Only 300 miles from the Pacific Ocean, Idaho's major moisture source is maritime air from the prevailing westerly winds. This maritime influence is strongest in Northern Idaho where air arriving through the Columbia River Gorge carries more moisture than westerly flow at lower latitudes. Eastern Idaho's climate is more continental in character than Western and Northern Idaho resulting in a greater range between winter and summer temperatures. Eastern Idaho does not have a distinct wet winter-dry summer precipitation pattern.
The highest average annual temperatures are in the lower elevations of the Clearwater and Little Salmon River Basins, and in the Snake River Valley from Bliss downstream to Lewiston (including the open valleys of Boise, Payette, and Weiser Rivers). Swan Falls, in Elmore County, has the highest mean annual temperature of 55°F and Obsidian, in Custer County, has the lowest at 35°F with other high elevation stations not far behind. In general, mean monthly temperatures 32°F or lower occur as follows:
|Above 5,000||November - March|
|4,000-5,000||November - February|
|3,000-4,000||December - February|
|2,000-3,000||One or two months|
The magnitude of diurnal range varies with the season. It is lowest in winter when cloudiness is much more prevalent and greatest in the warmer part of the year. Periods of extreme heat or cold extending beyond one week are rare; the normal progress of weather systems across the State usually results in a change at rather frequent intervals.
The largest source of moisture for precipitation in Idaho is the Pacific Ocean. In summer, moisture comes in from the Gulf of Mexico at high levels producing thunderstorms, particularly in Eastern Idaho. Because of the greater moisture supply over Northern Idaho and the greater frequency of cyclonic activity, average valley precipitation is greater there than in Southern Idaho. High elevations in many sections of the state average high amounts of annual precipitation.
Large areas including the northeastern valleys, much of the Upper Snake River Plains, the Central Plains, and the lower elevations of southwestern valleys receive less than 10 inches annually.
Seasonal distribution of precipitation shows a distinct pattern of winter maximum and midsummer minimum in the northern and western parts of Idaho. In Eastern Idaho, many stations show maximum monthly amounts in summer and minimum amounts in winter. In the northeastern valleys and eastern highlands, more than 50% of the annual rainfall occurs from April through September. In the same time frame, Northern Idaho receives less than 40% of its annual rainfall and portions of the Boise, Payette, and Weiser River drainages receive less than 30% of their annual amount.
The growing season (freeze-free period) varies greatly throughout the state because of differences in elevation, topography, and vegetative cover. The area in the immediate vicinity of Lewiston has the longest growing season, approximately 200 days, followed by the areas in the central Snake, lower Boise, Payette, and Weiser River Basins with 150 days or more. Growing season shortens to approximately 125 days upstream along the Snake River in the Idaho Falls and Pocatello area. A few high valley locations have no month without freezing temperatures.
- The Western Regional Climate Center has historical climate information, with specifics by state.
- Idaho Climate Summaries for more than 150 reporting stations, from the Western Regional Climate Center.
- Idaho State Climate Services from our state climatologist at the U of I Biological and Agricultural Engineering Department.
- Idaho NRCS Water Supply Information with Water Supply Outlook reports and streamflow forecasts.