Observational and regional modeling analyses show that summertime convective outbreaks like the 2012 D.C. “derecho” have been occurring more frequently in recent decades and will increase in future warmer climate.
The northwest flow type of severe weather outbreaks like the 2012 “DC derecho” is a common mid-summer feature, but climate models have failed to depict it due to the tendency for models to treat the relatively weak storm environment as fair and dry weather. We developed metrics to capture these northwest flow storms in climate models and found a connection between such severe weather and warming of the Gulf of Mexico. Our finding points to the possibility that mid-summer severe weather outbreaks impacting the northeastern U.S. will become more frequent.
The 1-in-1000-year precipitation event in late June 2016 over West Virginia caused tremendous flooding damage. Similar events like the 2012 mid-Atlantic derecho that blacked out much of the D.C. area can be traced to small, mid-tropospheric perturbations (MPs) embedded in the large-scale ridge pattern. Under this “weakly-forced” ridge pattern, severe weather outbreaks that move across the central and eastern U.S. occur alongside eastward propagating MPs acting as a triggering mechanism. Forecasting of such weakly-forced yet severe weather events is difficult in both weather and climate time scales. The present diagnostic analysis of MPs is the first step toward developing metrics that can identify and analyze weakly-forced severe weather outbreaks in multi-model projections. We report a pronounced seasonality change in the MP climatology associated with the changing circulation features over North America. Both sea surface temperatures within the Gulf of Mexico and mid-level high pressure over the central U.S. exhibit strong correlations with the MP climatology. Regional climate modeling projects an increase in MP frequency and associated convective precipitation towards the mid 21st century.