A synoptic weather pattern and sounding-based climatology of freezing precipitation in the United States East of the Rocky Mountains

An analysis of 411 winter storms that produced freezing precipitation events in the United States east of the Rocky Mountains over the 25-yr period of 1970-94 is presented to identify specific weather patterns associated with freezing precipitation and to determine their frequency of occurence. Seven archetypical weather patterns are identified associated with freezing precipitation. Four patterns (arctic fronts, the warm front-occlusion sector of cyclones, cyclone-anticyclone couplets, and the west quadrant of anticyclones) are not associated with specific topographic features. Three patterns (East Coast cold-air damming with an anticyclone, cold-air damming with a coastal cyclone, and cold-air trapping during approaching continental cyclones) are associated with freezing precipitation in and along the Appalachian Mountains. The frequency of occurrence and duration of each of these patterns are presented, and variability within patterns is discussed. In the second part of the paper, the vertical structure of the atmosphere during freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period of 1970-94. Statistical summaries of soundings from each archetypical weather pattern and from the entire dataset are presented for 1) the depth and minimum temperature of the cold surface layer, 2) the depth and maximum temperature of the warm layer aloft, 3) stability characteristics of air above the inversion, 4) layer thickness for the 1000-500-mb and 1000-850-mb layers, and 5) wind speed and direction at the surface, the 850-mb level, and the 700-mb level.