Bronco Posted May 4, 2012 Posted May 4, 2012 Detailed Avalanche Forecast Northwest Weather and Avalanche Center Seattle Washington 1308 PM PDT Fri May 04 2012 This forecast applies to back country avalanche terrain below 7000 feet and does not apply to developed ski areas or highways. Please note that regular forecasts for this season have ended. Special forecasts will be issued this spring only if unusually severe avalanche conditions develop. This special forecast applies from Friday May 4th through Tuesday May 8th and will be updated as conditions warrant. Zone Avalanche Forecasts Olympics, Stevens Pass, Snoqualmie Pass, White Pass, WA Cascades near and west of crest - north of Stevens Pass, WA Cascades near and west of crest - between Stevens and Snoqualmie Pass, WA Cascades near and west of crest - between Snoqualmie and White Pass, WA Cascades near and west of crest - south of White Pass, East slopes WA Cascades - north of Stevens Pass, East slopes WA Cascades - between Stevens and Snoqualmie Pass, East slopes WA Cascades - between Snoqualmie and White Pass, East slopes WA Cascades - south of White Pass, Mt Hood area Forecast Friday Through Tuesday: Strong spring storms this week have deposited significant new snow amounts above about 5 to 6000 feet. This snow should become increasingly unstable through the weekend with gradually increasing sunshine and warming expected. Expect locally high avalanche danger developing above about 5 to 6000 feet, especially on sun exposed terrain and higher elevations and on the volcanoes. Snowpack Analysis Recent Weather: Several strong fronts and associated upper level troughs have moved across the Pacific Northwest since Sunday. Snow levels have remained relatively low throughout the week mainly near 3-4000 feet in the north Cascades and 4 to 5000 feet in south Cascades and Mt Hood area. Total precipitation amounts over the past four days (Monday through early Friday morning) along most west slope areas and volcanoes range from about 2 to 3 inches of water equivalent with Timberline on Mt Hood receiving the greatest amount of 4.5 inches in four days and 5.2 inches in 5 days! Less water was received along the east slopes, mostly between .5 to 1 inch. This precipitation has mainly been in the form of snow above 4000 feet in the north and 5000 feet in the south. Recent snowfall totals during the week range from 1 to 2 feet of snow at study plots from 4-6000 feet. Significantly greater new snow is likely above 6-7000 feet, especially on the volcanic peaks. Snow Pack and Avalanche Activity: There have been numerous field observations over the past several days, especially from WSDOT crews working above Chinook Pass in the central WA Cascades and from patrol at Alpental and Mt Hood Meadows ski areas. Crews on Chinook Pass reported by early afternoon, they had already received about 15 inches of new snow through the day. This new snow became increasingly wet and unstable through the afternoon with natural slides releasing on most slopes even with mostly cloudy and cool conditions. Slides were also releasing on relatively low angled slopes of about 30 degrees, with most slides remained above a thin crust layer. However of potentially greater concern is the snowpack below the recent snow and thin crust layer. Below the new recent snow and any remaining thin crust, about 1 to 2 meters of large grain wet to saturated snow remains above the older finer grained winter snowpack. Control results and cornice drops earlier in the week did produce some larger wet slab releases involving these deep layers of wet snow. Other field observations from both Alpental and Mt Hood Thursday as well as back country skier reports this week confirm the deep layer of wet unconsolidated snow ranging from 1 to 2 meters. Just received snowpack conditions from Alpental pro patrol Friday morning. New recent snow becoming wet with ski cuts easily triggering wet avalanches. These slides are running far and fast and entraining wet snow underneath as well as carrying over shallow angled terrain and benches. What does this mean? It means we do not have a stable spring snowpack as yet. It also means that slides releasing easily in the new snow from this week, up to 1 to 2 feet or more, may break down and involve the deeper 3 to 6 feet of wet snow layers. These slides would become very large very quickly, travel fast and far and be potentially destructive! Cornice failures also remain a major concern, as these may trigger large wet slab releases on slopes below. Detailed Forecasts Friday and Saturday The coolest air mass of the week is over us Friday and into early Saturday as an upper level low pressure system moves through the area. This should maintain light snow showers and moderate onshore westerly flow through early Saturday with a diminishing trend in showers overnight and early Saturday. High pressure should begin building Saturday with modest rises in freezing levels. However, sun breaks should allow for daytime temperatures to respond quickly. This weather should continue to wet and weaken the recent snow received this week resulting in continued unstable conditions. Both natural and triggered slides should remain likely, especially during the warmer part of the day and on slopes receiving direct sunshine. Such slides could entrain deeper wet snow as they descend especially in chutes, gullies and canyons that may funnel larger avalanches from higher terrain. Concern #1: Wet loose or isolated wet slab avalanches especially mid and lower elevations and sun exposed terrain. Concern #2: Cornice failures. Concern #3: Glide cracks where full snow pack releases to smooth underlying surface such as rock faces are possible. Sunday through TuesdayMonday High pressure should become strong over the region Sunday through Tuesday. This should cause significant warming with freezing levels climbing to 9-10,000 feet late Sunday and 10-11,000 feet by Monday into Tuesday. Mostly sunny warm weather with light winds are expected Sunday through Tuesday with high clouds spreading over the area Tuesday. This should cause deep wet surface conditions in most areas. As a result, natural wet loose or wet slab avalanches should remain likely during this period, especially from late morning through the afternoon hours each day. Cornice failure remains likely as well. Some wet slides may become large and become potentially destructive, running to the valley floors. Even travel on relatively flat terrain at lower elevations may be dangerous as slides beginning at higher elevations could easily become very large and quickly reach lower elevation areas. As a result of this potentially large spring avalanche cycle, back country travel is not recommended late this weekend into early next week. As freezing levels rise, the avalanche danger should quickly spread to higher elevations, especially on the volcanoes where more recent snow this week as accumulated. Concern #1: Wet loose or isolated wet slab avalanches especially mid and lower elevations and sun exposed terrain. Concern #2: Cornice failures. Concern #3: Glide cracks where full snow pack releases to smooth underlying surface such as rock faces are possible. Quote
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