Avalanche Forecast

Archived

Feb 27th, 2017–Feb 28th, 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.

UPDATED - Stronger storm than forecast has created dangerous avalanche conditions. Widespread large wind slabs near and above treeline. Unstable storm slabs in all elevation bands. Cautious routefinding and conservative decision-making essential.

Detailed Forecast

A break between disturbances is expected during the day Tuesday before the next weather system arrives Tuesday afternoon. Cold temperatures, light to moderate winds and increasing light snow showers by late Tuesday afternoon. Winds in exposed terrain should be able to transport loose surface snow easily and build fresh shallow wind slabs Tuesday, especially near and above treeline.

Watch for fresh wind slabs on lee aspects and cross loaded features. Any newly formed wind slabs should mainly be confined to the terrain near and above treeline, but in some areas wind slabs may have formed in the upper elevations of the below treeline, so evaluate the snow and terrain carefully Tuesday.

Watch for loose dry avalanches on steeper slopes in non-wind affected terrain. Be especially wary of fast running loose dry avalanches near terrain traps.

Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.

Storm slabs will not be listed as an avalanche problem, but watch for any slightly stiffer surface storm snow that may be able to release as a soft storm slab.

Snowpack Discussion

Weather and Snowpack

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

About 2 feet of snow accumulated at the NWAC Mt Hood stations from Sunday, 2/19 through Wednesday 2/22.

Weak weather systems Sunday and again Monday, deposited about another foot of low density snow by Monday evening with light to moderate westerly transport winds near and above treeline.

Recent Observations

A widespread loose dry avalanche cycle was observed by the Meadows pro-patrol in steep terrain up to 8000 feet on Saturday.

On Sunday, the Meadows pro patrol reported pockets of fresh 12-18" wind slab on NE and E aspects near and above treeline that were easily ski triggered but not running far during avalanche control work.

...updated Tuesday morning 2/28 - Mt Hood pro patrol reported 20 inches of new snow in 24 hours with sensitive storm layers. Very strong westerly winds and deep storm snow have created dangerous avalanche conditions!

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.