Winter is back, with our first measurable snowfall since February 22. The likelihood of natural and human triggered slab avalanches has increased. Stability tests yesterday and today at treeline reveal increased reactivity, poor structure featuring a growing storm slab on top, and low strength at the new snow/old snow interface.
From Tuesday (Mar. 10) through Thursday morning, the peaks received about 1.5 to 2" of snow water equivalent (SWE), primarily in the form of wet, dense snow above 9,500 ft. Rain levels were highly variable this week, with saturation of the snowpack occurring below @10,000', resulting in sometimes unsupportable travel conditions.
By Friday morning, temperatures dropped and Snowslide SNOTEL reports ~6" of snow accumulation as of 5 am, and 12" + reported at treeline, 11,500'. Moderate southwest winds accompanied the storm, with more snow and wind forecast through Saturday morning.
Prior to this week's precipitation, skiing was highly variable and snow travel was likely to require crampons and an ice axe. These tools may still be needed.
This week has been somewhat similar to our Feb. 22/23 storm, coming in cool, then warm, then cold again. That system caused at least two D1.5-D2 sized avalanches on ~northerly slopes. This recent low pressure system is larger and slower. No new avalanches have been reported since the February 22 storm.
Click Read More for a few things to be aware of this weekend and into next week.
- 90% of human triggered avalanches happen during or within 24 hours of snowfall. Waiting 24 hours before traveling on or under steep (>30°) slopes will decrease your likelihood of triggering an avalanche
- Venture into this new snow with an "assessment" mindset by selecting conservative terrain in which to gather information
- Check the bonding and reactivity of new storm and wind slabs
- Look for signs of recent avalanches
- Watch for instabilities like cracks shooting out from your skis or board as you skin or ride in fresh snow
- Listen for collapses (whomping) underfoot
- Sunny/warm weather on Sunday and Monday may destabilize new slabs
- Make good decisions upon your observations
- The extended forecast suggest the possibility of more unsettled weather next week - keep your guard elevated.
Near and Above Treeline:On Thursday, it was noted that the new wet snow was struggling to bond to the old snow surface, at treeline on a southerly aspect.
12" + has fallen since early this morning at treeline, with poor bonding of the storm snow with the old snow surface. Assess conditions based on slope size and consequences...stability tests also revealed reactivity of the near surface facet sandwiches in the snowpack.
There's enough new powder snow and southwesterly winds to build wind slabs. Approach ridges and leeward terrain with caution. Wind can deposit snow 10 times more rapidly than snow falling from the sky multiplying the load on the snowpack. Be suspicious of any steep slope with recent deposits of wind drifted snow.
Near treeline, new snow has accumulated on a mix of surfaces: sastrugi, melt/freeze crusts, rain crusts, and rocks. Ice axe and crampons are recommended for travel within the Kachina Peaks Wilderness. Some windward areas of the peaks were stripped of snow during the February high-wind events, while other areas have retained a 1-2 meter snow depth.
Below Treeline:New snow will hide previously exposed rocks and logs. Coverage has improved, but our mid-winter drought made egress from the backcountry more difficult as many standard routes are a bit burned out, requiring more hiking. Plan accordingly.
You can reduce your risk from storm slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Open slopes and the large bowls above treeline deserve vigorous assessment and respect of the higher consequences of an avalanche. Debris piles could be large with significant destructive power due to the dense, wet snow this week.
There's enough new powder snow and southwesterly winds to build wind slabs. Winds are forecast to remain southwesterly through the weekend.
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 are usually confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas.
Join KPAC and crew for the Annual Mikee Linville Fundraiser event at Agassiz Lodge, Arizona Snowbowl, on Saturday March 28, 12-5 pm. Huge raffle of amazing prizes, including an unlimited Snowbowl Power Pass! This is our main education fundraiser and we hope to recoup the $8400 in scholarships awarded this winter...Thank you for your support and hope to see you there!
Always carry the 10 essentials and avalanche rescue gear for wintertime wilderness travel. Submit your observations here. You may save a life!
For information on uphill travel within the Arizona Snowbowl ski area, please refer to www.flagstaffuphill.com and https://www.snowbowl.ski/the-mountain/uphill-access/ for details.
Weather updated Friday March 13
The weather over the past week has been a complex interaction between a cutoff low pressure cell and its influence over the tropical Jet stream. On Tuesday and Wednesday the pressure system meandered off the coast of southern California, drawing moist warm air from the equatorial Pacific Ocean.
Plumes of warm saturated air and low-lying clouds streamed across northern and central Arizona bringing 1-2 inches of rain to elevations below 8500- 10,000 feet. Mid elevations received either rain, or very dense snow throughout the week, with the freezing altitude varying by day. Eight inches of high density snow fell at 10,800’ on the Peaks early in the storm.
After a brief and intermittent reprieve on Wednesday afternoon and evening, a resumption of precipitation and cooler temperatures are predicted on late Thursday and Friday as the trough finally passes through. Cooler air will lower the snowline to about 7000 feet. Twelve to sixteen inches of new snow are possible by the storm's end. Treeline temperatures have been in the mid 20s to mid 30s °F throughout the last week.
Relatively benign weather will characterize the weekend, but model guidance shows another upper level low developing in the Great Basin. This will bring wind, cool temperatures and possibilities for more precipitation early in the workweek.
Arizona Snowbowl reported a 64" ( 163cm) base at 10,800 feet. Snowslide SNOTEL is reporting a questionable depth of 51" (130cm) at 9,730 feet. So far this winter, 182" (462 cm) of snowfall has been reported at 10,800 feet.
Since Friday March 6th, SNOTEL temperatures have ranged between 49°F on March 12th and 24°F on March 9th. ASBTP station (11,555 ft) reported a low of 15°F on March 9th and a high of 40°F on March 6th.