Idaho Annual Temperatures and Extremes

Idaho Temperature Records

Hottest temperature ever recorded: 118 F, Orofino, northern Idaho, 7/28/1934 Coldest temperature ever recorded: -60 F, Island Park Dam, eastern Idaho, 1/18/1943 Hottest location ranked by highest average annual temperature: Swan Falls, southwest Idaho, 55.4 F Coldest location ranked by lowest average annual temperature: Obsidian, central Idaho, 34.4 F Idaho's record for most consecutive days with the high temperature equal to or exceeding 90 degrees was set in June - September 1967 with 77 at Slate Creek R.S., Idaho. The record for most consecutive days with the low temperature equal to or below 32 degrees was set in August 1966 - May 1967 with 267 days at Galena, Idaho.

    

The highest annual averages in Idaho are found in the lower elevations of the Clearwater and Little Salmon river basins, and in the stretch of the Snake River Valley from the vicinity of Bliss downstream to Lewiston, including the open valleys of the Boise, Payette and Weiser rivers. At Swan Falls the annual mean is 55 degrees Fahrenheit (º F), the highest in the State. Obsidian, at an elevation of 6,780 feet in Custer County, has the lowest annual average, 35.4º F, of any reporting station, with such places as Sun Valley, Chilly Barton Flat, Grouse, Island Park Dam and Big Creek not far behind. The range between the mean temperatures of the coldest and warmest months of the year varies from less than 40º F at a number of northern stations, to well over 50º F at stations in the higher elevation of the central and eastern parts of the State. In the basin of the Snake River and its tributaries, between Twin Falls and Idaho Falls, monthly mean temperatures of 32º F or lower persist from December through February, while downstream from Twin Falls, at the lower elevations, monthly mean temperatures are freezing or below only in December and January. Low-level stations like Riggins and Lewiston show no month in the year with mean temperature 32º F or lower. In general, it can be said that month means are 32º F or lower at stations above 5,000 feet from November through March; between 4,000 and 5,000 feet, November through February; 3,000 to 4,000 feet, December through February; and 2,000 to 3,000 feet, only one or two months. 

The diurnal range of temperature is, of course, most extreme in high valleys and in the semiarid plains of the Snake River Valley. The magnitude of diurnal range varies with the season, being lowest in winter when cloudiness is much more prevalent and greatest in the warmer part of the year. At Boise, for example, the average diurnal range is only 14º F in January, but exceeds 30º F in July through September. Official temperature extremes have been measured as low as -60º F at Island Park in January 1943, and as high as 118º F at Orofino in July 1934. The coldest monthly mean minimum temperature has been -20, and the warmest monthly mean maximum 104º F. In summer, periods of extreme heat extending beyond a week are quite rare, and the same can be said of periods of extremely low temperatures in winter. In both cases the normal progress of weather systems across Idaho usually results in a change at rather frequent intervals. In the realm of extremely low temperatures, two winters stand out in the records for the State, 1937 - 1938 and 1948 -1949. The lowest monthly mean temperatures on record occurred through the State in January 1949, and many stations registered their absolute lowest temperature on record during that month.

Idaho precipitation averages and extremes, precipitation and temperature data for all U.S. states and Top 10 U.S. climate extremes
Data source: National Data Climatic Center