The increased economic and societal demands to know the potential for winter precipitation as far in advance as possible lead to the development
of the Weather Prediction Center (WPC) Day 4 through 7 probabilistic weather outlook. The product was designed to provide a probabilistic outlook
for the possibility of winter weather in the medium range time period, and to be used by WFO forecasters and weather sensitive users. The forecasts
are prepared by WPC medium range forecasters who utilize the expertise of WPC QPF and precipitation type data from the GFS
and Canadian Global ensemble systems, hereafter referred to as GEFS and CMCE, respectively.
The QPF component of the product is derived from the WPC Day 4-5 and Day 6-7 deterministic QPF. Both 48-hour QPFs are disaggregated into 6-hour
amounts by using a mean of the 6-hour and 48-hour QPF from the most recent 2 runs of the GEFS mean and CMCE mean.
Where the 48-hour mean from the ensemble
systems is > 0, the 6-hour mean from the ensemble system is divided by the 48-hour mean for each respective 6-hour period. The
resulting percentage is then multiplied by the 48-hour WPC QPF, and assigned to the respective 6-hour time stamp. Where the ensemble system mean
48-hour QPF = 0, the WPC 48-hour QPF is multiplied by 0.125 for each 6 hour period. The 6-hour forecasts are then summed to make a 24-hour deterministic
QPF for each day of the medium range period.
The WPC 24-hour QPF is then used as a mean, and the 24-hour QPF from each ensemble member of the most recent
2 runs of GEFS and CMCE (80 members) is used as
variance to compute a cumulative distribution function (CDF) of the WPC deterministic QPF reaching or exceeding 0.25 inches.
The thermal or frozen precipitation component is addressed by computing an ensemble probability of frozen precipitation using precipitation type fields
for snow, sleet, and freezing rain in each member of the ensemble. The precipitation type fields for each ensemble members are computed
using a WPC algorithm consisting of 2-meter temperature and several mandatory level temperatures.
The ensemble probability of snow/sleet is then multiplied by the probability of WPC QPF > 0.25" to arrive at a probability of snow/sleet accumulating
> 0.25" liquid equivalent. WPC medium range forecasters will modify these probabilities then issue as a forecast twice daily.
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