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This Bulletin Valid Until: Saturday April 16, 2017 @ 6 pm.
Confidence: Moderate- limited alpine snow pack observations
Main Concerns: (Avalanche problems)
Wind Slab- Over 28 cm reported in the last 48 hours accompanied by moderate south east wind have formed wind slab in both the alpine and treeline. These wind slabs will initially be touchy to light triggering but are expected to settle and bond. At higher elevations expect in isolated areas with excellent protection from the sun these wind slabs to remain active during this forecast period. Wind slabs will be found on lee aspects (North west thru to North east) and located in terrain features such as at the base of steep cliffs, below any cornice features, ridge top areas and steep convex rolls. Avalanches from wind slabs could be large (size 2).
Cornice- Past precipitation and wind combined with warm temperatures have provided ideal conditions to continue to form and grow cornices.These hazardous snow features can be found on ridge tops and on primarily north west thru to north east aspects and in both the alpine and at treeline. As temperatures and freezing levels rise cornices will be fragile, unpredictable and when failing will deliver a heavy load to slopes below and may trigger in isolated terrain a much larger avalanche (size 3). Give these hazardous features a wide berth if deciding to travel either above or below them.
Loose Wet- These avalanches will become prevalent on Sunday as temperatures begin a rapid rise and the sun will be in full effect. Loose wet will be found on all aspects but especially solar and at all elevations tree line and below. Loose wet avalanches could gain enough mass be large in size (size 2) and will be very touchy to human triggers during the height of daytime warming. Loose wet avalanches are likely to be touchy to human triggers in steep, thin, and unsupported terrain features and when exposed to sun will become immediately unstable and very touchy to triggering.
Travel/Terrain Advice: Freshly loaded lee slopes will remain unstable during and after snowfall expect a rapid change in snow pack on Sunday due to rising temperatures and solar effect on the upper snow pack. Evaluate snow and terrain carefully and identify features and areas of concern. Plan to avoid steep open slopes and unsupported features,choose simple terrain that avoids over head hazards. Be aware of your position within the terrain as even a small loose wet avalanches may have enough mass to push a skier or hiker from a safe stance into hazards such as cliffs or crevasses below.
Past Weather: Over the last 72 hours, mountain alpine weather has seen air temperatures swing from minus six to zero degrees.A total of 30 centimeters has fallen with moderate south east wind.
Avalanche Summary: Ski cutting on steep west aspect terrain at tree line was touchy and produced numerous small (size 1 ) wind slab with crowns down 20 cm and propagating easily.Day time warming produced widespread loose sluffing and point releases of the upper surface snow on all aspects with the exception of due north.
Snowpack Description: Moderate temperatures, new snowfall and freezing levels rising mid day to above 1500 meters is settling new snow rapidly and producing almost daily, a surface crust. A total of over 100 centimeters has fallen above 1000 meters in the last week and a number of shears within this upper snow can be found at varying depths and are producing moderate resistant planar results when tested. The shears are found to be failing on a density change with preserved snow between at least two identifiable well settled slabs. The melt freeze crust buried on April fourth is as well producing moderate resistant planar results and can be found down between 80 to 100 cm and likely deeper than 100 cm in isolated lee aspect terrain. Below this crust a variety of both sun crust and wind crust have been observed and are dependent on orientation to both the sun and wind.
Three significant crusts exist in the mid snow pack with the mid February down approximately 250 cm and this being the most pronounced and supportive with facets still being found above and below this crust.The lower mid and lower snow pack continue to settle and with this settlement an increase in density and strength had been noted.
Surface- New snow has buried a well developed surface crust.
Upper- Numerous melt freeze crusts exist in the upper snow pack.
Mid- Well settled with lingering persistent weaknesses found between 100 and 200 cm
Lower- Well settled.
Weather Forecast: A low pressure centered off the north west of the forecast area continues to push storm pulses east, bringing moderate south east wind and light snowfall. Expect this system to dissipate early Saturday, and in its wake a ridge of high pressure will bring to the region clearing sky and north wind. Sunday will see solar effect at its strongest and alpine temperatures and freezing levels will begin to rise rapidly before mid day.
Fri- up to 15 cm new snow. Winds SW to 40 km/hr.
Freezing level to 1200 m.
Sat- up to 5 cm new snow. Winds S to 25 km/hr.
Freezing level to 1200 m
Sun- Clear sky Winds NE to 15 km/hr.
Freezing level to 1500 m.
Prepared by Jesse Percival