Average annual temperature has increased about 1.5°F since the beginning of the 20th century. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Winter and spring precipitation is projected to increase over the next century. However, naturally-occurring droughts are projected to be more intense because of warmer conditions, increasing the frequency and severity of wildfire occurrence.
Higher temperatures are projected to cause more of the increased winter and spring precipitation to fall as rain instead of snow. These seasonal changes in precipitation may result in increased flood risks.
Idaho’s northerly latitude and location in the interior of North America gives it a climate with large seasonal differences in temperature, from cold winters to pleasantly warm summers. The low elevation areas of southern Idaho are shielded by mountains to the east and west, reducing the amount of moisture that can penetrate the area and resulting in generally low amounts of precipitation. The higher elevations of northern and central Idaho receive up to four times the amount of rain and snow compared to the southern portion of the state. The majority of precipitation occurs during the cool season (November-May), with the state being reliant on mountain snowpack for water storage.
Temperatures in Idaho have increased about 1.5°F since the beginning of the 20th century (Figure 1). The year 2015 was the second hottest (after 1934) since records began in 1895, with a statewide average temperature of 46.9°F. Average high temperatures in Boise on a July day are around 92°F (individual July monthly averages have ranged from 79.6°F in 1993 to 98.6°F in 2007) and much lower in the northern town of Coeur D’Alene, where temperatures on a July day average 83.5°F (76.4°F–94.5°F). In January, average low temperatures are colder in Boise (23.6°F, range: 11.3°F–32.5°F) than in Coeur D’Alene (25.7°F, range: 20.5°F–31.9°F). Both the number of very hot days (maximum temperature above 95°F, Figure 2) and the number of very warm nights (minimum temperature above 70°F, Figure 3) have been above average in the 2000s. A winter warming trend is reflected in a significant decline in the number of very cold nights (minimum temperature below 0°F), which has been below the long-term average (1900–2014) since the early 1990s (Figure 4).
Average annual total precipitation at long-term monitoring stations ranges from more than 40 inches at some northern mountain sites, to less than 10 inches in the southwest part of the state. There is substantial variability but no overall trend in average annual precipitation for the 118-year period of record (Figure 5). However, the number of extreme precipitation events (days with precipitation greater than 1 inch) has been above the long-term average over the past decade. The record number of events (more than two per year) occurred during the period of 1995–1999 (Figure 6). Over the entire historical period (1895–2015), the statewide-average driest year on record was 1935 with an annual average total of 16.18 inches of precipitation, while the wettest year was 1996 with 32.10 inches. The driest multi-year period was in the 1930s, and the wettest periods were in the early 1980s and late 1990s (Figure 5). The driest 5-year period was 1928–1932 with an average of 20 inches per year and the wettest was 1980–1984 with an average of nearly 29 inches per year. Mean annual snowfall ranges from about 10 to 20 inches in the southern lowlands to more than 100 inches in the higher mountain locations. Snowpack accumulation in the mountains is the major source of water. This is highly variable from year to year and has generally declined since the middle of the 20th century (Figure 7).
Extreme weather and weather-related events that commonly occur in Idaho include severe winter storms, wildfires, floods, droughts, and heat and cold waves. Flooding frequently occurs in Idaho, with an estimated 90% of damage from natural disasters each year attributable to riverine flooding, flash floods, or ice/debris jam flooding. The winter of 1996–1997 brought tremendous amounts of snow in some parts of the state (80–100 inches). The snow in combination with heavy rains and unusually warm temperatures produced significant amounts of snowmelt, resulting in disaster declarations for one-third of the state’s counties due to severe flooding and mudslides. Flash flooding typically occurs following intense thunderstorm events in the spring and summer. In 2012, Idaho experienced one of its most active fire seasons to date, with over 1.5 million acres burned. Additionally, 11 of the state’s 44 counties were designated as primary natural disaster areas due to damages and losses caused by drought, excessive heat, and high winds. Extreme weather cost the state an estimated $480 million in property damages in 2012 alone.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. In addition, the intensity of heat waves is projected to increase while cold wave intensity is projected to decrease.
Projected rising temperatures will increase the average lowest elevation at which the snow pack reliably accumulates throughout the winter. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at lower elevations. Warmer spring temperatures will also result in earlier melting of the snowpack, further decreasing water availability during the already dry summer months.
Winter and spring precipitation are projected to increase in Idaho over the 21st century (Figure 8), while summer decreases are possible, especially in the southeastern portion of the state. However, even if overall precipitation increases, future naturally-occurring droughts will likely be more intense because higher temperatures will increase the rate of loss of soil moisture during dry spells. The earlier melting of mountain snowpack may also lead to a reduction in soil moisture during the summer months. As a result, the frequency of wildfire occurrence and severity is projected to increase in Idaho.
Heavy precipitation events are projected to become more frequent. As temperatures increase, the proportion of winter and spring precipitation falling as rain rather than snow are also projected to increase. The combination of more heavy events and more precipitation falling as rain will increase the risk of flooding during the cold season.