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Avalanche Forecast

Archived

Feb 24th, 2017–Feb 25th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Sunshine Saturday may lead to shallow wet snow conditions on steep solar aspects during the late morning to afternoon. Isolated shallow fresh wind slabs are possible near and above treeline, but will become less sensitive to human triggering Saturday. Older hard slabs in wind loaded terrain above treeline will be very stubborn to human triggering, but would be very consequential if released. 

Detailed Forecast

A brief period of high pressure should build over the area Saturday. This will allow for clearing and plenty of sunshine Saturday. Winds should remain light with very cool temperatures. 

Old hard slabs formed earlier in the week should continue to heal, isolated to specific terrain features, and very stubborn to human triggering Saturday.

Watch for any shallow fresh wind slabs that may have formed early Friday, 2/24 during moderate westerly winds above treeline. 

More sunshine is expected Saturday than previously this week. Direct sunshine and some daytime warming should gradually increase the potential for loose wet avalanches Saturday, mainly confined to steep solar aspects during the warmest part of the day. Watch for wet surface snow deeper than a few inches if you find yourself on sun exposed slopes during any extended sun breaks.

Loose dry avalanches have been triggered this week, but will not be indicated as an avalanche problem. You can do tests for loose dry avalanches by pushing snow onto small safe test slopes.

Avoid areas along ridges where there may be a cornice and slopes below cornices, since cornices can fail at any time.

Snowpack Discussion

Weather and Snowpack

The latest of several warm, wet SW streams of moisture this season arrived on Valentine's Day 2/14, causing heavy rain, avalanches, crusts and significant snowpack consolidation through Thursday 2/16 at Mt. Hood. About 5-8 inches of snow fell at Mt Hood during the tail end of the storm.

A short period of fair weather on Friday, 2/17 caused another surface crust at Mt Hood.

A storm Sunday night and Mondaydeposited 10-12 inches of snow at the NWAC Mt Hood stations with a warming trend. Mid-mountain winds were strong W-SW above treeline for much of Monday, forming hard slabs on lee terrain above treeline.  

Light to moderate snowfall Tuesday changed to showers in the afternoon with moderate W winds. More light snow showers occurred Wednesday, bringing the 4 day snow total to about 2 feet at NWAC Mt Hood stations by Wednesday evening.

A slightly unstable weather pattern this week caused a mix of sun and light snow showers with cold temperatures and light winds from Wednesday through Friday. The weather this week helped to freshen the surface with a few, up to several inches of snow. This weather has all tended towards increased snowpack stability through the week, while maintaining some excellent touring conditions, especially on shaded terrain.

Recent Observations

On Tuesday 2/21, the Mt Hood Meadows patrol triggered isolated pockets of large hard slabs with explosives above treeline on lee easterly aspects. One wind slab avalanche had a crown up to 5 feet! Near treeline, 6-12” storm slab was sensitive and showed good propagation with explosives and ski cuts. Below treeline, small loose dry was the only avalanche concern on steep slopes.

The Mt Hood Meadows patrol reported isolated wind slab releases from explosive control Wednesday 2/22. These recent 6 inch to 1 foot wind slabs were above treeline on easterly facing terrain and unlikely to have released by human trigger. Shallow wet snow conditions developed during sun breaks, but no wet snow avalanches were reported.

By Thursday 2/23 more stabilizing had taken place and the Mt Hood Meadows patrol reported no results from ski test within the ski area, but there was still some reactivity of wind slab layers in the 10-35 cm down on a ENE slope at 6600 ft.

NWAC observer, Laura Green on Friday, 2/24 reported about 20 inches of low cohesion snow on wind sheltered north aspects, providing excellent conditions with no avalanches. On solar aspects, a less desirable breakable sun crust was covered by the last few days of snow received during showers.

 

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Wind Slabs

Wind Slab avalanches are the 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.