Avalanche Forecast

Archived

Dec 18th, 2017–Dec 19th, 2017

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

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.

A peak in the avalanche danger should be seen Tuesday. You may encounter a greatly increasing avalanche danger with elevation on Tuesday. Avoid travel in terrain connected to higher start zones.

Detailed Forecast

Increasing strong southwest flow aloft will carry a major deepening low pressure system and cold front across the Northwest on Monday and Tuesday. Expect very strong alpine winds Monday night and Tuesday at Mt Hood, about 8-12 inches of new snow above treeline by Tuesday early morning, and more heavy new snow at lowering snow levels Tuesday.

New wind slab will form on lee slopes mainly near and above treeline Monday night and Tuesday morning. This is most likely on northwest to southeast aspects but is possible on other aspects. Watch for firmer wind transported snow. Due to the strength of the winds winds extensive wind slab is likely.

New storm slab is likely to form above treeline Monday night and then to the near and below treeline Tuesday in areas with rapidly accumulating new snow.

A peak in the avalanche danger should be seen Tuesday. You may encounter a greatly increasing avalanche danger with elevation on Tuesday. Avoid travel in terrain connected to higher start zones.

The recent warm weather has melted much of the early season snowpack. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.

Snowpack Discussion

The warm and sunny weather the first half of December did a number on the early season snowpack at Mt Hood, gradually consolidating snow and returning many areas to very shallow early season conditions. The most consistent snowpack remains on shaded northerly aspects in the upper below treeline to near treeline.

Above freezing temperatures, windy conditions, and periods of light rain over the weekend likely erased persistent grain types that were observed prior to Friday on Mt. Hood.

The overall snowpack is stable with settled old snow sitting over the strong Thanksgiving rain crust, buried about 1-3 feet.

Observations

No human-triggered avalanches have been reported in over a week.

But on Monday the Meadows pro patrol reported a major freezing rain event with up to 6-8 inches of ice in the 6000-7300 foot elevation range, with rain softened snow at lower elevations.

Ice coated tree limbs at Mt Hood Meadows on Monday December 18th. Photo Nick Burks.

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.