Avalanche Forecast
Issued: Apr 9th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
More snow than rain will come during the day Wednesday with cooler temperatures and strong southwesterly winds. Use caution at upper elevations around recently wind loaded slopes 35 degrees and steeper. Wet loose avalanches could occur within new snow by the afternoon.
Summary
Discussion
In the past 24 hours, the Heather Meadows weather station reported .50in of water, bringing the total water we have received in the past week up to 4.5in. This precipitation has come as almost all rain below 4000ft causing our low elevation snowpack to quickly disappear. Between 4000-5500ft, about 1ft of wet snow accumulated this past weekend, but was rained on and then re-froze Monday and Tuesday making for nasty travel conditions. Above 5500ft, winter is hanging on with deep drifts of dry snow and blustery conditions.
The coolest temperatures we have seen in several days are expected Wednesday, with snow levels set for 3000ft and another .50-.75in of water arriving during the day. Expect a refresh of new snow near and above treeline and increasing avalanche danger as the snow accumulates throughout the day. Strong WSW winds will drift the new snow into shallow wind slabs, and thicken older wind slabs that formed over the weekend.
The only avalanche activity in the past week has been isolated and small loose wet slides (D1) on slopes 40 degrees and steeper. Poor visibility has limited observations above treeline where uncertainty exists regarding the sensitivity of the wind slab problem. Keep in mind that melting cornices, widening glide cracks, and opening creeks continue to pose a hazard during this spring transition. Give these a wide berth and be heads up as the recent snow may hide these and other obstacles.
Snowpack Discussion
April 9th, 2019
Spring Conditions
The snowpack and weather have shifted solidly to spring-like conditions. A major warm-up started in mid-March with a prolonged period of strong sun and warm temperatures. This created a major difference between the snowpack on sunny slopes and that on shaded aspects. More recently, warm, wet, and sometimes weak spring storms have brought more rain than snow. The bulk of the precipitation with these storms focused on the southern forecast zones. Even so, mid-elevation rain established a dramatic snow line (about 4-4,500ft) below which the snowpack is minimal to non-existent in most zones. Going into the second week in April, intense snow and wind drove a prolonged period of High danger at Mt Hood.
A crown of a very large avalanche (D3+) above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.
Challenging Weather Forecasts
The Cascades have been experiencing unsettled spring weather with rain to many low and mid-elevation slopes and snow at upper elevations. Spring weather forecasts in the Cascades are notoriously challenging. With these storms, the weather models have been inconsistent and the accuracy has been limited to 12-24 hours, at best. A trend has been significant precipitation amounts for the Mount Hood area and other south-central Cascade volcanoes.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
Shrinking Snowpack
From the peak height of snow in mid to late February through early April, mountain weather stations in the 4,000-5,000ft range showed an average of 27% decrease in height of snow. The percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. If you’re 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. On upper elevation shaded slopes there’s still potential for large wet slab avalanches with prolonged warm temperatures or high elevation rain events.
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:
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Can you trigger avalanches due to new snow?
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If so, would they be storm slabs or wind slabs? And where?
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Can you trigger avalanches due to warming or rain?
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Will recent snow be warmed enough to result in loose wet avalanches?
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Will these avalanches be predictable point releases or more destructive wet slabs or gouging loose wet avalanches?
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What are the recent high and low temperatures and the forecasted temperatures during the time you’ll be in the mountains?
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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?
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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 has created 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.
The last daily avalanche forecast for all zones will be issued for April 14th. Statewide mountain weather forecast and weekly avalanche condition advisories will continue through May. The weather station data is available year round. Keep checking the advisories and help us out by submitting observations when you are in the mountains.
Glide avalanches and holes opening up in rocky terrain on an east aspect of Mount Herman. 4/3/19 Photo: Andrew Kiefer
Problems
Wind Slabs
A round of colder snowfall paired with strong southwesterly winds will build fresh wind slabs and thicken existing slabs in leeward terrain near and above treeline. Expect changing conditions with elevation and as more snow accumulates by Wednesday afternoon. Carefully assess the new/old snow interface and use caution around older stiffer slabs that formed during the weekend storms. Pay attention as you transition into wind affected terrain. Use visual clues such as snow drifts, uneven snow surfaces, and firm hollow sounding snow to help you identify and steer around large wind-loaded slopes steeper than 35 degrees.
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: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
New snow could lose strength and become wet throughout the day Wednesday. Rain will saturate our soggy, disappearing snowpack below treeline. Loose wet avalanches could occur within new snow or even gouge into deeper layers. Rollerballs and pinwheels are good signs that loose wet avalanches may follow. Even small loose wet avalanches can have severe consequences if they carry you into terrain traps like gullies or over cliffs. Use caution near slopes 40 degrees and steeper if the snow surface becomes wet and sticky.
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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 10th, 2019 11:00AM
