Synoptic Precursors to Subseasonal to Seasonal Extreme Precipitation Events across the United States

In the United States and throughout the world, extreme precipitation events are a major cause of loss in life, property, and economic progress. Although the science of hydrometeorology has made significant improvements to the prediction and understanding of these events in recent decades, there is still much to learn about these events in the subseasonal to seasonal (S2S) timescale. This presentation focuses on identifying the synoptic precursors ahead of an extreme precipitation event over the lower 48 as part of the NSF PREEVENTS Prediction of Rainfall Extremes at Subseasonal to Seasonal Periods (PRES²iP) Project. First, we provide a robust definition for “extreme precipitation events,” based on 14- and 30-day running precipitation totals from Parameter Elevation Regression on Independent Slopes Model (PRISM) daily precipitation data. Criteria for the events include exceedance of percentile thresholds, spatial extent, and consideration to the distribution of rainfall over the event period. The CONUS is partitioned into different geographic regions in order to compare and contrast the synoptic patterns associated with events in different parts of the country. Using these identified events, atmospheric variables from reanalysis (e.g., geopotential height, zonal and meridional winds, sea level pressure, and moisture flux) are composited to understand the evolution of the atmospheric state before and during an S2S extreme precipitation event. Common signals seen include deep troughing to the west of the region several days before event onset, an energized subtropical jet stream, and anomalously strong moisture fluxes from the south. Regional differences in frequency and in seasonality of events and precursors are also discussed along with the extension of this work to other regions of the US. Taken together, our preliminary results represent a step forward in helping forecasters to identify possible extreme events with long-leads in order to mitigate loss due to extreme precipitation events.