An Objective Forecast Study for Northwest Flow Snow Events in the Southwestern Pennsylvania Region

This study establishes a forecast bias in the Hydrometeorological Prediction Center's (HPC) Winter Weather Desk 24 hour accumulation forecasts for northwest flow snow (NWFS) events in the Pittsburgh National Weather Service's forecast area. The Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) and Cooperative Observers Program (COOP) daily snowfall data were obtained for 17 sites for the winter months (October-March) years 2008-2011. These data were analyzed to only represent events with northwest flow with a wind azimuth degree of 300° to 330° by using past sounding data for the 850-hPa mean wind at 00Z and 12Z. The analysis yielded 36 NWFS events through the data period. The recorded snowfall from each station for each NWFS event were compared to the HPC forecasts and a percentage of accuracy were produced for each station along with percentages of over and under forecasted days. The results are intended to be utilized as a guide to Pittsburgh forecasters during NWFS events while consulting the HPC's 24 hour snowfall accumulation forecast.