Average annual temperature has increased approximately 1.4°F since the early 20th century. This increase is most evident in winter warming, which has been characterized by a below average occurrence of very cold days since 2000. 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. Heavier spring precipitation, combined with a shift from snow to rain, could increase the potential for flooding.
Higher temperatures will increase evaporation rates and decrease soil moisture, leading to more intense future droughts. The frequency of wildfire occurrence and severity is projected to increase in Wyoming.
Wyoming’s climate is determined to a large extent by its location in the middle latitudes, in frequent close proximity to the jet stream, and in the interior of the North American continent far from oceanic moisture sources. While its proximity to the jet stream brings frequent storm systems, the lack of easy access to moisture sources leads to a mostly semi-arid climate. It has large climatic variations due to its geographic diversity and altitudinal range. The eastern portion of the state lies within the Great Plains, while a series of mountain ranges and basins comprises the central and western parts. Elevations across the state range from about 3,100 feet to 13,800 feet, and this large range causes a wide variation in temperatures. The state has mild to warm (depending on elevation) summers and cold winters. Average winter minimum temperatures are mostly in the range of 0°F to 15°F. For most of the state, average summer maximum temperatures range from upper 70s to the upper 80s, although temperatures are much lower at higher elevations. Due to the state’s semi-arid climate, temperatures can vary widely from day to night. The year 2012 was the state’s hottest on record, with a statewide annual temperature of 44.8°F, 3.9°F higher than the long-term average (1895–2014).
The 21st century has been the warmest period on record for Wyoming, with a net warming of 1.4°F since the beginning of the 20th century (Figure 1). This warming has been observed in all seasons; since 1995, winter and summer temperatures have averaged 1.9°F and 1.2°F above the historical average, respectively (Figure 4a). The state has experienced an above average frequency of very hot days (days with maximum temperature above 95°F) since 2000 (Figure 2). Wyoming rarely experiences warm nights (days with minimum temperatures above 70°F), but the early part of the 21st century has seen an above average number of such nights (Figure 3). In addition to the overall trend of higher average temperatures, the state has experienced a below average number of very cold days (days with maximum temperatures below 0°F) since 2000 (Figure 4b).
Wyoming’s topographic variability also causes large regional variations in precipitation across the state. The average annual precipitation at long-term observational sites varies from 6.34 inches at Lovell (Elevation: 3,837’) in the north central part of the state to 30 inches on the Snake River at the South Entrance to Yellowstone National Park (Elevation: 6,882’). Temporal variability is also large. Statewide average precipitation totals have varied from a low of 10.96 inches in 2012 to a high of 20.50 inches in 1927. The driest multi-year periods were in the 1930s, 1950s, and 2000s, and the wettest in the 1940s and 1990s (Figure 4c). The driest 5-year period was 1931–1935 and the wettest was 1995–1999. Most of the state’s precipitation falls during the spring and summer months, although some areas in the mountains experience a peak during the winter. Some mountainous regions receive more than 200 inches of snowfall annually.
During the summer months, the state experiences frequent thunderstorms, which can produce hail, lightning, and strong winds. Southeastern Wyoming lies in “Hail Alley,” the most hail prone area in the entire country. Since 2010, the state has averaged 32 severe hail (size 1 inch or larger) events annually. On August 1, 1985, a severe thunderstorm in Cheyenne dropped more than 6 inches of rain in just over 3 hours and produced hail up to 2 inches in diameter. In some areas of the city, the hail piled up into 4–8 foot drifts.
Wyoming’s northern location and associated proximity to the jet stream also makes it susceptible to the impacts of winter storm systems, including heavy snows, high winds, and low wind chill temperatures. In early 1949, Wyoming experienced a series of winter storms, which devastated the region, killing 17 people along with hundreds of thousands of livestock. More recently, eastern Wyoming was hit by an early season blizzard (October 3–5, 2013), which knocked down power lines and closed many roads in the state. Among long-term weather observing stations, Bates Creek reported the highest snowfall amounts, with 25 inches over the 3-day period, 20 inches of which fell on October 5th.
Chinook winds—warm and dry downslope winds that occur along the eastern slope of the Rocky Mountains—are a hazard unique to Wyoming and western mountain states. During the winter months these winds can persist along the eastern slope of the Continental Divide and often bring large temperature increases, leading to snowmelt. Chinook winds can often gust strong enough to cause property damage, and can also cause flooding due to the rapid melting of snow.
Wyoming serves as a major source of water for other states and changes in precipitation can have broad impacts beyond the state’s boundaries. Water from the state’s rivers flows into four major river basins: the Missouri-Mississippi, Green-Colorado, Snake-Columbia, and Great Salt Lake. Yearly variation in late season snowpack depths has implications for water availability across the west as snowmelt from the winter snowpack feeds many rivers and streams (Figure 5). In years with heavy snow cover, heavy rains during the spring thaw can cause severe flooding by causing rapid melting of the snowpack (Figure 4d).
Wyoming, like the rest of the Great Plains, is susceptible to droughts, which are occasionally severe. From 1999 to 2008, large portions of the state experienced drought conditions. The state then experienced several years of above average precipitation until 2012, which was Wyoming’s driest year since historical records began in 1895. By October 2012 almost 90% of the state was in severe drought. The drought, along with high temperatures and high winds, resulted in one of Wyoming’s worst wildfire seasons with over half a million acres burned (more than 7 times the yearly average of 75,000 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.
Winter and spring precipitation is projected to increase (Figure 6). Projected rising temperatures will increase the average lowest elevation at which snow falls (the snow line). This will increase the likelihood that some of the precipitation events now occurring as snow will fall as rain instead, reducing water storage in the snowpack, particularly at lower elevations. Higher spring temperatures will also result in earlier melting of the snowpack, further decreasing water availability during the drier summer months. Heavier spring precipitation, combined with a shift from snow to rain, could increase the potential for flooding.
The intensity of future droughts is projected to increase. Even if precipitation amounts increase in the future, rises in temperature will increase evaporation rates, resulting in a decreased rate of loss of soil moisture during dry spells. Thus, future summer droughts, a natural part of the Wyoming climate, are likely to become more intense. This in turn will increase the risk of wildfires, which are projected to become more frequent and severe.