PPQPF Product Description

The National Weather Service (NWS) River Forecast Centers (RFC)
require percentile amounts that can be accumulated for arbitrary
periods up to 72 hours.  A rigorous statistical approach would
require development of percentile amounts for a staggering number
of different accumulation intervals to meet this need.  The suite
of Possible Percentile Quantitative Precipitation Forecasts (PPQPF)
meets this need based on a simple approach.

The simple approach entails the following:

    1.  Compute the percentile amounts for the 72-h accumulation
        in the same manner as done for the 6-h Probabilistic
        Quantitative Precipitation Forecasts (PQPF) as described
        here:

        http://www.hpc.ncep.noaa.gov/pqpf/about_pqpf_products.shtml

    2.  Disaggregate the 72-h percentile accumulations into
        plausible 6-h accumulations that are in the same ratio to
        the 72-h percentile accumulation as the 6-h HPC QPF is to
        the 72-h HPC QPF.

This simple method is consistent with the assumption applied for the
creation of both the 6-h and 72-h PQPF:  The HPC QPF is the most likely
outcome.  Although this approach cannot claim to find the most probable
breakout of the 72-h percentile amounts into 6-h amounts, it is based
on the reasonable assumption that such a disaggregation exists and may
be approximated in the manner described.