The melting effect as a factor in precipitation-type forecasting

The process of atmospheric cooling due to melting precipitation is examined to evaluate its contribution to determining precipitation type. The " melting effect'' is typically of second- order importance compared to other processes that influence the lower- tropospheric air temperature and hence the type of precipitation that reaches the ground. In some cases, however, cooling due to melting snowflakes can emerge as the dominant agent of temperature change, occasionally surprising forecasters (and the public) by inducing an unexpected changeover from rain to heavy snow. One such case occurred on 3- 4 February 1998 in east- central Tennessee and surrounding areas. Commonly applied considerations for predicting precipitation type had convinced forecasters that significant snowfall was not likely with this event. However, real- time observations and a postevent analysis by forecasters at the Storm Prediction Center led to the hypothesis that the melting effect must have provided the cooling necessary to allow widespread heavy snowfall. To test this hypothesis, the Pennsylvania State University- NCAR Mesoscale Model was used to generate a mesoscale- resolution, four- dimensional dataset for this event. Diagnostic analysis of the model output confirmed that cooling due to melting snowflakes was of a sufficient magnitude to account for the disparity between observed and forecasted lower- tropospheric temperatures in this case. A simple formula is derived to provide a " rule of thumb'' for anticipating the potential impact of the melting effect. In addition, guidelines are provided for identifying meteorological patterns that favor a predominance of the melting effect.