GFS forecast precipitation, as accessed by OpenDAP, now is accumulative over the entire forecast. Previously it was accumulation per timestep.
The GFS variable ( apcpsfc , surface total precipitation [kg/m^2]) exists with the same identifier, but now is in this different structure.
The fix is to add <externUnit parameterId="PC.nwp" unit="mm" cumulativeSum="true"/> to the precipitation data import. Please note that this fix will work for 2018.02. For 2017.02 a new patch is required.
This change was intentional but not very advertised. A notice can be found here effective June 12:
II) New total precipitation and convective precipitation, with continuous accumulation, will be added to the output. Examples of the new variables: APCP:surface:0-10 day acc fcst ACPCP:surface:0-10 day acc fcst. It is confusing that the same variable query from the NAM forecast returns the accumulation per timestep.