HPC Guidance

As mentioned in the introduction, the QPF process begins at the Hydrometeorological Prediction Center (HPC). They provide the HAS forecasters with the initial QPF guidance on a schedule of twice-daily final 3-day QPF products, as well as several intermediate preliminary updates.

The HPC QPF issuance time schedule is as follows:

Table 1.
HPC QPF issuance times (QPFRSA)

At HPC, there are three operational forecasters preparing the QPF for the entire CONUS for three days in 6-hour time steps. The workload is divided between the three forecasters with each responsible for a 24-hour period. The QPF is prepared using NMAP software that uses a contouring program to draw isohyets of QPF in intervals of 0.01, 0.25, 0.5, 1.0, 1.5, 2.0 inches or more. The contoured data is converted to point data through bilinear interpolation to a non-uniform set of 58 QPF points scattered throughout the CNRFC area. It is this data that is used as input to the Mountain Mapper software, allowing the HAS forecaster to make adjustments to the HPC guidance. Why 58 points? This dates back to the late 1990s when WFOs issued the initial QPF guidance for the CNRFC. Each WFO was responsible for issuing QPF for about 4-5 points within their HSA boundaries. For continuity, these points continue to be used today with additional points added in Nevada to provide a more even spread of points throughout the CNRFC area of responsibility.

HPC also provides the CNRFC with freezing level data for use in the NWSRFS hydrologic models. The GFS models provides the freezing level data for the same 58 points, and it is derived by searching down vertically in the model grid layers to find the highest occurrence of 0 degrees centrigrade. The snow level for the river models is approximated by subtracting 1500 feet from the freezing level. The snow level in the models determines the elevation above which precipitation falls as snow and does not contribute directly to runoff.

The QPF and freezing level data are combined into a single product that is sent to the CNRFC as AWIPS text product RNOQPFRSA. This product is encoded in SHEF format and properly configured to load directly into Mountain Mapper software, which is then used to view and edit the data. At the CNRFC however, the data is stored in the database first and later extracted in the same format to run using an automated version of the Mountain Mapper software.

Table 2.
CNRFC Reissuance Times for HPC QPF

Note in the table above that the CNRFC only utilizes HPC QPF for days 1-3 anchored on the current hydrologic day (12UTC-12UTC). The 1100UTC issuance is regarded as the primary HPC guidance for the 3-day forecast period with later updates from HPC treated as updates to the same 3-day period (even though their guidance can and does go beyond the current 3-day period)..

Once the HPC QPF data is processed automatically by the Mountain Mapper software, there are several QPF text products generated and made available to AWIPS so that WFOs can view the original HPC QPF guidance in various formats. These AWIPS text products include:

As stated in National Weather Service Instruction 10-911, "RFCs coordinate with HPC and affected WFOs when operational significant changes are made to QPF guidance." However, during the hours that the RFCs are closed, this becomes the responsibility of the WFOs. Thus, it is important to understand that the WFO forecaster has the obligation to review the preliminary HPC QPF forecasts issued at 0615 and 0700 UTC and coordinate any significant changes to the preliminary HPC QPF prior to their 1015 UTC final QPF issuance. This coordination can be accomplished either by phone or using 12Planet chat. The HPC QPF forecasters are available as "hpc_QPF1" on 12Planet. As freezing level guidance is taken directly from the GFS model, there is no mechanism in place for HPC to adjust freezing levels.

Page last modified: January 22, 2004