An Analysis of the Recent Extreme Winters in the Contiguous United States

Abstract

Analysis of monthly-mean temperature and precipitation data for each of the 48 contiguous United States for the 1976?77 through 1978?79 winter seasons shows that the temperature and precipitation departures from the long-term means were extreme. The consecutive occurrence of such severely cold winters is unprecedented in the available 85-year record. Variability of temperature and precipitation has increased in the past 5-year period, compared to previous pentads, mainly as a result of much greater frequency of extreme anomalies. An ?extreme anomaly?is defined as a mean monthly or seasonal value exceeding two standard deviations from the long-term mean. Statistical estimates of average return periods of winter mean temperatures equal to or lower than the actual values recorded for the past three seasons are close to the empirical values. However, the implausibly low probabilities for the occurrence of consecutive severe winters suggest that the development of large-scale anomalies in atmospheric circulation, which these low temperatures represent, may have a common dynamical forcing and that these forcing mechanisms possess time scales on the order of several years.