How are normal high and low temperatures determined?
The National Weather Service calculates normal temperatures based on a 30-year period. The current 30 year normal temperature period is from 1981 through 2010. Those normals will be used for ten years, but they will next be updated using the period from 1991 through 2020.
The average high and low temperatures at a specific location on a specific date, say January 1, are obtained by adding the 30 daily highs and 30 daily lows for all years from 1981 through 2010. The same is done for January 2, and for all other dates through the year. The daily values, which bounce up and down a few degrees from one day to the next, are then smoothed. Those smoothed values are adopted as the normal temperatures for that location.