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NBM- National Blend of Models: (also referred to as the “model blend”) is a composite ensemble model that is run every hour. The NBM combines many major models (HRRR, RAP, GFS, NAM, GEFS, HIRESW, GFS MOS, Canadian GDPS, European ECMWF, Australian and others) into a statistical group or “ensemble”. The NBM incorporates specific constituent models into its calculations on a predetermined hourly basis and outputs the new forecast. 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.

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