The WPC produces 6-hour quantitative precipitation forecasts (QPFs) for forecast
projection days one through three at 6-hour intervals (72-hour duration). Deterministic
forecast models, including the National Centers for Environmental Prediction (NCEP) Global
Forecast System (GFS), the NCEP North American Mesoscale (NAM) model and the global model
from the European Centre for Medium-Range Weather Forecasts (ECMWF), along with
Short-Range Ensemble Forecast (SREF) system produce forecasts covering this
time period. These model runs constitute an ensemble from which uncertainty
information is obtained to construct a probability distribution about the WPC
QPF. This distribution is utilized to generate probabilistic forecasts of
precipitation. The 6-hour QPFs are summed to obtain 24-h QPFs, which are the basis
for 24-h probabilistic QPFs generated using the same multi-model ensemble and the same
method as for the 6-h probabilistic QPFs.
A binormal probability distribution (density) function (PDF), which allows skewness,
is constructed such that the mode is the WPC QPF and the variance is that of the
ensemble. The skewness is based on the position of the WPC QPF in the ensemble
distribution. This approach to estimating the three parameters for the binormal
PDF is a variation on the method of moments.
The probabilistic QPF forecasts provide information in two different forms:
- Probabilities of exceeding a threshold show filled contour levels
of probability that the 6- or 24-hour accumulation of precipitation will
equal or exceed the given threshold. As an example, consider the
.50 inch threshold. If a point of interest falls within the 40%
contour on the probability map, then the chance of precipitation
exceeding .50 inch is 40% or greater. As the threshold values
increase, the probabilities of exceeding them decrease.
- Percentile accumulations show filled contour levels of precipitation
amount associated with a given percentile in the distribution. The
percentile value is the percent chance of precipitation accumulating
less than the depicted amount. From the opposite perspective,
100 minus the percentile is the chance of precipitation exceeding
the depicted amount. For example, there is a 25% chance of
precipitation accumulating less than the amounts shown on the
25th percentile accumulation map; while, there is a 75% chance that
precipitation will exceed the 25th percentile accumulations. Thus,
lower percentile values are associated with smaller accumulations
than are higher percentile values.
Both forms of information are presented on the
WPC probabilistic QPF
. The products described under 1 above are found under the
"Probability of Precipitation of at Least a Specific Amount" tab on web page. The products described under 2
above are found under the "Precipitation Amount by Percentile" tab on the web page.