Avalanche Forecast
Dec 19th, 2018–Dec 21st, 2018
Alpine
3:
Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2:
Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2:
Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2:
Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2:
Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2:
Moderate
The avalanche danger rating below treeline will be moderate
Regions: Mt Hood.
The Bottom Line: Dangerous avalanche conditions persist at higher elevations following strong storms over the past two days. Warming, precipitation, and strong winds mean you could trigger a large slab avalanche Thursday, particularly in high elevation start zones.
Overview
On Tuesday morning, Mt. Hood Pro Patrol found high avalanche danger at all elevations. One notable slide occurred naturally on a path that normally only goes in east wind loading events. Given our predominant southerly and southwesterly winds during the recent storm, this event was surprising and suggests the possible reactivity of persistent grain types (although the bed-surface of this slide was not identified due to poor visibility).
Weak snow formed and buried earlier in the month can still be found in the upper snowpack. On Monday, Mt. Hood Meadows pro-patrol found buried surface hoar most intact in sheltered locations from 5500 to 6500 ft with the surface hoar layer below 5500 ft. While this layer was preserved intact 16-20" below the snow surface, it was neither reactive in snowpack tests nor involved in any recent avalanche activity. Several rain crusts were found above this layer. We will continue to track this layer and there is a small chance that with loading or rain it could produce a wet slab.
Regional Synopsis
Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern and widespread persistent weak layer.
For perspective, the approximate snow totals from 12/9-12/19 are:
In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause in numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess … in short, the scenario is tricky and getting more dangerous by the day.
For perspective, the approximate snow totals from 12/9-12/19 are:
- Mt Baker: 102â€
- Washington Pass: 55â€
- Stevens Pass: 76†mid-mountain
- Snoqualmie Pass: 68†mid-mountain
- Crystal Mountain 70†Green Valley
- Paradise: 78â€
- Mt Hood Meadows: 44†mid-mountain
- Olympics: 48â€
In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause in numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess … in short, the scenario is tricky and getting more dangerous by the day.
Higher snow totals along the Hwy 542 corridor /Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south and we have not yet experienced a widespread cycle …
When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.
Be patient and continue to stick to lower angle slopes with nothing above you. This is a good time to avoid areas where avalanches can start, run, and stop.
We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.
Weather Synopsis
A strong warm front lifting over the area this morning is directing heavier precipitation into the Olympics and north Cascades. Precipitation will fill in for areas further south later this morning and the trailing cold front should swing through the Washington Cascades around 1 PM. This storm system will bring heavier precipitation amounts to the volcanoes and Crystal Mt, and less precipitation for the Cascade Passes. Easterly flow will suppress snow levels in the Passes early this morning, but warmer air should push snow levels above Pass level for Snoqualmie and White Passes by late morning.
With the frontal passage strong winds aloft may mix down, creating very windy conditions in the afternoon. Snow level will lower rapidly late afternoon and overnight as colder air is advected into the region. Light to moderate snow should accumulate along the west slopes of the Cascades and Mt. Hood overnight. Westerly post-frontal winds should be blusterly overnight and ease off during the day on Friday.
Shortwave ridging will build over the area on Friday representing a relative break in the action, but there will still be a chance of showers for the mountains with lingering instability and weak onshore flow.
Avalanche Problems
Wind Slabs
Recent and expected strong winds, precipitation and warming will maintain dangerous and large wind slabs that will be reactive to human trigger. Natural avalanches are possible and may come down from the higher slopes on Mt. Hood. Watch for scoured ridge-lines, rounded snow pillows, and sastrugi as confirmation that wind slabs are present in the terrain.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 2 - 2
Loose Wet
Warming and rain to above treeline will cause small wet loose avalanches. If enough recent snow is available, some of these avalanches could be large enough to cause injury.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 2