Is the RRFS as good as the NAM was?

The NAM was retired on November 3, 2026, after 20 years as NCEP's regional model. Its replacement is the Rapid Refresh Forecast System (RRFS): 3 km, all of North America, out to 84 hours. Its ensemble, REFS, runs to 60 hours. Forecasters long treated the NAM's 48-hour 500 mb pattern as its "wheelhouse," so this page grades the RRFS on that test against the global models, including ECMWF's AI model (AIFS), which many now consider the best of them. Every 00Z and 12Z run is archived from NOMADS and verified against the ECMWF and GFS analyses. Built .

Anomaly correlation (ACC) measures how well the forecast's pattern of ridges and troughs, relative to normal, lines up with the analysis. 1 is perfect, and 0.6 is the classic floor for a useful forecast. At 48 hours, good models score above 0.95, so the differences that matter are in the second and third decimal places.

Day by day at 48 hours

Each dot is one 48-hour forecast. Lines are a 7-day running mean (14 forecasts). The S1 score (Teweles and Wobus, 1954) compares height gradients, so it rewards placing troughs and ridges correctly whatever their depth. It's the score NCEP used for decades to track its regional models, including the NAM. RRFS has no scores on the North America–Atlantic sector: its domain doesn't cover enough of it.

NAM's last season vs. RRFS now

Raw scores rise and fall with the weather (winter patterns are larger and easier to score well on), so each model is shown relative to the GFS over the same period. The top row is the NAM's final cool season, to , from the NOAA archive on AWS. The bottom row is the last 30 days. If RRFS sits where the NAM did, or to the right of it, it has kept the NAM's edge.

Skill by lead time

Mean score by forecast hour, using the score chosen above. RRFS runs to 84 h and REFS to 60 h, so their lines stop there. The dotted line marks 48 h.

Where the 48 h heights run high or low

Mean error on the 1° grid, against the ECMWF analysis, over the last days. A persistent blue patch means the model digs troughs too deep there (or builds ridges too weakly). Over a few weeks this reflects the weather regime as much as the model itself; the pattern only means something once it holds over a season.

Is REFS confident for the right reasons?

Each dot is one 48-hour REFS forecast: its member spread against the error of its weighted mean. A well-calibrated ensemble scatters around the dashed line. Points well above it are overconfident misses, where the members agreed and were wrong together. Convection-allowing ensembles usually have too little spread at 500 mb.

RRFS's worst 48 h forecasts

CONUS, lowest anomaly correlation first. A bust is more interesting when the global models did well at the same valid time: that points to the RRFS itself rather than a hard pattern.