Avalanche Danger Forecast
Issued Monday, March 1, 2021 at 11:15am for the greater Anchorage area Western Chugach Mountains (i.e. Chugach State Park). View map of primary forecast area here.
Monday:
Tuesday-Thursday:
Avalanche danger is increasing today due to snow and wind. It may reach considerable in areas where stronger wind is accompanying the snowfall (e.g. it’s blowing moderate with strong gusts at Arctic Valley this morning).
Danger is expected to decrease Tuesday through Thursday due to the quiet weather forecast: relatively clear skies with light to moderate winds. Be mindful of any uptick in outflow northerly wind that accompanies the midweek clear skies, as they will load leeward slopes with the few to several inches of new snow received Monday.
A rain crust formed to summit elevations sometime late Friday to early Saturday. We are not sure how widespread it is, but it may exist across the park. As it will be buried Monday, this layer will serve as a slick bed surface and/or weak interface for the new snow above it. Keep this in mind as it may make wind slabs and sluffs from Monday’s new snow especially reactive to human triggers.
Mountain Weather:
Monday: snow, light to moderate east to south wind, temps in the teens
Tuesday: decreasing clouds, light to moderate east to south wind, temps in the teens to low 20s
Wednesday: sunny, light to moderate northerly wind, temps in the teens
Thursday: increasing clouds, light to moderate east to south wind, temps in the teens to low 20s
Discussion:
Snow trickled in last week and provided some improvement in surface conditions for riding. A rain crust formed at the beginning of the weekend and was observed to summit elevations in the southeastern Front Range. Strong northerly wind affected the entirety of the park Sunday.
The mountains will receive a few to several inches of light, dry snow Monday (more in southern and eastern areas of the park). Most areas’ new snow is expected to remain unscathed from significant wind damage, and midweek conditions are promising both in terms of weather and surface conditions for riding.
The extent of the rain crust in Chugach State Park is not currently known beyond its existence to summit elevations in the southern Front Range and Turnagain Arm areas, but we do know it has also been found in the Girdwood area to the south and east and Hatcher Pass area to the north. It’s expected to summit elevation in all areas where it exists.
Unlike the drizzle crusts that capped the Chugach State Park snowpack a few times earlier this season (that didn’t have a significantly negative effect on riding conditions and disintegrated readily once buried), the rain crust formed early this weekend is stout enough to have an adverse affect on turn quality and will likely serve (once buried) as a low friction bed surface, weak interface, and/or relatively impermeable layer that negatively impacts the temperature gradient exacerbating faceting above and below it.
Another noteworthy persistent weak layer was buried last week. Wind slabs formed over near surface facets found buried by new snow and wind loading late Tuesday into early Wednesday in the South Fork Eagle River area were very reactive to human triggers (see the past forecast for photos and discussion).
Avalanche Problems:
Wind slabs up to D2 in size above 2500′ on leeward terrain steeper than 35º.
Wind will shift through the forecast period and due to the terrain of the park channeling winds in varying directions, watch for small wind slabs on all aspects.
Active wind loading, recent avalanches, and shooting cracks are red flag indicators of wind slab danger. Wind slabs are most likely to exist below corniced areas and along the lee sides of ridges and cross-loaded features like gullies:
Relatively densely-packed (firm and “punchier” feeling) snow and pockets of deeper snow (especially with a bulbous or fat appearance) are indicators of wind slabs. If you look closely at the snow surface you may be able to distinguish where wind slabs exist due to the snow having a more textured, densely-packed, consolidated, cohesive, and/or striated appearance. Be especially wary of (and avoid) steep, convex, and/or unsupported wind loaded terrain features.
Pole probing and hand pits are a quick and effective means of assessing this problem as you travel. Use pole probing to quickly feel out areas of denser, wind-packed snow overlying looser and weaker snow. Use hand pits to quickly assess how near-surface layers of snow are bonded.
Digging a snowpit and conducting a compression test and/or extended column test will provide an even better assessment of bonding and instability before you travel on terrain of consequence.
You can also assess wind slab instability via safer “test slopes” that are representative of higher consequence terrain.
Persistent slabs up to D2.5 in size above 2500′ on terrain steeper than 35º.
While persistent slabs may exist on all aspects, expect the problem to be the most pronounced on recently wind loaded terrain (where buried persistent weak layers may be “stressed” and haven’t adjusted to the recently added load) and shadier and colder northerly aspects (where the temperature gradient is relatively adverse to bonding and stabilization of buried persistent weak layers).
The persistent slab problem is lower probability, but potentially much higher consequence, than the wind slab problem. Persistent slabs triggered this forecast period may be hard slabs that break deeper down in the snowpack and release above and around a human trigger (rather than at the trigger’s feet) which can make escape difficult. Hard slab avalanche debris is also more likely to cause trauma than debris from soft slabs.
Be on the lookout for red flag warnings of persistent slab danger: recent avalanches, collapsing (aka “whumphing“), and shooting cracks.
Diverse and widespread persistent weak layers exist in the snowpack. Faceted snow exists above and below crusts in some areas, sandwiched between wind packed layers in many areas, and a basal weak layer of advanced facets and depth hoar is widespread. Surface hoar may also exist in isolated areas, although dangerous terrain where it’s been buried intact is likely very limited. That said, keep it in mind if you’re traveling around mid elevation terrain traps.
Dry loose snow avalanches (human triggered sluffing and naturally triggered spindrift) are possible above 3000′ on steep terrain (38º+) across all aspects.
While these relatively small avalanches (D1) aren’t much of a concern in regard to burial, they do have the potential to cause a fall or loss of control. While not likely, it’s possible that a loose snow avalanche could trigger a larger wind or persistent slab as it descends a slope.
Don’t let a loose snow avalanche catch you off guard. Manage your sluff if descending via ski or snowboard steep and wind-sheltered terrain with loose and uncohesive snow. If it’s windy and you’re climbing steep and/or exposed terrain, be mindful of the potential for heavy spindrift that could knock you off your feet and take you for a (potentially deadly) ride.
Cornices are large and overhanging in some areas above 2500′. New snow and wind will make them more prone to failure and falling.
Give cornices a wide berth and don’t approach the edge of a ridge to look down slope unless you’re sure it’s not corniced. Cornices can break off further back than expected (even beyond the true edge of the mountain ridge beneath the snow surface). In addition to the danger of taking a nasty fall due to a collapsing cornice, a falling cornice may “bomb” the slope it lands on and trigger an avalanche.
Terrain management is always the best way to deal with avalanche problems: eliminate or reduce your exposure to terrain capable of producing an avalanche. Before traveling on or under terrain that has the potential to avalanche think about the consequences and have a plan (to escape the avalanche, for re-grouping, and rescue).
Always be mindful of terrain traps that can make even a small avalanche deadly.
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