Issued: 6:00 PM PST Saturday, January 21, 2017
by Kenny Kramer

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

Watch for new wind slabs forming early Sunday on exposed lee slopes at higher elevations. Allow recent storm and wind slabs further time to stabilize Sunday. Wind slabs are most likely on NE-W-SW aspects. Storm slabs should continue to slowly stabilize Sunday, but still possible in the wrong spot.

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Avalanche Problems for Sunday

Wind Slabi

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.

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Storm Slabsi

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.

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Snowpack Analysis:

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun-sheltered areas during this period. Recent observations on Thursday, Friday and helped confirm that we aren't dealing with any older layers formed during this period.  

An atmospheric river moved over the Northwest Tuesday and Wednesday. Heavy rain was seen up to about 5500 feet in the Hurricane Ridge area. 3 day precipitation totals through noon Thursday were about 2.5 inches at Hurricane Ridge. An avalanche cycle likely occurred in the Olympics during this period.

The NPS reported a storm total of 15 inches of lower density snow at the Hurricane Ridge manual snow plot Thursday morning. Showers continued during the day Thursday, but our precipitation gage didn't record any precipitation after 9 am, possibly due to equipment problems. About a foot of snow fell at the Waterhole NRCS Snotel over a 24 hour period ending Friday morning. 

A very pleasant day Saturday allowed temperatures at Hurricane to reach the freezing mark. No showers had arrived by late Saturday afternoon.

Recent Observations

NPS personnel reported debris in the Old Faithful track down to the Hurricane Ridge road at 3500 feet. Debris piled up 10-15 feet deep in this narrow slide path. Plow drivers discovered the debris midday Wednesday with the avalanche likely occurring Tuesday or Tuesday night. 

Pro-observer, Matt Schonwald, was at Hurricane Ridge Friday. Matt found 10" (25 cm) of settled recent storm snow generally well bonded to the underlying rain crust and not reactive. SE winds were forming shallow wind slabs on lee slopes near treeline. The recent rainfall had penetrated well into the snowpack. At a limited number of test sites, the 12/17 PWL was found to be rounding and unreactive in snowpit test and well over a meter down. We feel comfortable removing the persistent slab from the avalanche problem list moving forward. 

Detailed Forecast for Sunday:

Occasional light snow showers are expected Sunday with a period of moderate to strong S-SE winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on NE-W-SW aspects due to recent and expected SE transport winds. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution.  

Storm slabs should be more stubborn to trigger, but still possible in the right spot on Sunday, especially near and above treeline and can be avoided by sticking to lower angled slopes. Storm slabs may extend into the upper portion of the below treeline band, but the overall avalanche danger will be rated as low.