Extreme precipitation events in the Southeast United States: Climatology, environmental properties, and predictability

The Southeast U.S. can experience extreme precipitation in all seasons in connection with a variety of phenomena, making quantitative precipitation forecasting in this region difficult. An enhanced understanding of the key meteorological processes and the forecast challenges associated with extreme precipitation in the Southeast can likely provide useful guidance to operational forecasters in identifying and predicting the occurrence of extreme precipitation.

In this study, a climatology of extreme precipitation events in the southeastern United States during 2002–2011 is derived using daily (1200 UTC–1200 UTC accumulations) 4-km NCEP Stage-IV quantitative precipitation estimates. Events in the climatology are classified as “tropical” if they were produced directly by a tropical cyclone or its remnants and “non-tropical” otherwise. Results of the climatology indicate that non-tropical extreme precipitation events in the Southeast occurred most frequently in the spring and fall and least frequently in the summer. In the winter and spring, non-tropical events occurred most frequently in the interior Southeast, west of the Appalachian Mountains, often in connection with strong synoptic-scale weather systems, while in the fall non-tropical events were most frequent east of the Appalachian Mountains. Tropical events occurred most frequently in the late summer and early fall and predominately affected the eastern portion of the Southeast.

Synoptic-scale composites are produced in order to examine the key environmental properties of non-tropical extreme events, with a focus on differentiating events featuring strong water vapor transport from low latitudes and strong dynamics from those featuring weak water vapor transport and weak dynamics. Subsets of non-tropical events are selected for composite analysis based upon the magnitude of time-integrated vertically integrated water vapor transport (IVT) associated with each event. The composite synoptic-scale environments of “strong IVT” and “weak IVT” events are then examined. In general, the strong IVT events feature a deep upstream upper-level trough, strong low-level winds, and a plume of moist, unstable air extending poleward from low latitudes, whereas the weak IVT events feature minimally amplified upper-level flow, weak low-level winds, and very moist and unstable conditions.

Lastly, verification of deterministic and probabilistic precipitation forecasts from the Hydrometeorological Prediction Center as well as the NOAA/Earth System Research Laboratory Reforecast ensemble is conducted for each event in the climatology in order to explore the general predictability of extreme precipitation events in the Southeast and to identify scenarios associated with exceptionally high/low predictability. This verification analysis motivates further observation- and numerical model-based investigations of the physical processes and environmental properties associated with high- and low-predictability events.