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date: 26 May 2017

Real-Time Flood Forecasting in the United States

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Natural Hazard Science. Please check back later for the full article.

The responsibility for official flood forecasting in the United States falls on the National Weather Service (NWS), a line office of the National Oceanic and Atmospheric Administration (NOAA), an agency of the U.S. Department of Commerce. It is carried out at thirteen river forecast centers (RFCs) of the National Weather Service. It involves a series of coordinated steps leading to the publication of the final product. The first step is the acquisition of observations and forecasts. These always include precipitation and temperature observations and forecasts, and river flow and stage observations, including reservoir discharge observations and proposed operations, where available. Additionally, some RFCs also use additional information in a qualitative way, such as snow water equivalent, snow areal coverage, soil moisture, and soil temperature.

The next step in the process is the data quality control. Although some of the quality control is performed automatically, a considerable amount of effort is dedicated to a manual quality control process of precipitation and temperature observations. Precipitation observations from rain gages are used jointly with radar observations to arrive at mean areal quantitative precipitation estimates over individual watersheds.

The initial source of quantitative precipitation and temperature forecasts (QPF and QTF) is the numerical guidance that is prepared by the National Centers for Environmental Prediction (NCEP). That numerical guidance is refined further at the RFCs in coordination with the Weather Forecast Offices (WFOs). With the precipitation and temperature observations and forecasts on hand, it is possible to execute the hydrology models and issue forecasts but only at non-regulated watersheds.

The hydrologic forecasting system the NWS uses is known as the Community Hydrologic Prediction System, or CHPS. It is based on the Flood Early Warning System (FEWS) developed by the Dutch non-profit Deltares. The NWS forecasters make deterministic runs of CHPS by river groups, starting with the upstream-most watersheds and moving downstream. The forecasters then assess the model performance by comparing past forecasts with streamflow observations, and, optionally, modify model states, precipitation or temperature input, or even model parameters. This interaction between forecaster and model results is a key component of the forecasting process. Finally, the deterministic runs serve as “hot states” for the probabilistic forecasts.

There are three types of probabilistic forecasts, depending on the source of the precipitation and temperature forcings. The first and oldest technique is the Ensemble Prediction System. A more recent ensemble forecast approach is known as the Multi Model Ensemble Forecasting System (MMEFS). The most recent approach is the Hydrologic Ensemble Forecasting System (HEFS)

The final steps are the communication of forecasts and the performance evaluation. Forecasts are communicated via a number of means: internet, NOAA radio, specific arrangements for major government users, etc. Forecast performance is assessed internally and is used to decide on what watersheds are in need of recalibration due to significant changes to the basin characteristics, such as urban development, deforestation, reforestation, etc.