Avalanche Forecast Mt Hood

Date Issued: Valid Until:

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche Forecast

Tue Apr. 9th · 2:36AM

Alpine

Danger Ratings High

Treeline

Danger Ratings High

Below Treeline

Danger Ratings Considerable
Wind Slabs Wind Slabs

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Moderate

Below Treeline

Danger Ratings Moderate

UPDATE 04/09/19 at 9 AM due to high rates of wind transport and snowfall along with widespread shooting cracks near treeline in the Mt. Hood Meadows area with a wind slab problem developing into below treeline terrrain. Strong winds and cool temperatures are eliminating the potential for loose wet avalanches.

Very dangerous avalanche conditions continue to exist above treeline on Mt. Hood where high winds continue to load slopes at all elevations and very large avalanches are likely to continue to release near and above treeline. Lowering snow levels and continued winds are building wind slabs at all elevations on Tuesday. Tuesday is a good day to keep things simple and avoid areas where these avalanches can start, stop or run and all terrain steeper than 35 degrees at lower elevations.

Discussion

Snow and Avalanche Discussion

A widespread cycle of very large natural avalanches occurred on Mt. Hood during an atmospheric river event that brought literal buckets of water to Mt. Hood. Strong winds continue to sustain high avalanche danger near and above treeline. With the help of a cooling trend, wind slab danger will increase into the near treeline band.

Very large crown (D3.5-D4) on the upper slopes of Mt. Hood above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.

These avalanches are deadly and they may travel tremendous distances from start zones typically above or well above treeline.

Over the last 4 days, multiple storm systems produced 5-8” of rain (much of it Sunday through Monday). Snow levels oscillated between 5000-7000' during the storm. After a deluge of rain Saturday night and Sunday morning with snow levels rising to 7000', several inches of wet snow accumulated down to 5300' Sunday afternoon, then snow levels rose back to 7000 ft Sunday night, dropping down the mountain to below Mt. Hood Meadows base during the day on Monday. Winds were strong at times during the event. Snow will continue to accumulate to lowering elevations Monday and Monday night.

Moderate to heavy rain Sunday night into Monday morning caused wet loose avalanches on steep slopes that experienced a transition from snow to rain Sunday night. This threat has been negated by cooling temperatures and strong winds on Tuesday.

Snowpack Discussion

April 3rd, 2019

Spring snowmelt

The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.

Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg

Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. Traveling in the mountains the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers.

NWAC climatological snow depth data from April 1st. You can view it on our website here.

 

A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus

 

Spring avalanche considerations

As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:

  1. Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?

  2. What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?

  3. How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?

Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg

Other considerations

In addition to daily avalanche hazard, the early snowmelt is creating some other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.

We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.

Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer

Problems

Wind Slabs

Wind Slabs

UPDATED 9AM due to strong wind transport and active wind loading in all elevation bands. Very dangerous avalanche conditions exist near and above treeline. Widespread shooting cracks in approximately 10" wind-altered snow were reported around 6000 ft on Mt. Hood on Tuesday morning. This adds to our concern about the potential for large to very large natural or human-triggered avalanches.

Avoid all areas where avalanches can start, stop, and run on Mt. Hood. Strong winds and heavy snowfall produced very large, powerful and very sensitive wind slabs which released naturally during the storm and we think will continue to do so for one more day. Strong westerly winds on Tuesday will continue to have plenty of snow (including fresh snow) to transport.

Before venturing into the storm and likely poor visibility, know your exposure to large avalanche paths (like the major canyons) with start zones located much higher on the mountain. Small to large wind slabs will build near treeline through the day on Monday. Slab avalanches can entrain wet snow at lower elevations and become even more powerful and destructive.

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

very likely

Expected Size

1 - 2