Investigating the July 2018 Mid-Atlantic Floods with NASA GMAO Forecast and Reanalysis Models

Despite dry conditions for the first half of the month, July 2018 ended as the wettest on record in the Mid-Atlantic region of the United States as a multi-day extreme precipitation event unfolded over large parts of Pennsylvania and Maryland with extensive flooding. This particular event was associated with an atmospheric river (AR) that originated in the Gulf of Mexico positioned in a roughly south to north orientation along the east coast of the United States, culminating in deep tropical moisture convergence over the Mid-Atlantic. Extreme precipitation events such as this are rare, yet becoming more common under a changing climate. Using NASA’s Goddard Earth Observing System (GEOS) and the Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), the extreme precipitation event of late July 2018 will be investigated from the perspective of numerical weather prediction, relation to prior extreme events in the MERRA-2 record, and attribution to features indicative of a changing climate. It will be demonstrated that long range forecasts in the GEOS system indicated a mid-level height anomaly pattern analogous with those of prior notable long-duration and major Mid-Atlantic rainfall events leading into the event. The rainfall accumulation with then be put into perspective of past events in the region using the RX5Day index developed by the CLIVAR Expert Team on Climate Change Detection and Indices (ETCCDI) and indicates the maximum five-day accumulation within a month as well as the number of five-day periods exceeding an accumulation of 50mm. Finally, the attribution of select features that played a role in the culmination of the multi-day event and the position and intensity of the AR will be examined. Particular attention will be given to the sea surface temperature in the Gulf of Mexico and the Gulf Stream as well as the anomalous position of the North Atlantic subtropical high.