Forecast Tools: The NCEP Models

The National Centers for Environmental Prediction (NCEP), in conjunction with groups such as the Earth System Research Laboratory and Geophysical Fluid Dynamic Laboratory, develops and runs the operational computer forecast models used by the National Weather Service and other public and private groups across the United States.  The most widely used for forecasting in the western United States are the Rapid Update Cycle (RUC), the North American Mesoscale (NAM) forecast system, and the Global Forecast System (GFS).

Many computer forecast models divide the atmosphere into a series of vertically layered grid cells.  Some numerical models use other techniques to divide the atmosphere, but this approach is the easiest to describe conceptually.  The distance between the centers of each of these grid cells is called the grid spacing.  The smaller the grid spacing, the better the resolution of the model, and the more detailed the model terrain.  Higher resolution models, however, also require more calculations (and hence computer power), and this is what ultimately places a limit on model resolution.

Source: NOAA/Wikipedia Commons

The RUC has 13-km grid spacing and produces forecasts for North America out to 18 hours every hour.  The NAM has 12-km grid spacing and produces forecasts for North America out to 84 hours every 6 hours.  The GFS is not a grid-cell model, but has an effective grid spacing of 27 km and produces global forecasts out to 384 hours every 6 hours (the portion of the forecast beyond 192 hours is run with a larger grid spacing).   In January 2012, the RUC is scheduled to be replaced by the Rapid Refresh, which will feature an improved numerical modeling and analysis system.

NCEP also runs two ensemble modeling systems, the Short-Range Ensemble Forecast (SREF) system and the Global Ensemble Forecast Systems (GEFS).  An ensemble modeling system features several forecasts produced by models with somewhat different formulations (e.g., different ways of modeling precipitation processes) or initial analyses.  Because several forecasts are produced, ensemble modeling systems are run with larger grid spacings.   The idea, however, is to use the spread in the forecasts to better evaluate forecast uncertainty and the range of possibilities during the forecast period.  Spaghetti diagrams are often used to examine the diversity of forecasts produced by the ensemble.

"Spaghetti" plots of 10C 700-mb temperature analyses (top) and
48-h forecasts (bottom ) from the NCEP SREF.
Source: NOAA/SPC.

The SREF covers North America, features 21 forecast members, and is run four times daily out to 87 hours. The GEFS produces global forecasts, features 21 members, and is run four times daily out to 384 hours.

Products from these modeling systems are available at http://www.ral.ucar.edu/weather, http://www.spc.noaa.gov/exper, http://www.meteo.psu.edu/~gadomski/ewall.html, and http://weather.utah.edu.  In the case of the latter, we are only providing the NAM and GFS, but hope to do more in the future.

The modeling systems at NCEP undergo frequent updates and, unfortunately, I do not know of an easily accessible web site that clearly describes the configuration of each of the NCEP modeling systems.   Thus, the information above may get stale quickly and it is based on the best information that I could locate.  If anyone knows of a single site that clearly describes the configuration of the NCEP models, let me know.

Update: 16 Oct 2011

One of our readers points out that a great site for updated information on the NCEP models is http://www.meted.ucar.edu/nwp/pcu2/.  Access requires registration, but it's free.