CHARACTERISTICS OF SNOWFALL OVER THE EASTERN HALF OF THE UNITED-STATES AND RELATIONSHIPS WITH PRINCIPAL MODES OF LOW-FREQUENCY ATMOSPHERIC VARIABILITY

Monthly data from 206 stations for the period 1947-93 are used to exam ine characteristics of snowfall over the eastern half of the United St ates and relationships with precipitation and the maximum temperature on precipitation days. Linkages between snowfall and modes of low-freq uency circulation variability are diagnosed through composite analyses , based on results from a rotated Principal Component Analysis (PCA) o f monthly 500-hPa geopotential height. Results are examined for the 2- month windows of November-December, January-February, and March-April. The three dominant PCAs for each window capture regional components o f the Pacific-North American (PNA), Tropical-Northern Hemisphere (TNH) , and east Pacific (EP) teleconnection patterns. Two general snowfall regimes are identified: 1) the dry and cold upper midwest, Nebraska an d Kansas, where snowfall is strongly a function of precipitation: and 2) the Midwest, southeast, and northeast, where snowfall is more close ly tied to the mean maximum temperature on precipitation days. The PNA (the dominant circulation mode) and the EP pattern are both associate d with strong snowfall signals, best expressed for November-December a nd January-February. Snowfall for the PNA over the southeast, midwest, and mid-Atlantic states Increases (decreases) under positive (negativ e) extremes, when the eastern United States is dominated by a strong 5 00-hPa trough (zonal how or weak ridge) with associated lower (higher) precipitation-day temperatures, Snowfall signals are more extensive u nder positive PNA extremes where the lower temperatures have a greater impact on precipitation phase. An opposing precipitation-controlled s nowfall signal is found over the upper Midwest. The positive phase of the EP pattern, describing a western ridge-eastern trough, is associat ed with negative snowfall signals clustered over the midwest and upper midwest. Opposing signals are found under the midwestern trough-easte rn ridge pattern of the negative mode. These signals are primarily pre cipitation controlled, which for the Midwest are counter to the climat ological control by temperature. TNH snowfall signals are fairly weak except for March-April, when significant differences are found for the upper Midwest and from Missouri northeast into New England. No cohere nt trends are observed in snowfall or in the strength of the circulati on patterns derived from the PCA.