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Avalanche Forecast

Archived

Mar 8th, 2017–Mar 9th, 2017

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

Yet another storm will impact Mt. Hood on Thursday with increasing rain and snow, wind and rising snow levels. Travel in avalanche terrain is not recommended on Thursday due to the very dangerous avalanche conditions expected.  

Detailed Forecast

A weak low pressure system should produce another 12-18 inches of snow for Mt. Hood Wednesday afternoon and night. A warming trend Wednesday night should produce a spike in avalanche activity overnight. 

However the main event will be the storm system forecast Thursday through Friday morning.  A low pressure system tracking north across Washington State will bring increasing precipitation during the day Thursday to Mt. Hood. This will be accompanied by a continued warming trend with snow levels peaking around 8000 feet Thursday afternoon. 

All of these weather factors will stress the upper snowpack still struggling to settle and stabilize. The warming trend and increasing snowfall at higher elevations will increase the likelihood of new storm slab layers.

Deep wind slab should be watched for on all aspects but should be found mainly on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.

Loose wet avalanches will become more likely as rain becomes heavier and pushes higher up the mountain. Loose wet avalanches on steeper slopes may become large and entrain a significant amount of recent snow. 

Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time.

Snowpack Discussion

Weather and Snowpack

The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost very strong rain crust in our snowpack. 

Strong southwest flow carried a strong front across the Northwest on Friday evening March 3rd. At Mt Hood this caused strong southwest alpine winds, heavy, moist, dense new snow above about 4000 ft and wet snow or rain below about 4000 ft.

NWAC stations at Mt Hood have had about 4 ft of snow in the past 3 days during the ongoing storm cycle.

Recent Observations

The Meadows patrol on Tuesday morning reported strong winds and heavy snowfall but with limited avalanches. Upside down wind slab layers were building with lots of snowpack cracking and whoomping due to wind slab on ridges. Patrollers released 2 sensitive cornices remotely.

Warming, strong W-SW winds and heavy snowfall Tuesday caused a very large natural avalanche cycle according to the Mt. Hood Meadows pro-patrol by early Wednesday morning.

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.

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.