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Avalanche Forecast

Archived

Dec 6th, 2017–Dec 7th, 2017

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

Regions

Mt Hood.

Thursday is a day to pay close attention to the weather and snowpack conditions as they evolve due to a high degree of variability and an unusual December warm-up impacting an evolving, but settling snowpack. Watch for small areas of firmer, wind-transported snow on isolated, lee terrain features. Watch for loose-wet snow avalanches releasing from sun exposed rock faces or below cliffs or rock bands.

Detailed Forecast

Continued sunny and mild weather is expected Thursday with crest level easterly winds decreasing significantly overnight and becoming light SW Thursday afternoon.  The winds will slacken overnight and shift to southeast and eventually to a gradually increasing SW wind on Thursday. Generally decreasing ESE wind speeds on Thursday, combined with increased warming aloft, will allow temperatures in the Mt. Hood zone to top out 10 degrees or so warmer than today and approach 50F in many locations.

Expect a high degree of variability depending on aspect and degree of warming from location to location. Expect fresh wind slab size and sensitivity to vary considerably near and above treeline in the Mt. Hood area, but to generally be found on westerly aspects. Warming throughout the day should help older wind slabs continue to stabilize, but fresher recent wind slabs have been forming and will need another day or two of warm temperatures to stabilize. A general decreasing danger is expected as relenting winds and warm temperatures mitigate further wind transport and wind slab formation. Stubborn wind slab may exist in specific locations near and above treeline.  

In all areas, watch for small loose wet avalanches on steep sun exposed terrain releasing during the late morning or afternoon, especially on unsupported slopes or near rocks and where loose wet avalanches might carry you into terrain traps or other hazards such as trees or low-snow hazards. Loose wet avalanches are expected to become larger as warming continues and winds slacken throughout the day.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

A series of frontal systems from post-Thanksgiving through the first weekend in December produced significant snowfall in the Mt. Hood area. There are no snowpack concerns below the Thanksgiving crust and in general new snow received post-Thanksgiving has reportedly bonded well. 

A series of frontal systems produced snow over the week following Thanksgiving: NWAC stations at Mt. Hood received 20-23" of snow. 

The active weather pattern was replaced with a high pressure system which brought plenty of sunshine and initially cold temperatures Sunday-Monday, followed by a significant warming trend from Monday through Wednesday. Generally northeasterly winds in the 10's and 20's with gusts to 30+ mph at Mt. Hood sites have kept this region from seeing as much warming as sites west of the Cascade crest in the Washington Cascades and temperatures topped out on Wednesday in the upper 30's at many Mt. Hood NWAC stations. The graphs below illustrate the warming trend and low-level surface-pressure-driven winds. 

Observations

On Monday, Patrol at Hood Meadows saw evidence of a natural wind slab that released after dark Sunday.

 

Natural wind slab release sometime Sunday night, 12/3. ESE facing wind loaded slope about 6600 feet. Photo: Brian Murphy

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.

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.