Avalanche Forecast
Issued: Feb 27th, 2018 12:39PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
A significant storm will impact Mt. Hood on Wednesday with stormy conditions developing reactive wind slabs near and above treeline and creating small, but touchy storm slabs in wind-sheltered areas. As conditions deteriorate, step back your terrain selection; avoid travel on obvious wind drifts, pillows of snow on leeward and crossloaded slopes or terrain 35 degrees and steeper. Continue to watch your exposure large avalanches that start from above.
Summary
Detailed Forecast
You will be likely to trigger an avalanche on Wednesday, particularly in the afternoon, as winds and snowfall ramp up throughout the day with a slight warming trend.
Fresh wind slabs will become reactive to human trigger and may be large by afternoon given the anticipated strong moderate to strong winds.
Older wind slabs deposited last weekend have mostly settled and stabilized. These slabs are quite large, so Continue to watch for areas that received significant wind loaded snow, such as steep slopes below ridges, mainly facing North to Southeast. Any triggered avalanche could become very large as there is a significant amount of storm snow available to become involved in an avalanche.
Storm slabs may develop if heavy snowfall rates and higher density snow fall on lower-density older snow. This problem will be mitigated somewhat by the variable nature of the existing snow surfaces created by recent winds.
Back country travelers should continue to travel with extra caution and best to travel in shallower angled terrain well away from avalanche paths or runout zones where avalanches release from above.
Snowpack Discussion
Several days of relatively quite weather have allowed dangerous avalanche conditions to partially stabilize.
Very strong winds and heavy snowfall intensities since late Friday have built large to very large wind slabs near and above treeline. These slabs are gradually stabilizing, but still threaten travelers exposed to Mt. Hood's very large slide paths which originate high on the mountain.
Older low density snow from last week has been loaded by back to back strong storms over the weekend with about 3 ft of new storm snow accumulating by Sunday evening. These storm slab layers should have stabilized by Wednesday.
The deepening storm snow now sits over a strong crust layer formed and buried on Saturday 2/17. This crust has been reported up to 6600 feet by professionals in the region.
Observations
On Tuesday, Mt. Hood Meadows pro patrol reported wind buff and soft wind-blown powder with crusts on solar slopes.
On Monday, evidence of the Sunday storm was in plain view with several very large crowns from natural avalanche releases were visible above Mt Hood Meadows above treeline. Storm slabs were still being triggered near treeline Monday.
Huge natural slab avalanche in Newton Canyon above Mt Hood Meadows releasing Sunday, 2/25. Image, Laura Green
By Sunday morning, MHM professional patrol reported "monumental" snow drifts! These conditions have only become larger by Monday.
MHM professional patrol on Saturday reported white out conditions above treeline with significant wind transport occurring. Below treeline the surface snow was forming unstable storm slabs. Still relatively shallow storm slab avalanches were releasing with explosives Saturday morning.
NWAC pro-observer Laura Green traveled in the Mt Hood backcountry Friday. Laura reported stable conditions prior to Friday’s incoming storm. She found a right-side-up snowpack with loose unconsolidated surface snow.
Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 28th, 2018 12:39PM
