LOW avalanche danger is expected today, but if more snow falls than forecasted, the danger may rise to MODERATE. Pay attention to changing weather. Watch for shallow wind slabs forming in the afternoon with 1-2″ of new snow and increasing winds. Avoid travel under glide cracks and give cornices a wide berth.
Good travel habits, such as exposing one person at a time, watching your partners and re-grouping in safe zones are key ways to minimize risk.
Special Announcements
Don’t miss an evening with renowned snow scientist Ed Adams tonight, March 7th!! The American Society of Mechanical Engineers is hosting an evening lecture delving into snow micro-structure and how it relates to the white stuff we love to play on. More details HERE.
Heading to Hatcher Pass? Be sure to check out the Hatcher Pass Avalanche Center mid-week snow and avalanche summary HERE.
Thu, March 7th, 2019
Alpine
Above 2,500'
1 - Low
Treeline
1,000'-2,500'
1 - Low
Below Treeline
Below 1,000'
1 - Low
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Travel Advice
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Natural avalanches likely; human-triggered avalanches very likely.
Natural and human-triggered avalanches certain.
Avalanche Size and Distribution
Small avalanches in isolated areas or extreme terrain.
Small avalanches in specific areas; or large avalanches in isolated areas.
Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.
Large avalanches in many areas; or very large avalanches in specific areas.
Very large avalanches in many areas.
Avalanche Problem 1
Wind Slabs
Historic (D4-5)
Very Large (D3)
Large (D2)
Small (D1)
Size
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question.
Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to
hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are
relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure,
or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.
Following a two-week stretch of sunny skies and spring-like conditions we await a weather pattern shift. Several storms are on tap tomorrow and into the weekend. Winds will be the first thing to change today. Expect light Easterly winds to build to Moderate (10-25mph) by the late afternoon and 1-2” of new snow possible. Shallow wind slabs may form on leeward features, but we don’t expect the avalanche danger to rise until tomorrow with more snow. However, pay attention to changing conditions and if more snow falls anticipate thicker wind slabs.
The storm tomorrow could produce up to a foot of snow and more is on the way Saturday into Sunday. In anticipation of this weather shift, we have been closely mapping surface conditions. Surface hoar has been observed from valley bottoms to the Alpine and is resting on a variety of a sun crusts on Southerly facing slopes. On shaded aspects surface hoar and 4-8” or so of soft near surface facets sit over a firmer base. Along ridgelines and areas affected by the NW wind a few weeks ago, there is hard sastrugi, wind crusts and/or rime crust. This set-up does not bode well for future bonding and will be our next concerning layers once buried.
Things to keep in mind if you are headed into the backcountry today:
Wind slabs – Look for drifting snow on leeward features this afternoon and pay attention to slab thickness. Older loose snow is available for transport, in addition to a few inches of new snow today. Adjust your plans if you see shooting cracks and slabs thicker than 6” — evidence the avalanche danger is increasing.
Glide avalanches – These types of avalanches are highly unpredictable and not associated with human triggers. It’s always best to watch for and limit exposure under glide cracks.
Dry-loose sluffs – Watch your sluff on steep Northerly slopes where a sun crust doesn’t exist.
Cornice fall – As always, give cornices a wide berth.
An outlier slab avalanche – Although it is unlikely a person could trigger a slab avalanche, the mountains can harbor surprises, especially in thin snowpack areas. South of Turnagain in the Summit Lake and Silvertip zones there is a shallow snowpack with a generally poor structure. A variety of old weak layers (facets and buried surface hoar) sit in the mid and base of the snowpack. The most suspect place to trigger an avalanche is steep terrain with old, hard wind slabs sitting on weak snow. Increased winds may add more stress to these old layers in Summit Lake area and Johnson Pass.
Surface hoar sits on a variety of surfaces: loose snow, firm sun crusts, and hard wind board. Our current surface conditions will become our next layer of concern with more snow tomorrow through the weekend.
Weather
Thu, March 7th, 2019
Yesterday: Skies were partly cloudy. Temperatures were in the 20s F to low 30s. At lower elevations temps bumped into the mid-30s F during the day. Ridge tops winds were light from the West shifting to the East overnight. No precipitation fell.
Today: Expect overcast skies and winds to increase this afternoon. Ridge tops winds from the East will start out light 5-15mph and increase into the 20s mph by the afternoon. 1-2″ (0.12″ SWE) of new snow is possible today and 2-6″ new snow overnight. Temperatures in the upper elevations will be in the 20s F. Sea level temps will increase into the 30s F and rain/snow line may reach 500′.
Tomorrow: Snow showers are expected to increase tomorrow morning and into the afternoon with 4-8″ of new snow possible. Easterly ridge top wind will be in the 30s mph with gusts in the 40s mph. Temperatures will be in the 20s F in the upper elevations and 30s F near sea level. Expect snow to transition to rain at lower elevations.. A more powerful storm is expected Saturday into Sunday with another shot of snow accumulation, stronger winds and warmer temps.
This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area.
This advisory does not apply to highways, railroads or operating ski areas.
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