Snoqualmie Pass as it climbs into the Cascades passes through a microclimate characterized by considerable precipitation, and at times hazardous conditions for travelers. The average annual precipitation is over 100 inches; snowfall averages over 400 inches. The average annual number of days with measurable precipitation is over 170. The rapidly changing conditions require special cautions, relayed to motorists via variable message displays along I-90. Depending on traction they may call for tire chains to be installed, usually on large trucks but occasionally on smaller vehicles as well. Chain-up areas are provided along the side of the Interstate to facilitate the placement of chains. The pass has been subjected to closures when weather conditions become extreme. A snow shed, constructed in 1950 when the road was known as US 10, formerly covered the westbound lanes, but it has been replaced by avalanche bridges that stand away from the slope to allow slides to pass under the road, as of April 2014. The use of sheds is an admission that plowing cannot keep up with snowfall and avalanches. WSDOT maintains cameras at selected locations along the pass to monitor weather conditions. Some of these cameras can be viewed via the internet.
Snoqualmie Pass was well known to the Native Americans of the region. Hudson's Bay Company trappers and traders were active in the Snoqualmie and Yakima valleys during the early 19th century. They knew about Snoqualmie Pass but information about their use of it is vague. A possible early use of the pass was that of A.C. Anderson, who drove cattle across the Cascades in 1841, via a pass he called "Sinahomish Pass". Captain George B. McClellan and his lieutenant Abiel W. Tinkham explored the Snoqualmie Pass region in 1853 and 1854. Their goal was to find a pass better suited for a railroad than Naches Pass, where the Naches Trail crossed the Cascade Mountains. They explored from the east side of the mountains, reaching the vicinity of Yakima Pass. Tinkham continued down the west side via the Cedar River. McClellan decided not to examine Snoqualmie Pass itself because of unfavorable reports from Natives. In 1856, Major J.H.H. Van Bokkelen, then of the Washington Territory Volunteers, crossed Snoqualmie Pass on a scouting mission. In 1858 several large pack trains bound for mines east of the Cascades crossed the pass. In 1865 a number of Seattle citizens, including Arthur A. Denny, explored the Cedar River, Snoqualmie Pass, and Naches Pass. They reported that Snoqualmie Pass route was a better choice for a road than the old Indian trail over Yakima Pass. By 1867 a toll road had been built over Snoqualmie Pass. Intended to be suitable for wagons, for years the road was only usable by pack trains and for cattle The Chicago, Milwaukee, St. Paul and Pacific Railroad completed a line through Snoqualmie Pass in 1909, part of its Pacific Extension. This grade was soon replaced in 1914 by the 2¼-mile Snoqualmie Tunnel, from Hyak due west to Rockdale, at an approximate elevation of, more than below the pass. The rail line was abandoned in 1980, and the tunnel is presently a multi-use trail for bicyclists and hikers, part of Iron Horse State Park. During tunnel construction, an improved wagon road was built over the pass. Near the original rail line, the Sunset Highway was opened through the pass in 1915. In 1927, the road over the pass became U.S. Route 10; it began to be plowed and kept open during winter in 1931. By 1933, the first alpine ski hill was cleared at Snoqualmie Pass, and U.S. 10 was finally paved in 1934. In 1946, tunnel engineer Ole Singstad proposed the construction a tunnel under Snoqualmie Pass to avoid the most treacherous section of the route. In 1969, construction of U.S. 10's replacement, Interstate 90, began across the pass. Since 1991, the Mountains to Sound Greenway Trust has acted to protect the scenic value of the I-90 corridor over Snoqualmie Pass.
