2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 7. AVALANCHE 7-5 – Wet snow—Rainstorms or spring weather with warm, moist winds and cloudy nights can warm the snow cover, resulting in wet snow avalanches. Wet snow avalanches are more likely on sun-exposed terrain (south-facing slopes) and under exposed rocks or cliffs. – Wind is the most common cause of avalanches. Wind can deposit snow 10 times faster than snow falling from storms. Wind erodes snow from the upwind side of obstacles and deposits snow on the downwind (lee) side. This is called "wind loading". • Terrain: • Ground cover – Large rocks, trees and heavy shrubs help anchor snow, but also create stress concentrations between anchored and unanchored snow. • Slope profile – Dangerous slab avalanches are more likely to occur on convex slopes that produce stress concentrations within surface snow due to varying creep rates. • Slope aspect – Leeward slopes are dangerous because windblown snow adds depth and creates dense slabs. South facing slopes are more dangerous in the springtime due to increasing solar effects. • Slope steepness—Snow avalanches are most common on slopes of 30 to 45 degrees. • Snowpack: • Snow texture—the feel, appearance, or consistency of the snow determined by the shape, size and attachment of snow grains that comprise the particular snow layer. A layer of small grained moist snow has a distinctly different texture—much more cohesive and able to make snowballs—than well faceted snow that falls apart in one’s hands and exhibits very little internal cohesion. • Snow layering – The snowpack is composed of ground-parallel layers that accumulate over the winter. Each layer contains ice grains that are representative of the distinct meteorological conditions during which the snow formed and was deposited. Once deposited, a snow layer continues to evolve under the influence of the meteorological conditions that prevail after deposition. • Snow bonding—in the absence of strong temperature gradients within a dry snowpack, this is the normally stabilizing or “rounding” process whereby individual snow grains or layers come into contact and gradually strengthen the ice skeleton or snow layer(s) through sintering or the formation of ice “necks” between the grains. The associated redistribution of water vapor results in inter-granular attachments or bonds between grains through an expanding ice matrix, and typically results in gradual strengthening of the surrounding snowpack structure. In observations and tests, the hardness of a faceting snow layer decreases with time and it becomes easier to penetrate and pull individual faceted grains out of a snow pit wall. The common factors contributing to the avalanche hazard are old snow depth, old snow surface, new snow depth, new snow type, density, snowfall intensity, precipitation intensity, settlement, wind direction and speed, temperature, and subsurface snow crystal structure. 7.2.5 Warning Time The time of an avalanche release depends on the condition of the snow pack; which can change rapidly during a day and particularly during rainfall. Research done at Snoqualmie Pass showed that most natural avalanches occurred less than 1 hour after the onset of rain; in these cases the snow pack was initially weak (Washington Emergency Management Division, 1996). In cases where the snow pack was stronger,