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RegisterMar 6th, 2014–Mar 7th, 2014
Snoqualmie Pass.
Wet snow avalanches near and below treeline will be the primary concerns Friday. Be wary of increasing sign of surface snow instability and avoid terrain traps where even a small but heavy wet snow avalanche could be dangerous. Not much is known about recently formed wind slab above treeline.
Rain and snow showers should taper down overnight and into Friday morning. There should be some partial clearing in the afternoon. Despite some cooling relative to the last couple of days wet snow avalanches will continue to be the primary concern Friday, primarily in the below and near treeline zones. Direct sunshine especially on solar aspects should help accelerate this process, so use increasing caution in the late morning and afternoon hours.
Steep slopes that recently received heavy snowfall should be primed for wet loose avalanches. Natural pinwheeling of surface snow is usually a precursor to this type of avalanche. Expect wet loose avalanches to potentially entrain the most recent storm snow and become potentially large and dangerous. If you are sinking in more than ankle deep, expect wet loose avalanches to be possible on steeper slopes.
Wet slab avalanches are possible if the saturated slab above is poorly bonded to the early March crust and this could allow for a much larger slide on all aspects. Wet slab avalanches are harder to predict than wet loose releases and may not be tied to direct sunshine or the warmest temperatures. This will not be listed as a concern because of the low predictability. In the same vein cornices are expected to still be sensitive on Friday.
While the generally warm temperatures should have helped bonding, new storm slab formed Thursday and Thursday night may have lingering instabilities through Friday on all aspects. Not much is known about how sensitive new wind slab near and above treeline is following this most recent storm cycle. Approach lee slopes cautiously and avoid steeper wind loaded slopes above treeline.
Recent Weather
An extended mild and dry period occurred the latter half of January generally forming a crust in all areas. Very cold weather followed in early February which produced light low density snowfall and facets just above the late January crust.
A two week storm cycle ended on 2/25 producing 7-12 feet of snowfall and many avalanche cycles near and west of the crest.
Warm dry weather was seen late last week from Wednesday through Saturday causing wet snow avalanches and melt/freeze crusts throughout the region. Surface cold air also redeveloped east of the crest last weekend. Temperatures dropped significantly by Sunday prior to the onset of a new storm cycle. This caused poor bonding of the initial snowfall to the newly formed crust layer in most areas.
A series of fronts have moved across the area since Sunday at the rate of at least one per day. Snow levels have been generally on the rise through these events causing rain or wet snow conditions to about 5500 ft in the north, 6000 ft central and 7000 ft or higher in the south. Snow levels moderated a bit on Thursday's storm to 4000 - 5000 ft. Five day water equivalents are around 6-8 inches through Thursday afternoon and snowfall of about 3-6 feet.
Washington Cascades near and west of the crest
The latest storm cycle began on Sunday. An avalanche cycle was seen on Sunday night and Monday morning. DOT crews at Stevens and Snoqualmie reported widespread natural and triggered 1-2 foot avalanches Sunday night and Monday morning which closed Snoqualmie Pass. Poor bonds to the melt-freeze crust from late last week have been reported as a likely bed surface as well as colder lower density snowfall preserved at the onset of precipitation likely providing a point of failure within the most recent storm snow.
Snow levels have been rising and temperatures warming Tuesday afternoon through Wednesday with heavy rain or wet snow producing several avalanche cycles. Alpental ski patrol and Snoqualmie and Stevens Pass DOT reported another round of natural avalanches Tuesday night and Wednesday morning with mostly loose wet snow avalanches. However some larger natural releases were noted above cliff bands where unsupported slabs released above rock faces slopes. Loose wet snow avalanches releasing during control work entrained quite a bit of the underlying storm snow. Crystal mountain reported similar reports Thu AM with a wet snow avalanche off of Rockface gouging down to deeper layers and running a fair distance.
3-5-14 Point release (wet loose) that gouged to the ground, Spiral Butte nr White Pass, Photo John Stimberis
The late January crust and faceted layers for early February are now very deeply buried and have been getting tested by the recent very heavy precipitation. In the Snoqualmie and Stevens Pass area this interface has been reported 2-3 m deep around 4000 feet. While large avalanches have been seen down to this layer along the west slopes over the last 2 weeks, including a close call at Chair Peak near Alpental on 2/22, we are close to dropping the deep persistent slab concern. This is due to how deep this layer is and how unreactive during the latest round of natural avalanche cycles. However, we will wait until the recent warm and wet period comes to an end. Downhill creep in the upper snowpack and the possibility of large natural triggers due to cornice failures remain an outside possibility to trigger a slide this deep. This layer will continue to be tracked as we transition into spring. Here is a link to more information about deep persistent slabs which are unusual in our area and not predictable. It's still very much worth thinking about the terrain you are connected to and what a deep persistent slab release would mean.