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 event s 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 occurrence. Sev en archetypical weather patterns are identified associated with freezing pr ecipitation. Four patterns (arctic fronts, the warm front-occlusion sector of cyclones, cyclone-anticyclone couplets, and the west quadrant of anticyc lones) are not associated with specific topographic features. Three pattern s (East Coast cold-air damming with an anticyclone, cold-air damming with a coastal cyclone, and cold-air trapping during approaching continental cycl ones) are associated with freezing precipitation in and along the Appalachi an Mountains. The frequency of occurrence and duration of each of these pat terns are presented, and variability within patterns is discussed. In the s econd part of the paper, the vertical structure of the atmosphere during fr eezing precipitation events is investigated by analyzing 972 rawinsonde sou ndings 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 patte rn and from the entire dataset are presented for 1) the depth and minimum t emperature of the cold surface layer, 2) the depth and maximum temperature of the warm layer aloft, 3) stability characteristics of air above the inve rsion, 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.