Variabili GFS relative alle precipitazioniacpcpsfc  ** surface convective precipitation [kg/m^2]  apcpsfc  ** surface total precipitation [kg/m^2]  cpratsfc  ** surface convective precipitation rate [kg/m^2/s]  pratesfc  ** surface precipitation rate [kg/m^2/s]  pwatclm  ** entire atmosphere (considered as a single layer) precipitable water [kg/m^2] 
Note:1 kg/m^{2} = 1mm of water (because of the density conversion  1000 kg of water per meter cubed). The values will be (almost exactly) the same because the density of water is approximately 1000 kg/m^{3}, so the total mass of a 1mm layer of water covering an area of 1 m^{2} is 1 kg. GFS total accumulated precip (apcpsfc) includes all forms of precipitation that reaches the surface during the accumulation period. For forecast hours evenly divisible by 6, the accumulation period is from T6h to T, while for other forecast hours (divisible by 3 but not 6) it is from T3h to T (Eric L. Altshuler). PRATE is actually the time average precipitation rate over the same periods that are used to accumulate APCP. In other words, PRATE is the average precip from T6h to T for T=6,12,18,24,... and from T3h to T for T=3,9,15,21,... The same "peculiarity" in APCP also applies to PRATE. PRATE and APCP are proportional to each other. For T=6,12,18,24... PRATE=APCP/21600 (6h=21600s). For T=3,9,15,21,... PRATE=APCP/10800 (3h=10800s)(Eric L. Altshuler).
