Utah features a dry, semi-arid to desert climate, although its many mountains feature a large variety of climates, with the highest points in the Uinta Mountains being above the timberline. The dry weather is a result of the state’s location in the rain shadow of the Sierra Nevada in California. The eastern half of the state lies in the rain shadow of the Wasatch Mountains. The primary source of precipitation for the state is the Pacific Ocean, with the state usually lying in the path of large Pacific storms from October to May. In summer, the state, especially southern and eastern Utah, lies in the path of monsoon moisture from the Gulf of California.
Most of the lowland areas receive less than 12 inches (305 mm) of precipitation annually, although the I-15 corridor, including the densely populated Wasatch Front, receives approximately 15 inches (381 mm). The Great Salt Lake Desert is the driest area of the state, with less than 5 inches (127 mm). Snowfall is common in all but the far southern valleys. Although St. George only receives about 3 inches (8 cm) per year, Salt Lake City sees about 60 inches (152 cm), enhanced by the lake-effect snow from the Great Salt Lake, which increases snowfall totals to the south, southeast, and east of the lake.
Some areas of the Wasatch Range in the path of the lake-effect receive up to 500 inches (1,270 cm) per year. The consistently deep powder snow led Utah’s ski industry to adopt the slogan “the Greatest Snow on Earth” in the 1980s. In the winter, temperature inversions are a common phenomenon across Utah’s low basins and valleys, leading to thick haze and fog that can sometimes last for weeks at a time, especially in the Uintah Basin. Although at other times of year its air quality is good, winter inversions give Salt Lake City some of the worst wintertime pollution in the country.
Previous studies have indicated a widespread decline in snowpack over Utah accompanied by a decline in the snow–precipitation ratio while anecdotal evidence claims have been put forward that measured changes in Utah’s snowpack are spurious and do not reflect actual change. A 2012 study found that the proportion of winter (January–March) precipitation falling as snow has decreased by 9% during the last half century, a combined result from a significant increase in rainfall and a minor decrease in snowfall. Meanwhile, observed snow depth across Utah has decreased and is accompanied by consistent decreases in snow cover and surface albedo. Weather systems with the potential to produce precipitation in Utah have decreased in number with those producing snowfall decreasing at a considerably greater rate.
Utah’s temperatures are extreme, with cold temperatures in winter due to its elevation, and very hot summers statewide (with the exception of mountain areas and high mountain valleys). Utah is usually protected from major blasts of cold air by mountains lying north and east of the state, although major Arctic blasts can occasionally reach the state. Average January high temperatures range from around 30 °F (−1 °C) in some northern valleys to almost 55 °F (13 °C) in St. George.
Temperatures dropping below 0 °F (−18 °C) should be expected on occasion in most areas of the state most years, although some areas see it often (for example, the town of Randolph averages about 50 days per year with temperatures dropping that low). In July, average highs range from about 85 to 100 °F (29 to 38 °C). However, the low humidity and high elevation typically leads to large temperature variations, leading to cool nights most summer days. The record high temperature in Utah was 118 °F (48 °C), recorded south of St. George on July 4, 2007, and the record low was −69 °F (−56 °C), recorded at Peter Sinks in the Bear River Mountains of northern Utah on February 1, 1985. However, the record low for an inhabited location is −49 °F (−45 °C) at Woodruff on December 12, 1932.
Utah, like most of the western United States, has few days of thunderstorms. On average there are fewer than 40 days of thunderstorm activity during the year, although these storms can be briefly intense when they do occur. They are most likely to occur during monsoon season from about mid-July through mid-September, especially in southern and eastern Utah. Dry lightning strikes and the general dry weather often spark wildfires in summer, while intense thunderstorms can lead to flash flooding, especially in the rugged terrain of southern Utah. Although spring is the wettest season in northern Utah, late summer is the wettest period for much of the south and east of the state. Tornadoes are uncommon in Utah, with an average of two striking the state yearly, rarely higher than EF1 intensity.
One exception of note, however, was the unprecedented F2 Salt Lake City Tornado that moved directly across downtown Salt Lake City on August 11, 1999, killing 1 person, injuring 60 others, and causing approximately $170 million in damage. The only other reported tornado fatality in Utah’s history was a 7-year-old girl who was killed while camping in Summit County on July 6, 1884. The last tornado of above (E)F0 intensity occurred on September 8, 2002, when an F2 tornado hit Manti. On August 11, 1993, an F3 tornado hit the Uinta Mountains north of Duchesne at an elevation of 10,500 feet (3,200 m), causing some damage to a Boy Scouts campsite. This is the strongest tornado ever recorded in Utah.