Avalanche Forecast
Regions: Mt Hood.
New wind slabs are the primary concern near and above treeline. The avalanche danger at lower elevations will continue to slowly decrease Thursday. The uncertainty will be wind transported snow near ridges and newly forming wind slabs on a variety of aspects. Watch for any wind stiffened surface snow and avoid large open slopes of consequence in higher terrain. Caution should still be the watchword over the next days.
Detailed Forecast
Cool weather with mostly clear skies or high clouds are expected Thursday with moderate NW winds at higher elevations.Â
This weather will continue to cause a decreasing trend in the avalanche danger as recent storm snow slowly stabilizes. At higher elevations, expect wind transport and building areas of wind slab, mainly above treeline and onto unusual aspects from the mostly NNW winds.
While the overall danger is decreasing, continue to travel with caution and watch for any slab like structure or cohesion in the upper snowpack, indicating the possibility of triggered slab avalanches.  Â
Watch for touchy recently formed cornices and avoid travel on steep unsupported slopes.
Remember that beneath snow received during this storm cycle, early season terrain hazards exist, especially at lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the season's snowcover.
Snowpack Discussion
A strong storm cycle began early Monday and tapered Wednesday. Storm total new snow ranges from 2-4 feet or more over the Mt Hood area as of Wednesday afternoon where between 3-5 inches of water equivalent has fallen since early Monday. Â Light showers at cool temperatures with light winds Wednesday added 1-3 inches additional new snow.
One or more widespread natural avalanche cycles have occurred during the heaviest intensities, especially Tuesday. Sensitive storm slab avalanches occurred Tuesday from natural, human and explosive triggers.Â
These avalanches all appear to have released within storm layers with no indication of slides releasing on the late November rain crust.Â
As of Wednesday, field observations indicate these previous very sensitive storm layers have settled and continue to stabilize. Cooling temperatures and light winds have left a favorable storm snow profile of increasing density with depth.Â
The old snow interface of 12/15 appears to be favorable as well, from observations received as of late Wednesday.
The main take away is the current upper snowpack in most areas below treeline, lacks a slab structure and remains of low cohesion. Thus, there is low propensity to release slab avalanches. However, it may only take some minimal wind effect to stiffen the surface snow to change all that.
There was a freezing rain crust reportedly forming Monday above 6000 feet at Mt Hood Meadows which should be noted, see image below.Â
Winds above treeline have shifted and strengthened from the N-NW Wednesday, transporting the abundant new snow and building wind slab layers on a variety of aspects, especially above treeline. Â
The snowpack near and above treeline may be very different as there are likely wind slabs lurking. Limited observations above treeline means a lower confidence on the forecast and hence travelers should approach with a high margin of safety.Â
There's plenty of great snow at mid and lower elevations so eliminating any potential wind loaded terrain would be wise.Â
Observations
The Meadows pro patrol on Wednesday reported less avalanche activity than Tuesday, however wind slabs continued to be released with explosives and active wind transport was occurring Wednesday near and above treeline with strong NW winds.
The Meadows pro patrol on Tuesday reported new wind slab forming near tree line and did not need to venture to the above tree line. New snow was bonding well to previous surface below tree line.
NWAC pro oberserver Laura Green was in the Heather/Mitchell drainages on Tuesday in low visibility conditions, her test results indicated several storm layers in about 1 foot of new snow but with low quality results. Total snow depths were quite varied and the new snow was bonding well to previous melt freeze snow.
On Monday the Meadows pro patrol reported a major freezing rain event with up to 6-8 inches of ice in the 6000-7300 foot elevation range, with rain softened snow at lower elevations.
Ice coated tree limbs at Mt Hood Meadows on Monday December 18th. Photo Nick Burks.
Avalanche 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: Likely
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1