Short- and Medium-Range Predictability of Warm-Season Derechos. Part I: Operational Predictability and Composites

Abstract This study analyzes the operational predictability of warm-season (May–August) progressive derechos. A subset of 47 derechos occurring between 2010 and 2022 were selected based on NCEI Storm Data and radar imagery. The Storm Prediction Center Convective Outlooks issued from 5 days before each derecho to the day of the event were used to determine whether the severe weather and derecho predictability was low, moderate, or high, for each case. The cases were also categorized based on the synoptic-scale midlevel flow pattern. Composite maps of synoptic patterns associated with the derecho cases and distributions of severe weather parameters in the derecho inflow region were generated. About 72% of the 47 derechos selected for this study were associated with low predictability, and the remainder were categorized with moderate predictability. None of the cases had high predictability. Most derechos occurring under northwest- and zonal-flow regimes (80% and 85%, respectively) had low predictability. Composites of low- and moderate-predictability derechos indicate that derechos formed in the equatorward entrance region of the upper-level jet, where low-level warm advection exists in conjunction with an equivalent potential temperature maximum. A midlevel trough is located upstream of the derecho initiation point in many of the moderate-predictability composites but is absent in the low-predictability composites, which indicates weaker synoptic-scale forcing for ascent in low-predictability cases. The distributions of severe weather parameters for low- and moderate-predictability derechos are similar, suggesting that these parameters alone are not particularly useful as a discriminator of derecho predictability. Significance Statement Derechos are characterized by swaths of damaging wind gusts caused by an organized group of long-lived convective storms. These events are more frequent in the summer and are often difficult to predict far in advance. This study analyzes a subset of derecho cases to better understand the differences between cases with lower and higher predictability. About 72% of the 47 derechos examined in this study had low predictability. In these cases, the development and maintenance of storms are typically associated with larger uncertainty, but if persistent storms do form, there is potential to produce a derecho. These findings are part of ongoing work that examines ways to improve derecho forecasts with greater lead time.