The Model Blend aka “the NBM”- (National Blend of Models) is more often referred to as the “Model Blend” because it now includes international models, is a composite ensemble of many models that is run every hour. The Blend is not strictly a distinct model but rather is a composite or statistically weighted blend of other model outputs.
The Model Blend combines many major models (HRRR, RAP, GFS, GEFS, GFS-MOS, Canadian GDPS, European ECMWF, Australian and others) into a statistical group or “ensemble”. The Model Blend incorporates specific constituent models into its calculations on a predetermined hourly basis and outputs the new composite forecast. The Model Blend begins (“initializes”) with the latest hourly HRRR model.
The Model Blend uses an advanced statistical treatment of its constituent models, weighted towards which model(s) have performed best in the previous 6 hours, as determined by comparison with actual adjusted observed measurements in what’s called the “URMA”. (URMA – Unrestricted Real Time Mesoscale Analysis)
Statistical means and variance (standard deviations) are calculated for numerous parameters including rain, high temperature, etc. The model Blend also includes severe weather composite parameters.
The NBM forecasts in hourly increments to 48 hours, then extends its forecast past 280 hours in 3 hour intervals.
As of May 2026, the current version of the Blend is NBMv5.0. The above hourly forecast increments have been increased from 36 to 48 hours. The Model Blend now includes AI model outputs.
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