North American Mesoscale Model
numerical weather prediction model
The North American Mesoscale Model (NAM) is a regional numerical weather prediction model run by the National Centers for Environmental Prediction (NCEP), part of the U.S. National Weather Service, for short-range operational forecasting over North America. NAM traces its lineage to the Eta model, which NCEP's predecessor, the National Meteorological Center, made operational in 1993; the system was renamed North American Mesoscale on 25 January 2005. Since 2011, NAM's dynamical core has been the Non-hydrostatic Multiscale Model on the B grid (NMMB); it previously ran on the Weather Research and Forecasting Non-hydrostatic Mesoscale Model (WRF-NMM) from 2006 to 2011. NCEP is scheduled to discontinue NAM on 6 October 2026, replacing it with the Rapid Refresh Forecast System (RRFS).
Operation
NAM runs four times daily, with forecasts initialized at 00:00, 06:00, 12:00, and 18:00 UTC. The 12 km parent domain, covering all of North America, produces forecasts out to 84 hours. Four fixed 3 km domains, nested one-way inside the parent domain and covering the contiguous United States (CONUS), Alaska, Hawaii, and Puerto Rico, run out to 60 hours. A relocatable 1.5 km fire-weather nest, placed within the CONUS or Alaska domain, runs out to 36 hours; its location is set by NCEP Service Centers or, during the summer wildfire season, the National Interagency Fire Center.
NAM uses a partially cycled, six-hour data-assimilation window with hourly analysis updates for the 12 km parent domain and the 3 km CONUS and Alaska nests, using the NCEP Gridpoint Statistical Interpolation (GSI) analysis system. The non-cycled Hawaii, Puerto Rico, and fire-weather nests are initialized from the parent domain's first guess. Since August 2014, the analysis has used a hybrid ensemble–variational method, incorporating background-error covariances from NCEP's global ensemble Kalman filter alongside the static variational background-error covariance.
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