Climate of the Weakly-Forced Yet High-Impact Convective Storms Throughout the Ohio River Valley and Mid-Atlantic United States

The 1-in-1000-year precipitation event in late June 2016 over West Virginia caused tremendous flooding damage. Like the 2012 mid-Atlantic derecho that blacked out much of the DC area, similar events can be traced to small, mid-tropospheric perturbations (MPs) embedded in the large-scale ridge pattern. Under this “weakly-forced” pattern, severe weather outbreaks commonly occur alongside eastward propagating MPs acting as a triggering mechanism for progressive mesoscale convective systems, which move across the central and eastern US. Forecasting of such weakly-forced yet severe weather events is difficult in both weather and climate timescales. The present diagnostic analysis of the MP climatology is the first step toward developing metrics that can identify and evaluate weakly-forced severe weather outbreaks in multi-model projections. We report a discernable, potentially pronounced subseasonal change in the MP climatology associated with the changing climate of North America. Both sea surface temperatures within the Gulf of Mexico and mid-level high pressure over the central US were found to exhibit strong correlations with MPs. Analysis of regional climate downscaling indicates a projected increase in MP frequency and the associated convective precipitation through the mid twenty-first century