NBM- National Blend of Models: a composite model that is run every hour. The NBM combines many major models (HRRR, RAP, GFS, NAM, GEFS, HIRESW, GFS MOS, Canadian GDPS, European ECMWF, and others) into a statistical group or “ensemble”. The NBM incorporates each of its constituent model into its output on a predetermined, hourly basis. The NBM begins (“initializes”) with the latest hourly HRRR model.
The NBM 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 NBM forecasts in hourly increments to 36-39 hours, then extends its forecast past 280 hours in 3 hour intervals.