New forecast hour units in the pressure GRIB file for " T170 L42 MRF Forecast "
For forecast hours beyond 252, the averaged or accumulated fields like precipitation in the pressure GRIB file had been mislabeled as instantaneous. This is because before now the forecast hour could not fit into one byte which could contain values only up to 255. Now, however, the fields can be labelled as averaged or accumulated by taking advantage of new forecast time units such as half-days. Therefore, for forecast hours beyond 252, the averaged or accumulated fields in the MRF and ensemble pressure GRIB files will have forecast hour units of half-days rather than hours. That is, PDS byte 18 will contain a decimal value of 12 rather than a decimal value of 1. For the hour 264 fields, then, PDS bytes 19 and 20 will contain decimal values of 21 and 22, respectively, indicating the fields has been averaged or accumulated over 21 to 22 half-days or forecast hour 252 to 264. The programmer should also llow for other new forecast time units such as 3-hours (byte 18 is 10) and 6-hours (11) as specified in NCEP Office Note 388. This change finally allows the accumulated precipitation as well as the average precipitation rate to both be encoded in all the MRF pressure GRIB files. This change does not affect files before hour 255 nor any instantaneous fields in the pressure GRIB file.Page Last Modified: September 3, 1999