A Statistical Analysis and Synoptic Climatology of Cold Air Outbreaks in the United States

Cold air outbreaks (CAOs) pose substantial hazards to economic activity and human health. Despite these hazards, studies regarding the climatological trends and behavior of CAOs are relatively few in number. As such, there remains some uncertainty regarding CAO frequency for the contiguous U.S. since most prior studies of CAO trends and behavior have been limited in spatial and temporal scope. Specifically, it is unknown if particular regions of the contiguous U.S. are experiencing increases or decreases in the frequency of CAOs, and it remains to be seen if there are regional differences in CAO evolution across various portions of the contiguous U.S. The purpose of this study is to determine regional and nationwide statistical trends in CAO frequency and to establish the regional developmental patterns of CAOs in order to improve forecasts of this high-impact phenomenon.

CAOs are identified using daily minimum temperature data extracted from 53 stations within the Global Historical Climatology Network-Daily dataset over a 68-year period from January 1948 through December 2015. Stations are evenly distributed across the contiguous U.S. and the nine National Centers for Environmental Information (NCEI) U.S. Standard Regions. Days that have minimum temperatures at a station fall below the 31-day centered moving average of the daily fifth percentile temperature are designated as anomalously cold days, and sequences of three or more such days are classified as CAOs. A regional CAO is diagnosed whenever two or more stations within the same NCEI U.S. Standard Region experience an overlap of the same sequence of anomalously cold days.

Preliminary statistical trends for the meteorological spring, summer, fall, and winter seasons show that there has been a decrease in the annual frequency of anomalously cold days for the northeastern U.S. across all four seasons. There has also been a decrease in the annual frequency of northeastern U.S. CAOs across all four seasons. Additional regions will be studied to determine annual statistical trends in the frequency of anomalously cold days and CAOs. CAO-relative composites using NCEP–NCAR reanalysis data will be constructed to identify region-specific patterns in the planetary- and synoptic-scale flow patterns that accompany the development of CAOs.