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Avalanche Forecast

Archived

Mar 13th, 2017–Mar 14th, 2017

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

Regions

Mt Hood.

We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches have diminished. However their destructive potential remains high. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.  

Detailed Forecast

The relatively high snow levels will continue with rain forecast below 7500-8500 feet through Tuesday morning at Mt. Hood. We should see a relative break in the steady precipitation Tuesday afternoon before another wave of moisture returns Tuesday night. Periods of moderate W-SW winds above treeline will continue to transport new and recent snowfall only for the upper portion of the above treeline band Monday night through Tuesday. 

We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches have diminished. However their destructive potential remains high. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.

Loose wet avalanches are likely on steeper slopes with the additional rainfall. While most loose wet avalanches will be shallow, a few may gouge down to deeper layers and become more powerful than you expect. 

Cornices will be weakened and prone to failure due to rain and mild temperatures. Very large cornices have been reported in many areas and have been involved in recent accidents and close calls. A cornice failure could trigger a large and destructive wet slab avalanche. 

Wet slab avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Glide cracks can indicate areas where wet slab avalanches are more likely to occur. Besides their destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected. 

If you manage to venture into the above treeline zone Tuesday, expect new or previous wind slab mainly on NW-SE aspects due to recent SW-W winds. Continue to watch for firmer wind transported snow on all aspects especially in areas of complex terrain.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC stations at Mt Hood piled up about 6-7 ft of snow.

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. On Thursday night rain pushed up to about 7000 feet at Mt Hood. By Friday morning NWAC stations at Mt Hood had about 1.6 inches of WE (water equivalent) but alas only rain to show for it. This gave the snowpack a huge test and caused an avalanche cycle at Mt Hood.

Another front crossed the Cascades on Saturday causing more SW-W winds, snow at higher elevations, and rain at low elevations. NWAC stations at Mt Hood had about 2-6 inches of new snow on Sunday morning with the most on the upper mountain.

A mild frontal system slowly sagged south across the Cascades on Monday with moderate to locally heavy rainfall. Rainfall totals through 5 PM PST (or 6 PM PDT)

Recent Observations

The Meadows patrol on Thursday reported that rainfall was saturating the upper snowpack allowing several feet of boot penetration by mid-day.

NWAC observer Laura Green was out on Friday and reported new large cornice triggered slab avalanches in White River Canyon on SE aspects in the 6500-8500 ft range, and in Heather Canyon on a NE aspect in the 6100-6800 ft range. Very large debris was seen from new avalanches in Newton Canyon. Laura reports that in the 5200-6600 ft range the upper snowpack of 4F wet snow with some lower density drier layers below.

Mt. Hood Meadows pro-patrol reported an increasingly saturated snowpack with increasing boot penetration from 5400' and down within their area early Monday morning. 

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Wet Slabs

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.