Well the famous James Spann says this about it:
"RPM: Rapid Precision Mesoscale model. A numerical weather prediction system based on the Advanced Research Weather Research and Forecast system (WRF-ARW). RPM generates forecasts out to 24 hours with updates every 3 hours in the United States and every 6 hours outside the United States. Precipitation forecasts are calculated from half-hourly instantaneous precipitation forecasts output by RPM. RPM model output is not available to the pubic; but we have full access and use it extensively in our forecast products."
The rest is here:
https://www.alabamawx.com/?p=38944