National Weather Service Flood Surveys & Post-Event Analysis of Hurricane Isaac

Author(s): Lincoln, W.

After the substantial impact to the United States East Coast from Hurricane Sandy, Hurricane Isaac may become the forgotten hurricane of 2012. With its above average size and slow forward motion, Isaac produced higher storm surge than typically seen by a storm of its wind category, and also dropped notably heavy rainfall across portions of southeast Louisiana and south Mississippi. Over a four day period from August 28th to August 31st, rainfall totals ranged from 10-15 inches across most of the area, with a few areas seeing more than 20 inches. This significant rainfall caused flooding of numerous rivers in the forecast area of the National Weather Service (NWS) Lower Mississippi River Forecast Center, especially areas within the county warning area of the New Orleans/Baton Rouge Weather Forecast Office. Because of the rare nature of the event, a team composed of NWS staff from multiple offices was assembled to record the impacts, survey flood crests when necessary, and discuss the event with local residents. Post-event flood surveys were conducted over a number of days in early September, 2012, particularly across the Wolf, Tchoutacabouffa, Biloxi, and Escatawpa River watersheds in Mississippi and the Tangipahoa River watershed in Louisiana. A vast amount of observations, anecdotal data, and recommendations were collected by the survey teams and summarized in a report for the River Forecast Center and the Weather Forecast Office. Flooding of numerous locations was of a magnitude seen only on very rare occasions and may have been the worst flooding yet-experienced by numerous long term residents. Luckily, due to the sparse population density in most of the river floodplain areas, impacts were not as severe as would typically be expected. Findings from the post-event flood surveys and analysis of data from numerous sources will be presented to further our understanding of Isaac’s hydrologic impact.

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