Development of a Comprehensive Gauge-Based Flood Frequency Analysis for the National Weather Service Baltimore/Washington Hydrologic Service Area

Gauge-based flood determinations in the National Weather Service (NWS) have been largely focused on river forecast points for many years. In the NWS Baltimore/Washington Weather Forecast Office Hydrologic Service Area (HSA), river forecast points account for only ten percent of all gauged locations. This yields over 200 gauged locations which have been providing real-time data, in some cases for decades, without context.

As part of this study, flood stages and impacts, where applicable, were determined at the remainder of the gauged locations using a combination of in-field survey work, FEMA flood insurance studies, high-resolution LiDAR imagery, and historical reports of flooding. Using peak flow and stage data from the United States Geological Survey (USGS), a comprehensive flood frequency analysis was performed to determine which locations are most susceptible to flooding. This analysis results in operational forecasters being focused on the locations with a history of frequent flooding while monitoring stream levels across the entire HSA.

Additionally, using the results of the flood frequency analysis, each gauge was further classified by the creation of a flood threat score. This classification allows operational forecasters improved situational awareness of locations that flood frequently. The collected impact statements for each location are available for use in flood warnings and statements when a site is threatened by flooding. The results of this study allow the gauge data to be utilized for verification of floods and flash floods, even when no other reports are received.