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Avalanche Forecast

Archived

Feb 18th, 2017–Feb 19th, 2017

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

Regions

Mt Hood.

Up to several inches of new snow should be seen on Sunday at Mt Hood. Evaluate snow and terrain carefully for possible wind or storm slab on Sunday.

Detailed Forecast

Moderately increasing south to southwest alpine winds and several inches of new snow should be seen on Sunday at Mt Hood. Temperatures shouldn't change much again on Sunday.

This should bring a slight avalanche danger increase on Sunday.

Watch for firmer wind transported snow on potential lee slopes. Alpine winds at the tail end of the last storm and on Sunday will have been mostly west to southwest so north to southeast aspects will be indicated.

Small areas of shallow storm slab might be possible in any areas that receive a few inches of rapidly accumulating snowfall.

Underlying wet snow especially at lower elevations should continue to drain and refreeze.

Snowpack Discussion

Weather and Snowpack

Yet another atmospheric river arrived Tuesday night, bringing heavy rain to Mt Hood. The storm total rain amounts ranged from over 2 to nearly 3 inches of water by Thursday morning. This weather caused some avalanches, lots of consolidation and the formation of surface crusts as temperatures began to cool at the tail end of the storm.

A slow cooling trend through late Thursday with light to moderate snow showers deposited 5-8 inches of new snow late Thursday and Thursday night at mid and upper elevations above the hardening rain crust. A period of strong westerly winds accompanied these snow showers late Thursday, building new wind slab on lee slopes near and above treeline.

A fair day was seen on Friday with light winds and no precipitation.

Increasing west to southwest alpine winds and light snow are seen at Mt Hood on Saturday.

Temperatures haven't changed much on Friday and Saturday.

Recent Observations

NWAC pro observer Laura Green reporting late Thursday from Mt Hood Meadows indicated the 5-6 inches of new snow was being rapidly transported, building fresh wind slabs on a variety of lee slopes from NE-SE facing below ridges. The bonding to the underlying wet snow appeared to be good initially.

Control results by the Mt Hood Meadows pro patrol early Friday produced 1 foot wind slab releases on lee terrain above treeline from artillery explosives. These slides would be the most recent wind slabs formed late Thursday. Elsewhere, controlled wind slab releases ranged from 6 inches to 1 foot were described as stubborn and isolated. Below treeline significant wet snow remained in the upper snowpack Friday.

A backcountry report from Laura from the White River drainage on Friday indicated a mix of surface conditions. Below treeline she found plenty of roller balls. Near and above treeline she found a firm rain crust near and above treeline with varying amounts of recent snow above depending upon wind exposure. Isolated wind slabs had formed on the leeward terrain below ridges, with one natural wind slab at about 7700 feet on an ESE facing wind loaded slope. Elsewhere she found firm conditions on an exposed hard rain crust.

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.