Climate of Salt Lake City - Precipitation

Precipitation

Both precipitation and humidity are highest from March through May and lowest from June through mid-September. Major sources of precipitation are winter snow storms originating in the Gulf of Alaska, late winter and spring rains from the Pineapple Express created in Hawaiian waters, and summer monsoons from the Gulf of California. The airport averages 16.5 inches (419 mm) of precipitation per year, with bench areas receiving up to 20 inches (508 mm), mainly due to increased snowfall. May is the wettest month, averaging 2.09 inches (53.1 mm) of precipitation. Average year-round humidity is 55%.

The summer monsoon rising from Mexico and Arizona passes through the region beginning in mid-July and continuing into September, bringing intense but short-lived thunderstorm activity. Tornadoes have been known to occur during this time period, the most notable recent example being in 1999 when an F2 tornado struck downtown Salt Lake City, causing extensive damage and resulting in one death. Many of these thunderstorms consist of dry lightning, which occurs when humidity is too low to support rainfall and the rain evaporates before reaching the ground. This is the main cause of wildfires in Utah during summer. Smoke from such fires in nearby areas sometimes make their way into the valley. Despite the intensity of the thunderstorms, the low humidity, short duration, and isolationism of the thunderstorms all play a part in making this the driest season. July is the driest month, with only 0.72 in (18.3 mm) of precipitation on average. In late fall, the Pacific Ocean usually begins to kick in again, and starting in late September precipitation begins to increase. During late summer and fall, remnants of tropical cyclones from the East Pacific occasionally reach Utah, resulting in heavy tropical rains. Weather events such as hail, freezing rain, and sleet are uncommon in Salt Lake City. When they do occur, they are of short duration and rarely accumulate. The record precipitation for a single month occurred in September 1982, with 7.04 inches (179 mm), largely due to the remnants of Hurricane Olivia. The record 24 hour precipitation occurred from April 22–23, 1957, when 2.41 inches (61.2 mm) fell.

The first snowfall of the season is usually recorded in October, although significant snowfall usually does not occur until mid-to-late November. However, snow has been recorded in every month of the year in the mountains. Snow fell in the valley as early as September 17 (in 1965). January averages the most snowfall of any month, with 13.6 inches (34.5 cm). Snowfall is common through mid-March, but the last snowfall of the season usually occurs in early or mid-April, although measurable snow has been recorded on the valley floor as late as May 18 (in 1977). However, significant snow can fall in March and in rare cases even April. Measurable snow falls on average from November 6 through April 18.

The airport also averages 61 inches (155 cm) of snow in a year, which is more than most Great Plains and Mid-Atlantic locations. However, bench locations near the mountains receive substantially more, reaching 80 to 100 in (200 to 255 cm) annually. The four ski resorts located in Big and Little Cottonwood each average more than 500" of snow per year. These high snowfall totals can be credited to the lake effect, in which snow storms passing over the Great Salt Lake are amplified by the warm waters of the lake. The lake effect is usually strongest from mid-fall through mid-winter, when lake waters are warmer and land temperatures are cold enough to support snow.

Blizzards are very rare in the Salt Lake City valley due to a rarity of extreme winds. Although average snowfall is relatively high, large snowstorms of more than about 12 inches (30 cm) are rare in the valley. In addition, average winter temperatures in the city are usually not cold enough to support snow cover all winter long. The maximum snow depth recorded in the city was 26 in (66 cm) on January 11, 1993. January 1993 was the snowiest month on record, with 50.3 inches (128 cm). The record 24 hour snowfall occurred from February 24–25, 1998, when 18.9 in (48 cm) fell, while the record single storm snowfall occurred from January 6–10, 1993, with 23.3 inches (59 cm). The snowiest season on record was 1951-1952, when 117.3 inches (298 cm) fell, while the least snowy winter on record was 1939-1940, with 18.5 inches (47.0 cm).

The airport averages around 37 and 97 days of snow and precipitation, respectively, each year. Daily sunshine hours range from a low of 3.6 in December to a high of 12.3 in July, all for a bountiful total of nearly 3030 hours per year.