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The operational WPC Winter Weather Desk (WWD) creates 24h forecasts of snowfall and
freezing rain accumulations for each of three consecutive 24h periods (days)
extending 72 hours into the future. These products are shared with the NWS Weather
Forecast Offices (WFO) in a collaborative process resulting in refinement of
the accumulation forecasts. After the 24h snowfall and freezing rain
accumulation forecasts are finalized, the WWD issues its public products: a
limited suite of
probabilistic
winter weather forecasts. These probabilistic forecasts are computed based on
the deterministic accumulation forecasts combined with ensemble information (see below) and are manually edited by the WWD forecaster.
The probabilistic forecasts found here on the WPC PWPF page are also based on
the deterministic WWD
accumulation forecasts, but are generated totally automatically using an ensemble of
model forecasts along with the WWD forecasts. The automatic nature of this product
generation allows a much more extensive set of displays of probabilities for
snowfall or freezing rain exceeding a number of thresholds and accumulations of snowfall
or freezing rain for various percentile levels.
1 NCEP North American Mesoscale (NAM) 12Z (day) or 00Z (night) operational run
1 NCEP Global Forecast System (GFS) 12Z (day) or 00Z (night) operational run 1 European Center for MediumRange Weather Forecasts (ECMWF) latest operational run 1 Canadian Model (CMC) latest operational run 1 ECMWF latest ensemble mean 1 NCEP Global Ensemble Forecast System (GEFS) latest ensemble mean (6h SLRs) 1 GEFS latest ensemble mean (24h mean SLR)
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28 Total members
Probabilities of exceeding a threshold show filled contour levels
of probability that the 24hour or 48hour accumulation of winter precipitation will
equal or exceed the given threshold. As an example, consider the
6inch threshold for snowfall. If a point of interest falls within the 40%
contour on the probability map, then the chance of snowfall
exceeding 6 inches is 40% or greater. As the threshold values
increase, the probabilities of exceeding them decrease.
Percentile accumulations for 24 or 48hour intervals show filled
contours of snowfall or freezing
rain amounts for which the probability of observing that amount or less is
given by the percentile level. For example, if the 75th percentile map
shows six inches of snow at a location, then the probability of getting up to
six inches of snow is 75% at that point. Conversely, there is only a 25%
probability of snowfall exceeding six inches at the location in this example.
Percentile accumulations increase as the percentile level increases.
