to , mainly due to increased snowfall. May is the wettest month, averaging 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 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 , largely due to the remnants of Hurricane Olivia. The record 24‑hour precipitation occurred from April 22–23, 1957, when 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 . 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 of snow in a year, however, bench locations near the mountains