NAM Hourly Grib Files

[Walsh, Brian J]

Hello,
 
I have a question about 12 km NAM hourly grib files from NCEP.  When I
download all hourly interval files out to 36 hours, there is a problem
with the total precipitation amounts (APCPsfc) contained in these files.
The amounts are coded differently in these files than most WMO standard
grib files.  I am attaching the wgrib output for the concatenated grib
file (hours 0-36).  As you can see, there are multiple outputs for
amounts for certain time steps (6, 9, 12, 18, 24, 21, 30, 33 and 36
hours).  For instance, for the 24 hour time step, there is an APCPsfc
variable for amounts from 12 to 24 hours, then another one showing
amounts from 21 to 24 hours.  How does grib2ctl.pl and GrADS
differentiate these values?
 
Any help would be appreciated.


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