After a short wave disturbance and cooler weather, natural and human triggered wet avalanche activity will be possible as warm weather returns by Tuesday, April 11th and later into the week.
At approximately noon on Wednesday, April 5th, a wet loose point release on a SE aspect in Humphrey's Cirque was triggered above treeline (see photo). With forecasted warming later in the work week, these wet avalanches may increase in probability.
New snow from last weeks storms have bonded well to lower layers. Note: dry slab avalanche debris was observed below the north aspects of North Core Ridge on 4/4/2017.
Near and Above Treeline:Crampons and ice axes may increase security on steep hard snow and ice that may be encountered at high elevations - especially during cooler weather or cold mornings. Mixed conditions exist above treeline, ski with caution. Warm weather returns after the weekend, and human triggered wet-avalanches will become possible.
Below Treeline:Natural and human triggered avalanches are unlikely other than small wet loose avalanches after warming. Unstable wet snow may develop on isolated terrain features.
Southerly and westerly aspects below 10,000' are quickly losing snow coverage as springtime warming continues. Rocks and logs are emerging just under the surface and may be hidden. As temperatures are still dropping below freezing, north, north east, and north west aspects are retaining snow coverage.
Warm temperatures return by midweek. Deep slush, snowballing, pinwheels, and small wet slides indicate the potential for larger wet loose avalanches or wet slab releases. Refer to the Agassiz Weather Station which is near treeline at 11,500' for temperature trends. If overnight temperatures do not drop below freezing then wet avalanche hazard will increase near treeline.
Quick warming after new snow can cause wet avalanches. Rain on snow can cause wet avalanches as well. Careful terrain choice relative to elevation and aspect, along with time of day will help in wet snow avalanche evaluation.
On Thursday, April 6th, collapsing and propagating wet slab layers were observed in flat meadows of the inner basin near 9800'. The slab was 1 to 2" thick and the collapsing sounded like a spray of wet corn snow. This phenomena could be explained by the following conditions; an interaction between diurnal temperature swings, water percolation, and/or the late March dirty snow layers (that become warmer due to lowered albedo; the reflective properties of snow). When temperatures warm back up, this phenomena could exacerbate wet slide instability.
Old storm and wind slabs resting on harder melt freeze crusts will also be suspect as the temperature warms.
Liquid water from snowmelt or rain-on-snow, moves through the layers of the snowpack at different rates. Wet Slab avalanches happen when a weak layer or interface becomes moist, wet, or saturated. The wet snow loses strength resulting in an avalanche.
Travelers are advised to exercise caution, make slope specific evaluations, and most of all know where you are going and be prepared for the unexpected.
As always, please treat this summary with appropriately guarded skepticism, make your own assessments, and contribute to our body of knowledge by reporting your observations.
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Last updated on Friday, April 7, 2017
Breezy and stormy conditions dominated the weather early last week with an accumulation of 13" of new snow recorded at Snowbowl (10,800'). These storm cycles were followed by average and below average temperatures. Temperature at the Agassiz Station (11,500') barely exceeded the freezing point and wind gusts in the 30s and 40s (mph) were recorded throughout the week. Temperatures have climbed, just ahead of another cold, low pressure system forecasted to arrive on Saturday afternoon and evening. Windy conditions will precede the storm developing on Friday out of the south and southwest. New precipitation is expected on Saturday evening and night. Accumulation of new snow this coming week is forecasted to be light at treeline followed by several days of cold temperatures and northerly winds. High pressure is forecasted to build, accompanied by a breezy and gradual warming trend later in the week.
The Inner Basin SNOTEL site (Snowslide) reported a snow depth of 59 inches (150 cm) at 9700'. Arizona Snowbowl reported 93 inches (236 cm) at 10800'. Since March 31st, SNOTEL temperatures ranged between 15° and 58° F. Agassiz station seems to have malfunctioned on April 3rd, with no new data.
Season snowfall to date is 326" (8.28 meters) at AZ Snowbowl, 10800'.