Weather 101: When to Expect the Coldest Day of the Year

To give you a better idea of the coldest time of year, on average, for your area, NCEI has created “Coldest Day of the Year” maps for the contiguous United States, Alaska, Hawaii, and Puerto Rico. The maps are derived from the 1991–2020 U.S. Climate Normals—NCEI’s 30-year averages of climatological variables including the average low temperature for every day. From these values, scientists can identify which day of the year, on average, has the lowest minimum temperature, referred to here as the “coldest day.”

The contiguous United States map reveals several interesting regional differences across the country. Most prominently, the portion of the United States west of the continental divide (the West) typically reaches its climatological coldest day in December, whereas most of the East reaches its seasonal minimum in January. The West is profoundly influenced by the flow of moderating air masses with clouds and precipitation coming off the Pacific Ocean and aligns more closely to the seasonal solar cycle, while the East is responding to cold air flowing south from Canada that is modified by snow cover reflecting solar radiation away from the surface.

Pockets of later coldest dates near the Rockies and Cascades/Sierra Nevada often correspond to stations located in deep valleys where calm winds and snow cover cause temperatures to become colder than nearby higher ground, a condition called a temperature inversion. Alaska tends to have January coldest day normals, while the low latitude islands of Hawaii and Puerto Rico can have normal coldest days later in February and even in early March.