The hot summer sun awakens wildflowers in the mountains and turns desert areas brown. Autumn brings pleasant weather to all elevations until the snow line begins to creep down the mountain slopes. Winter snowfalls provide excellent skiing and add beauty to the landscape but cause many mountain roads to close. On those roads that don't close, you'll need chains or snow tires. Spring tends to be unpredictable—wet and windy one day, sunny and calm the next—but it's then you'll find the deserts at their greenest.
Utah's mid-continent location brings wide temperature variations between the seasons. Only a small part of the south experiences winter temperatures that average above freezing.
Most of Utah is dry, with an average annual precipitation of 13 inches. Precipitation varies greatly from place to place due to local topography and the irregularities of storm patterns. Deserts cover about 33 percent of the state; the driest areas are in the Great Basin, the Uinta Basin, and on the Colorado Plateau, where annual precipitation is around 5-10 inches. At the other extreme, the highest peaks of the Wasatch Range receive more than 50 inches of annual precipitation, most of it as snow.
Winter and Spring Weather Patterns
Periods of high-pressure systems broken by Pacific storm fronts shape most of Utah's winter weather. The high-pressure systems cause inversions when dense cold air flows down the snow-covered mountain slopes into the valleys, where it traps moisture and smoke. The blanket of fog or smog maintains even temperatures but is the bane of the Salt Lake City area. Skiers, however, enjoy bright, sunny days and cold nights in the clear air of the mountain peaks. The blankets of stagnant air in the valleys are cleaned out when cold fronts roll in from the Pacific. When skies clear, the daily temperature range is much greater until the inversion process sets in again.
Most winter precipitation arrives as snow, which all regions of the state expect. Fronts originating over the Gulf of Alaska typically arrive every 6-7 days and trigger most of Utah's snowfall.
Summer and Autumn Weather Patterns
During summer, the valleys still experience inversions of cold air on clear, dry nights, but with a much less pronounced effect than in winter. The canyon country in the south has higher daytime temperatures than do equivalent mountain elevations because there's no source of cold air in the canyons to replace the rising heated air. Also, canyon walls act as an oven, reflecting and trapping heat.
Thunderstorms are most common in summer, when moist warm air rises in billowing clouds. The storms, though they can produce heavy rains and hail, tend to be erratic and concentrated in small areas less than three miles across. Southeastern Utah sees the first thunderstorms of the season, often in mid-June; by mid-July, these storms have spread across the entire state. They lose energy as autumn approaches, and by October they're supplanted by low-pressure systems at high altitudes and Pacific storm fronts, which can cause long periods of heavy precipitation. Hikers need to be aware that the highest mountain peaks can receive snow even in midsummer.
Rainwater runs quickly off the rocky desert surfaces and into gullies and canyons. Flash floods can sweep away anything in their path, including boulders, cars, and campsites. Do not camp or park in potential flash-flood areas. If you come to a section of flooded roadway—a common occurrence on desert roads after storms—wait until the water goes down before crossing (it shouldn't take long). Summer lightning causes forest and brush fires, posing a danger to hikers foolish enough to climb mountains when storms threaten.
© W.C. McRae and Judy Jewell from Moon Utah, 9th Edition