Snow and avalanche climate varies widely across the parts of the Cascade Mountains spanned by SR-20. Little information is presently available about temperatures and weather patterns over the passes, but a limited amount of snow depth data are available from snow courses. These snow data are presented and analyzed.
Several distinct zones of avalanche activity are identified, extending from Newhalem on the west to Early Winters Creek on the east. These zones each have distinct types of avalanche occurrence and patterns of hazard for the highway. Periods of high hazard for the different zones are not likely to coincide in time. Due to the wilderness nature of the highway and lack of public facilities, the whole highway probably will have to be closed each time hazard develops in any one zone. The most serious danger from avalanches to both the public and to maintenance crews exists in the Liberty Bell and Cutthroat Ridge zones between Washington Pass and Cutthroat Creek.
Although a limited application of avalanche defense structures might be technically possible, especially the selected use of snowsheds, any such measures will be severely limited by the dedication of this part of the Cascade Mountains to recreation and scenic uses.
The recommended method of dealing with avalanche hazards on SR-20 is the operational management of traffic flow, closures and artificial avalanche release by artillery fire. The percentage of time the highway can be kept open in the winter depends on a trade-off with costs and desired level of public safety. In general, a decrease in closure time can be obtained only at the price of increasingly complex and costly avalanche forecasting and control operations. Reducing closure time lower than 15-20% does not presently appear feasible. This level of operation should be sought only after several years of experience have been acquired with snow and avalanche conditions.