NWS Short-range Hydrologic Ensembles


The short-range hydrologic ensembles are intended to provide an outlook in advance of a hydrologic event, are produced automatically, and are not official forecasts. Official forecasts are provided by NWS Weather Forecast Offices (WFOs) in conjunction with the NWS River Forecast Centers (RFCs). These official forecasts are provided on the NWS Advanced Hydrologic Prediction Service web site (AHPS) or via each RFC's own web site.


Short-term hydrologic ensembles are provided by a system named the Meteorological Model-based Ensemble Forecast System, or MMEFS. The objective of MMEFS is to produce short lead-time (< 10 days) hydrologic ensemble forecasts using forcing fields provided by various meteorological ensemble systems as input to and processed by the NWS River Forecast System (NWSRFS) Ensemble Streamflow Prediction (ESP) system. At this time, MMEFS is used by the 3 NWS Eastern Region RFCs and the Southeast RFC in the NWS Southern Region to process model outputs from the National Centers for Environmental Prediction (NCEP) 12-member Global Ensemble Forecast System (GEFS) as provided via AWIPS and the 21-member Short Range Ensemble Forecast (SREF) system produced at NCEP. Efforts are underway to include the 42-member North American Ensemble Forecast System (NAEFS), which is a combination of the 21-member NCEP GEFS and the 21-member Canadian Metereorological Center Ensemble (CMCE). These results will be available for the 00 and 12 UTC forecast cycles. The 21-member NCEP GEFS will be available for the 06 and 18 UTC forecast cycles. An online course, designed to help understand numerical weather prediction (NWP) models, which includes a section on ensembles, is available from UCAR's COMET. Another online course from COMET provides an Introduction to NAEFS.

MMEFS was developed for several reasons:

  1. Hydrologic forecast uncertainty is closely linked to the uncertainties associated with precipitation and temperature forecasts used by hydrologic simulation models. MMEFS provides a range of hydrologic model results based on differing meteorological numerical weather prediction (NWP) model forecasts used as inputs.
  2. MMEFS explicitly uses short-range (days 1-7) NWP model output, eliminating the need for historical precipitation and temperature data for its results.
  3. Using the MMEFS provides a means to further users' understanding of the effects of model inputs used in hydrologic simulations.
  4. The hydrologic ensembles are useful surrogates for multiple contingency runs that are typically used by river forecast centers to convey quantitative precipitation forecast (QPF) or quantitative temperature forecast (QTF) uncertainty for worst case scenarios.

Hydrologic Ensemble Overview

A slide presentation of an Introduction for Ensemble Streamflow Prediction, based on this UCAR COMET distance learning module, is available.

Presentation Page Overview

How do you navigate your way around the web pages and what does all the information mean? Here's some help to understand it (More to come).


There are a few significant limitations with the use of the MMEFS for hydrologic forecasts:


MMEFS is a joint collaboration of the ER DOHs: Thomas Adams (NWS/OHRFC retired), Joseph Ostrowski (NWS/MARFC retired), and Robert Shedd (NWS/NERFC)

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