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Avalanche Forecast

Archived

Feb 20th, 2017–Feb 21st, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
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.

New wind loading of lee easterly aspects should occur near and above treeline Tuesday afternoon. Lingering storm slab may still be sensitive to skier triggering in less wind affected areas.

Detailed Forecast

Another low pressure system will track across Oregon Tuesday with another round of light to moderate snow for Mt. Hood and a slow cooling trend taking place during the day. NW winds will increase and become moderate in the afternoon. 

New wind loading of lee easterly aspects should occur near and above treeline Tuesday afternoon. Lingering storm slab may still be sensitive to skier triggering in less wind affected areas.

 

Snowpack Discussion

Weather and Snowpack

Hopefully the last atmospheric river of the 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 low pressure system that tracked from the Oregon Coast across the south Washington Cascades brought 10-12 inches of snow to Mt. Hood stations Sunday night and Monday along with a warming trend that peaked late Monday morning. Mid-mountain winds that were easterly in the morning switched to westerly Monday afternoon and were strong W-SW above treeline for much of Monday.  

Recent Observations

A backcountry report from NWAC pro-observer Laura Green from the White River drainage on Friday indicated a mix of surface conditions. Below treeline she found plenty of roller balls. Near and above treeline she found a firm rain crust near and above treeline with varying amounts of recent snow above depending upon wind exposure. Isolated wind slabs had formed on the leeward terrain below ridges, with one natural wind slab at about 7700 feet on an ESE facing wind loaded slope. Elsewhere she found firm conditions on an exposed hard rain crust.

Laura was out again on Sunday at Bennett Pass up to about 6000 feet and found recent snow well bonded to the 2/17 crust and observed no signs of avalanches.

The Mt Hood Meadows pro-patrol on Sunday found N-E slopes up to about 6600 feet a bit loaded with 8-10 inches of wind transported snow on the 2/17 crust but no significant wind slab.

Problems

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.