Improving Probabilistic Snow Guidance Over Complex Terrain: Collaboration Between A National Center and Local Forecast Offices
Darren Van Cleave, NOAA/NWS - Salt Lake City Weather Forecast Office, Salt Lake City, UT
Bruce Veenhuis, NOAA/NWS - Weather Prediction Center
Randy Graham, NOAA/NWS - Salt Lake City Weather Forecast Office
Glen Merrill, NOAA/NWS - Salt Lake City Weather Forecast Office
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Abstract
Recent research has shown the benefits of using probabilistic

snowfall forecasts in place of deterministic forecasts in communicating winter

hazards to decision makers. Beginning with the 2010 winter season, the Weather

Prediction Center has created probabilistic snow guidance  across the

continental United States each winter, known collectively as Probabilistic

Winter Precipitation Forecasts (PWPF). With the PWPF methodology, at each grid

point the mode of the predicted probability distribution is set to the WPC

deterministic snowfall forecast and the variance is derived using information

from a multimodel ensemble. The probabilistic output is available both as

accumulation percentiles and probability of exceedances in a gridded format. In

2013, several National Weather Service (NWS) Weather Forecast Offices (WFOs)

began to ingest this guidance, using a similar mode-match technique to align

with their local forecast. This produces the same outputs as the original WPC

PWPF grids, but is effectively tailored to local expertise. The project was

expanded to an organized national experiment beginning with the 2015/2016

season. WFO Salt Lake City (SLC) was the first office in the Western US to

officially experiment with this process, beginning that same 2015/2016 season.

 

Complex terrain introduces a number of issues with gridded forecast guidance and in particular the PWPF guidance, ranging from resolution limitations to different handling of terrain in forecast systems from local to national offices. Prior to 2015, the PWPF guidance had not been thoroughly evaluated in complex terrain. SLC’s testing and evaluation highlighted numerous issues, often related to topography, which prevented SLC from using the guidance to brief the public or decision makers.

With feedback and help from SLC and other WFOs, significant changes to the PWPF methodology at local WFOs were implemented for the 2016/2017 season. Currently, WPC is working on a higher-resolution ensemble based partially on feedback and testing at SLC. Both of these changes, along with other modifications, have resulted in PWPF guidance which is more useful for decision makers which

partner with SLC including officials in transportation and avalanche safety, and will ultimately be more appropriate to share with the general public.