The Coldest Day of the Year

Nice ArcGIS Online Web App (by NOAA): Coldest day of the year across the United States

The spatial pattern is interesting:

• The western parts of the USA reaches its climatological coldest day in December (matching the seasonal cycle of insolation). because of the moderating influence of the Pacific Ocean (clouds, precipitation, etc.). 
• The eastern parts of the USA (especially New England) reaches its seasonal minimum in later January because of the influence of cold air masses from Canada and the presense of snow cover that reflects solar radiation.
• Noticeable 'cold pockets' in the Rockies, Casades, and Sierra Nevada represent locations in deeper valleys with frequent temperature inversions.