Process-Focused, Multiscale, Integrated Hydrometeorological Assessments toward Understanding National Water Model Forecasts: A Case Study of the 27 May 2018 Ellicott City Flood

On 27 May 2018, extreme localized precipitation brought catastrophic flood damage and one fatality to central Maryland, particularly Historic Ellicott City, MD. Flood damage from road washout, land erosion, and localized landslides present challenges even now as the region continues to recover from the second such extreme precipitation event in less than two years.

The NOAA National Water Model (NWM) became operational in August 2016, producing the first real-time, distributed, continuous set of hydrologic forecasts over the Continental United States (CONUS) and significantly expanding forecast guidance, spatially and temporally, in previously underserved locations. With such a large forecast domain, understanding regional and local-scale errors and uncertainties for the relatively new NWM is no small task. The NWM also creates opportunities to complement existing hydrologic forecast guidance from river forecast centers (RFCs) and weather forecast offices (WFOs) but how these complements are best accomplished is not always clear. Here, a comprehensive case study assessment of the 2018 Ellicott City flood has been undertaken, focusing on hydrometeorological processes including the meteorological mesoscale precipitation forecasts, the NWM representation of land-surface and riverine hydrology, and experimental flood inundation maps. Results highlight that “good” meteorological forecasts do not necessarily guarantee accurate streamflow forecasts, and that NWM streamflow forecast skill on shorter timescales does not necessarily scale with basin size following traditional hydrologic arguments. Potential best-practice usage of NWM output alongside existing official forecast guidance will also be discussed.

This presentation will dissect the 2018 Ellicott City flood with the objectives of (i) evaluating the forecast utility of the short-range NWM forecasts for this event, (ii) disentangling hydrologic model error from atmospheric model (forcing) error, and (iii) demonstrating the importance of truly integrated (i.e., hydrologic and meteorological), multi-scale case study assessments to engender forecaster/user engagement on, and understanding of, the NWM as potential flood forecast tool.