Using gridded MOS techniques to derive snowfall climatologies

The Meteorological Development Laboratory (MDL) provides station-based Model Output Statistics (MOS) snowfall guidance for the contiguous United States and Alaska. For many MOS weather elements, including snowfall, station guidance is also analyzed to generate gridded MOS guidance as part of the National Digital Guidance Database, created in support of the National Digital Forecast Database. Efforts to improve the current MOS snowfall guidance are underway. The current development relies upon 24 h snowfall reports from the Surface Land Daily Cooperative Summary of the Day (COOP) observations, archived by NOAA's National Climatic Data Center (NCDC).

Climatology has proven to be a useful predictor when developing MOS guidance. Due to the relatively rare and non-continuous nature of snowfall in many parts of the U.S. (particularly heavy amounts), climatological relative frequencies of 24 h snow amount should serve as a potentially strong predictor in MOS forecasts of this element, particularly in the extended projections. However, snowfall climatology is not available at every site used in the MOS station-based development, thus it is necessary to develop a grid of climatological relative frequencies for the domain of the development stations. NCDC compiled a 30-year (1961-1990) snowfall climatology at approximately 3200 cooperative observing sites which had a long and consistent historical record. MDL grids these data by applying the same analysis technique used to create gridded MOS guidance from station MOS forecasts. Grids of monthly mean relative frequency are generated for the 24 h threshold amounts of 0.1” and 2.0” (the levels at which the available NCDC climatology and MOS categorical forecasts correspond). These two thresholds represent measurable snow and somewhat more significant snowfall, respectively. Values can be interpolated from these grids to development stations for use as predictors.

In this presentation the process for establishing the snowfall climatology grids will be discussed, with a focus on the techniques investigated for creating them. Plots depicting gridded climatology will also be shown.