The Utility of Thickness and Weather Type as a Rain–Snow Divide: A 30-Year Case Study at Albany, New York

Albany, NY is affected by winter weather systems that can produce significant snowfall with bitter cold temperatures, creating both social and economic hazards in the capital city. Consequently, it is beneficial for meteorologists to understand the relationship between the commonly used 5400 and 1300 GPM lines and weather/ air mass types to better forecast rain versus snow events. Other studies have looked into the use of the 5400 and 1300 GPM line but none have assessed the validity of this boundary in respect to weather type characterization at Albany. This study aims to determine the reliability of the widely referenced guides for depicting the rain – snow line, and improve forecast aids for the vertical atmosphere during winter precipitation events. The mean daily 500, 850, 925 and 1000 mb heights and weather type frequency of the Spatial Synoptic Classification between November and March of 1980 – 2012 are analyzed here. Results indicate that the standard vertical boundaries (5400 and 1300 GPM) are inaccurate indicators of a rain versus snow event in Albany. More reasonable rain – snow cut offs for the 1000 – 500 and 1000 – 850mb thicknesses are 5222 and 1262 GPM, and when analyzing the 1000 – 925mb level, 606 GPM is a good indicator of a rain – snow boundary. Further scrutinizing by weather type indicates that the rain – snow boundary also varies depending on what air mass is present on a given day. For instance, the most prominent type that can be observed over Albany during this case study is Dry Polar. At the 1000 – 850mb level, a boundary of 1241.5 GPM is found to be most representative for Albany on DP days. Results indicate only for the rarest of winter weather types, Moist Tropical, are the standard cut offs useful. Determining the reliability of this precipitation indicator at a specific station, like Albany, could enable meteorologists in other regions of the country to draw parallels between weather type, precipitation, and thickness in their forecast zones.