Temporal and spatial variations of freezing rain in the contiguous United States: 1948-2000

A new freezing-rain-days database was used to define the spatial and temporal distributions of freezing-rain days across the contiguous United States. The database contained 988 stations, spanning the period 1948-2000. Areas averaging one or more days of freezing rain annually included most of the eastern half of the United States and the Pacific Northwest. The national maximum is in portions of New York and Pennsylvania, a result of several weather conditions conducive to freezing rain. Other maxima included an east-west zone across the Midwest, an area along the eastern Appalachians, and the Pacific Northwest. The latter two maxima have high frequencies as a result of the mountains, which trap low-level cold air with warm air moving above, resulting in freezing rain. National maximum annual values during 1948-2000 were 3-5 times as great as annual averages, but the two patterns were similar. Average patterns for three discrete 17-yr periods between 1948 and 2000 were very similar, but the magnitudes of values differed greatly between periods. The earliest period, 1948-64, had many more freezing days than the latter periods. The high early values resulted in significant down trends for 1949-2000 in the Northwest, central, and Northeast regions. The 1965-76 period had the lowest frequency of freezing-rain days during 1949-2000. Months of first freezing-rain occurrences ranged from September to December, with November the predominant month in the eastern United States and October in the West. Months of last freezing events shifted latitudinally, with February being last along the Gulf of Mexico and April being last in the northern half of the United States. Nationally, peak months of freezing-rain days are December and January, and both have similar patterns. January averages are highest in the eastern half of the United States, and those in December are highest in the west. Freezing-rain days in these two months are more than one-half of those experienced each year in much of the United States.