSs, derechos are generally initiated in an environment with larger CAPE, higher 2-meter temperatures, larger 0-6 km wind shear, steeper lapse rates between 850 and 250 hPa, higher low-level specific humidity, larger moisture advection from the Gulf of Mexico, stronger low-level convergence and mid-level ascent, and a deeper upper-level trough in the northwestern US. During the genesis stage, derechos developed in an environment with significantly lower mid-level relative humidity than non-derecho MCSs, possibly facilitating stronger downdrafts by enhancing evaporative cooling from precipitation.