Avalanche Forecast

Cool weather with mostly clear skies or high clouds are expected Thursday with moderate NW winds at higher elevations. 

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.   

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.

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.  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. 

These avalanches all appear to have released within storm layers with no indication of slides releasing on the late November rain crust. 

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. 

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. 

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.  

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. 

There's plenty of great snow at mid and lower elevations so eliminating any potential wind loaded terrain would be wise. 

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.