The data contained here are a subset of accumulated precipitation output from the NOAA Earth System Research Laboratory (ESRL) Physical Sciences Division (PSD) Reforecast Project. The Reforecast project produces 15-member ensemble NWP forecasts from 1979 onward using initial conditions from the NCEP/NCAR Reanalysis and a recent, unchanging version of NCEP's global forcasting system (GFS, formerly known as MRF -- medium range forecast).
The subset included here is divided into two sections. The first, "ensemble15", includes accumulated precipitation output from daily 00Z forecast runs for all 15 ensemble members and for all forecast lead times from 12 hours to 360 hours for forecast start times beginning in August 2007 and continuing to the present. The second section, "ensemble_mean", includes ensemble mean forecast precipitation from 1979 to 2004 for lead times from 12 hours to 144 hours.
You can find additional information on the ESRL GFS Reforecast Project on the following page: ESRL GFS Reforecast Project. A web interface for downloading Reforecast output from ESRL is available here: Reforecast Ensemble Data.
Hamill, T. M, J. S. Whitaker, and S. L. Mullen, 2006: Reforecasts: An important dataset for improving weather predictions. Bull. Amer. Meteor. Soc., 87, 33-46.