Last week's dangerous and reactive wind slabs now appear to have gained some stability by bonding to the snowpack below. However recently deposited windslabs may be reactive to a skier's or boarder's weight, especially with warning temperatures later in the work week. Natural and human triggered avalanches seem unlikely, however pockets of sensitive windslabs may exist. Shaded north aspects may preserve weak layers and persistent slabs.
No new natural or human triggered avalanches have been reported since Humphrey's Cirque and Snowslide Canyon avalanches, discussed in last week's snowpack summary.
The Snowslide Avalanche took out 10" diameter trees. This would have been an unsurvivable avalanche.
Unseasonably warm daytime temperatures have contributed to snowpack bonding, strengthening and densification. Snow from the storm cycle that produced 93" has settled and sublimated reducing its volume and mass by 10% or much more in some places. On some north aspects, much of the new snow was stripped by winds.
Across the San Francisco Peaks, widely variable windslab and stormslab layers sit atop decomposing facets and hard rain event crusts...Southwest facing upper elevation terrain may have a reactive wind slab from last Friday- Sunday's extended northeast wind loading event.
Southwest and west winds of 15-25 mph over the last 48 hours have transported the meager remaining snow above treeline. Last weekend's strong northerly and northeast winds of sustained 40-50 mph transported large amounts of snow, loading southwest aspects with a dense surface wind slab above treeline and in exposed areas. This hard slab was reactive on a southwest aspect at 11,400' (ECTP11Q1; CT6Q2). more recent warming has left little powder snow available for transport above treeline.
Watch for relatively shallow and recently deposited wind slab on northeast, east, and southeast facing aspects. As in the case with snow falling out of the sky during a storm, wind blown snow also needs time to bond with the snowpack below.
A delayed report was received from riders who triggered a loose snow, point-release avalanche on a steep north facing cinder cone slope, below 8000 ft. This very small avalanche occurred on January 25th.
Near and Above Treeline:Following strong NE winds last weekend, calming and warming weather prevailed throughout much of the workweek. Wind out of the west picked up on Thursday and Friday transporting snow and potentially depositing pockets of new unstable wind slabs.
Stability tests conducted in upper elevation starting zones show strong thick slabs with low to moderate reactivity on aspects besides the loaded southwest above treeline terrain. This wind slab is perched on top of a highly variable lower snowpack structure. Presumably the strong slab (four finger to one finger hardness) from the trifecta storm event occurring January 20th-23rd now have the strength to bridge old weak layers below. Propagation tests have revealed low fracture probation likelihood. The snowpack structure below the trifecta slabs is highly variable, ranging from preserved crusts and facets deep in the snowpack to homogeneous wind slab (top to bottom) and "sastrugi" (patterned snow cover where wind has eroded most of it). In areas where the snowpack is thick, the conundrum is, how strong is the slab? Although the likelihood of a skier or boarder triggering an avalanche seem low, the consequences if it were to slide might be very high, given the high volume and mass potentially entrained.
These slab layers have recently been affected by wind and warming. The weather forecast shows potential daytime temps into the 40's (°F), which can destabilize slabs. So as always, approach >35° terrain and loaded slopes with caution and assess for instability based on current conditions and actively changing conditions, to help guide your choices. Please review the snowpack and avalanche observations forum for other recent observations - find it under in the snowpack menu. You too can contribute to our BC community!!!
Below Treeline:The powder below treeline on shady and wind protected aspects remains good. Melt/freeze crusts have developed on sunny and radiation affected open southerly slopes at lower elevations. Very warm temperatures mean that it is time to look for wet snow signs such as snow rollers, and wet loose snow sluffing. Steep south facing slopes and south facing aspects of low elevation gullies will be the first to show signs of rapid thaw instability. If the snow under your feet becomes sloppy, move onto cooler or flatter terrain.
Coverage is still acceptable down to 7000 ft. but changing rapidly. The soft fun snow below 8000 ft may become melted and gone with the predicted warming trends.
The good news is that not much snow remains available to move - it was stripped by last week's strong northerly winds, or locked up in melt/freeze crusts created by solar input. The storm predicted for Monday/Tuesday (Feb. 6/8) likely will not add significant new snow. But if it does, the the windslab problem may become more likely. Watch for poorly bonded windslabs that may become more sensitive during the warming cycle beginning mid-week.
Always keep in mind, wind slabs are unpredictable, and may support the weight of a skier or rider initially, and fail suddenly with tragic consequences. Avoid snow surfaces which are recently loaded, sound hollow, and look for signs of fracturing, cracking, or whoompfing sounds.
As the temperatures start warming, watch for slushy/sloppy cohensionless snow turning into loose wet slides, especially on south aspects. If things get really warm, then loose wet slides may become more destructive wet slab avalanches. Wet slides may be possible later in the week, primarily on southerly, sunny slopes below treeline.
Numerous rescues were conducted last week by the Coconino County Search and Rescue, and the Arizona Snowbowl Ski Patrol. Some of these could have been avoided by better planning and preparation.
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.
Arizona Snowbowl uphill policy.
Want to learn more safe backcountry habits? KPAC offers level I and II avalanche courses. They are filling up fast!!!
During winter, backcountry permits are required to access the Kachina Peaks Wilderness. More info
What difference a week can make. In the aftermath of last weekend's 40-60 mph northeasterly winds, high pressure conditions with temperatures significantly above normal characterized the workweek. High clouds and breezy westerly winds on Thursday and Friday have the effect of moderating midday surface melting, and also enhancing sublimation (the loss of snow mass back into the atmosphere). Cloud cover at night on Thursday and Friday will reduce energy loss to the atmosphere keeping temperature from dropping much below freezing. Breezy conditions and possibly a few snow flurries will occur as a modest storm passes to our north. The best chances of any precipitation will be on Monday/Tuesday, but accumulation is expected to be light.
Weather station information:
On the morning of Friday February 3rd the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 76 inches (193 cm) at 9700’, and Arizona Snowbowl reported 86 inches ( 219 cm) at 10,800’. Since January 27th, SNOTEL temperatures ranged between -1° and 48° F, and Agassiz station between 12 and 42° F.