Historical Depiction and Future Projection of Weakly Forced Yet High-Impact Convective Storms in Central U.S.

Severe weather outbreaks frequenting the eastern half of the United States such as the record precipitation leading to the 2016 summer extreme “1-in-1000 year” floods in West Virginia and Ellicott City, Maryland, as well as the 2012 Mid-Atlantic derecho event in late June incur tremendous costs and presents challenges to operational weather forecasting and disaster prevention and relief. These convective events are unique because they can be traced to progressive mesoscale convective systems (MCSs) that prevail under the weakly forced synoptic setting. Even though weather forecast models are generally capable of predicting the large-scale circulations, forecasting such weakly forced yet extreme weather events is difficult at both weather and climate timescales. In fact, rather than an increased storm activity as observed, forecast models tend to simulate dry anomalies under such weakly forced, large-scale circulation patterns.

The aforementioned progressive MCSs were traced to sub-synoptic-scale, mid-tropospheric perturbations (MPs) embedded in the large-scale ridge pattern. A set of metrics capable of capturing these MP features was applied to historical reanalysis where in recent years it was found that MP frequency and the associated convective precipitation exhibits an increasing trend. Given this historical trend, further exploration between historical hindcasts and future projections produced by ensemble regional climate models was conducted. Results here continued to demonstrate an increasing trend in future MP frequency and associated convective precipitation for the region under the changing climate. Results presented in this study suggest a growing influence from MPs over the region’s weather and climate. Given the difficult-to-forecast, weakly-forced conditions associated with MPs and their ability to produce extreme severe weather impacts, these results reinforce the need for a better understanding of the climatology and underlying mechanisms responsible for MP outbreaks.