ENSEMBLE
A collection of numerical model results that show slightly different possible outcomes.
Ensemble Forecast
Multiple predictions from an ensemble of slightly different initial conditions and/or various versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
Ensemble Hydrologic Forecasting
In hydrologic terms, a process whereby a continuous hydrologic model is successively executed several times for the same forecast period by use of varied data input scenarios, or a perturbation of a key variable state for each model run. A common method employed to obtain a varied data input scenario is to use the historical meteorological record, with the assumption that several years of observed data covering the time period beginning on the current date and extending through the forecast period comprises a reasonable estimate of the possible range of future conditions.
Ensembles
Reference to a set of computer models run under the concept of Ensemble Forecasting: multiple predictions from an ensemble of models with slightly different initial conditions used as input and/or slightly different versions of models. The objectives are to improve the accuracy of the forecast through averaging the various forecasts, which eliminates non-predictable components, and to provide reliable information on forecast uncertainties from the diversity amongst ensemble members. Forecasters use this tool to measure the likelihood of a forecast.
Global Ensemble Forecast System (GEFS)
A weather forecast model made up of 21 separate forecasts, or ensemble members. The National Centers for Environmental Prediction (NCEP) started the GEFS to address the nature of uncertainty in weather observations, which are used to initialize weather forecast models.
High Resolution Ensemble Forecast (HREF)
An ensemble of products from several different models running at ~3 km horizontal grid spacing.
Hydrologic Ensemble Forecast System (HEFS)
A probabilistic forecast tool with the goals to provide hydrologic forecasts including an analysis of “probable outcomes” and to minimize biases in the atmospheric models and in the hydrologic models.
Meteorological Model Ensemble River Forecast (MMEFS)
An automated short-term hydrologic ensemble forecast system which utilizes temperature and precipitation output from the National Centers for Environmental Prediction (NCEP) GEFS and NAEFS meteorological models as inputs to River Forecast Center hydrologic models.
North American Ensemble Forecast System (NAEFS)
An atmospheric ensemble of 20 members each from the NCEP GEFS and CMC EPS ensemble systems.

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