Average annual temperature has increased approximately 2°F since the early 20th century. This increase is most evident in winter warming, which has been characterized by fewer very cold days since 1990. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Montana’s mountains and river systems provide critical water resources not only for Montana but also for other downstream states. Projected increases in spring precipitation may have both beneficial (increased water supplies) and negative (increased flooding) impacts.
Higher temperatures will increase the rate of soil moisture loss during dry spells, leading to an increase in the intensity of naturally occurring future droughts. The frequency of wildfire occurrence and severity is projected to increase in Montana.
Montana, the fourth largest state in land area, has large climatic variations due to its geographic diversity and altitudinal range. The central and eastern portions of the state are part of the northern Great Plains, which experience warm summers and cold winters. The western part of the state is mountainous with snowy winters and cool summers. Elevations across the state range from about 1,800 feet to 12,800 feet, leading to large variations in temperature. Average January temperatures at valley and plains locations range from 11°F in the northeast to 22°F in the south-central portion of the state, while July temperatures range from 64°F in the southwest to 74°F in the southeast.
The 21st century has been the warmest period on record for Montana (Figure 1). Since 2000, the state has experienced its highest number of very hot days (days with maximum temperature above 95°F) since the extreme summer heat of the 1930s Dust Bowl era (Figure 2a). With an average annual temperature of 44.5°F (3.4°F above normal), 2012 was the third hottest year on record, surpassed only slightly by 1934 and 1987 which averaged 44.9°F and 44.6°F, respectively. Montana rarely experiences warm nights (days with minimum temperature above 70°F) due to its dry air and high average elevation, but the early part of the 21st century saw an above average number of warm nights (Figure 2b). In addition to the overall trend of higher average temperatures, the state has experienced a below average number of cold days (days with maximum temperature below 0°F) since 1990 (Figure 3). Additionally, the rise in average winter and summer temperatures has been quite pronounced (Figure 4).
Montana’s topographic variability causes large regional variations in precipitation across the state, with annual totals ranging from less than 7 inches in lowland and valley locations across the southern portion of the state to more than 35 inches in the mountainous northwest. Some of the higher mountain locations can receive much more, with late-season snow depths at high elevations averaging more than 40 inches of water content (Figure 5). Most of the state’s precipitation falls during the summer months, although some areas in the mountains experience a peak during the winter and spring months due to abundant snowfall induced by airflow over the mountains. Statewide average precipitation has varied from a low of 12.62 total inches in 1931 to a high of 26.15 inches in 1927. The wettest multi-year periods were the end of the 19th century through 1909, and since 2010. The driest multi-year periods were the 1930s and the early 2000s. The wettest 5-year period was 2010–2014 and the driest 5-year period was 1933–1937 (Figure 2c). Annual snowfall varies across the state, with some areas in the mountains receiving more than 300 inches annually.
During the summer months, the state experiences frequent thunderstorms, which can produce hail, lightning, and strong winds. Tornadoes are infrequent and occur almost entirely in the eastern third of the state. Montana’s northern location makes it highly susceptible to the impacts of winter storm systems, including heavy snows, high winds, and low wind chill temperatures. Blizzards are most common in the northeastern part of the state, occurring about five times per year. The eastern part of the state can experience bitterly cold temperatures (occasionally less than -30°F) due to the state’s northern location and the lack of mountain barriers to the north. The state’s record low of -70°F is the lowest of the contiguous 48 states. Alternatively, the state is also prone to summer hot spells, with an all-time high temperature of 117°F observed at Medicine Lake and Glendive. This record bests those of more obvious states such as Florida, and is a result of the dry air and downslope winds in the lower elevations of the eastern plains.
Chinook winds—warm and dry winds common along the eastern slopes of the Rocky Mountains—are a hazard to Montana during the winter months and often bring large temperature increases. Over January 14–15, 1972, Chinook winds caused the temperature in Loma to increase from -54°F to 49°F (a 103°F change), the largest 24-hour temperature change in U.S. history. Although these winds can bring pleasant warmer temperatures during the winter months, they can be accompanied by strong gusts and result in property damage, dangerous cross winds, and quickly melt snow cover even in mid-winter.
Montana serves as a major water source for other states, and changes in precipitation can have broad impacts beyond the state’s boundaries. Water from the state’s rivers flow into the Atlantic Ocean via the Gulf of Mexico (Missouri River system), the Arctic Ocean via Hudson Bay (Bellys, St. Marys, and Waterton Rivers), and the Pacific Ocean (Columbia River system), making the state one of the few regions globally whose waters feed three oceans. Streamflow in these rivers relies on meltwater from the snowpack in the late spring and summer.
Heavy rains during the spring thaw can cause severe flooding (Figure 2d). In 1964, a cool spring delayed melting of the snowpack. From June 8 to 9, heavy rains fell on the above average snowpack, with some areas receiving more than 10 inches in 36 hours. This was one of the worst flash floods in the state’s history, causing at least 28 deaths and damages of approximately $62 million (1964 dollars). In 2011, heavy spring rains, along with an extremely heavy snowpack, caused significant flooding across the state. Multiple stations reported more than 10 inches of rain during the month of May, making it the second wettest May in Montana’s history, and many rivers hit record levels. Below normal temperatures in the area caused pieces of floating ice to accumulate in some streams and rivers, leading to ice jams, which obstructed streamflow and resulted in additional flooding. By mid-June, 31 of the state’s 56 counties and 4 of the state’s 7 reservations were declared disaster areas. Total damages across the state were estimated at more than $60 million. Ice jam flooding can also occur during rapid spring warming.
As an agriculturally dependent state, Montana is particularly vulnerable to drought. The Dust Bowl is by far the most famous drought of the past 100 years, but recent severe droughts have occurred as well. In 2012, the state experienced the driest July to September period in the historical record, dating back to 1895. By October 2012, much of the southern half of the state was in severe drought. The drought provided ideal conditions for wildfires and in 2012 the state experienced more than 2,000 fires, burning more than 1.2 million acres.
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. Increases in heat wave intensity are projected, but the intensity of cold waves is projected to decrease. Projected rising temperatures will also increase the average lowest elevation at which snow falls (the snow line). This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at those lower elevations which are now on the margins of reliable snowpack accumulation. Higher spring temperatures will also result in earlier melting of the snowpack, further decreasing water availability during the summer months.
Although projections of overall annual precipitation are uncertain, winter and spring precipitation is projected to increase (Figure 6). Temperature increases will affect snowmelt patterns, shifting runoff to earlier in the year. Heavy spring precipitation could increase the potential for flooding. Increased spring precipitation can have both positive and negative impacts on Montana’s agricultural economy, improving soil moisture but potentially delaying planting and resulting in loss of yield. Changes in snowmelt patterns could also have impacts on other water-reliant industries, such as mining and tourism.
The intensity of future droughts is projected to increase. Even if precipitation increases in the future, increases in temperature will increase the rate of soil moisture loss during dry spells. Thus, future summer droughts are likely to become more intense, also leading to an increase in the occurrence and severity of wildfires.