Avalanche hazard has increased over the last 24 hours with the addition of at least 2.7 inches of snow water equivalent (SWE). St. Valentine delivered in the form of rain at and below 11,000', and high density snow above. This is an exceptional load on the snowpack and may overburden the weak faceted layer buried since December. This is the same layer that failed last week naturally in Snowslide Canyon due to wind loading, resulting in a significant avalanche.
Fortunately the snowline fell to below 9500' as the cold front passed through Thursday evening.
The storm cycle is forecast to continue. The potential for natural and human triggered avalanches will be likely on slopes exceeding 30 degrees. Problems will be new wind slabs, new storm slabs and persistent slabs. The addition of new snow may transition our persistent slab problem into more of a deep persistent slab problem as slab thickness exceeds the threshold of one meter.
For the near future, travel in avalanche terrain near and above treeline may be hazardous. Give the new high density snow a chance to bond with the snowpack below, by waiting it at least 48 hours, and moderating slope angles to near or below 30 degrees. Avoid wind loaded areas and test each slope for signs of instability.
Read more about rain on snow and the possibility of near surface facets.
Rain on snow may saturate the snowpack, percolating down to a weak layer and decrease cohesion between snow layers. Also, rain will increase the temperature of the surface, creating a temperature gradient suitable to produce near surface facets (NSF) when new snow does accumulate. NSF may prevent new storm/wind slabs from bonding well to the existing snow.
There was a crown line about 1/2 way down Jay's Slide - a NNW aspect on Fremont. Picture from Sat. Feb 9, 2019. This avalanche likely occurred on Tuesday or Wednesday, February 5 or 6. Estimated classification is U N R2D2 U.
Near and Above Treeline:Watch for new wind slab development on northeasterly and easterly aspects. Winds have blown from the southwest and are forecasted to blow southwest and west over the weekend. Cornices are starting to form on the lee side of ridgelines. Avoid cornices, as they may collapse unpredictably and send you downslope.
Persistent strong winds have characterized the last two weeks. Most recently, intense north wind up to 65 mph scoured windward terrain above treeline on Monday, February 11. Many southwest, west and northwest slopes are scoured above treeline.
Watch for reactive persistent weak layers. These may be buried under 2 to 5 feet of slab on east, northeast, north, and northwest aspects. Southeast aspects may be suspect as well, especially on slopes connected to easterly aspects.
Prior to this week's precipitation, wind scoured zones reduced the snowpack down to bedrocks in some areas. Crampons and ice axe will be helpful where new snow has been packed into hard wind board. You may find this on southwest and west slopes. Perhaps others.
Below Treeline:Persistent slab instabilities exist in our snowpack. A reactive, faceted weak layer exists about 30 to 50cm from the ground, primarily on east, northeast, north and northwest slopes.
Recent avalanche activity indicated that the buried December facets can be reactive with enough load. This weak layer will be re-energized by the addition of new dense snow and rain from the St. Valentine storm. Carefully assess run out zones and terrain traps.
You will find great skiing and riding on low avalanche hazard slopes if you stay off of and out from under slopes that are greater than 30° in steepness.
Watch for new wind slab development on northeasterly and easterly aspects. Winds are forecasted to blow southwest and west over the weekend. Cornices are starting to form on the lee side of ridgelines. Avoid cornices, as they may collapse unpredictably and send you downslope.
Post weekend winds may create slabs on other aspects. As always watch for cross loading in gullies. Slopes just below ridges and on the flanks of shoulders should be considered suspect.
New storm slabs and wind slabs will be your primary problem over the next week.
Keep an eye on the ASBTP weather station. Readings between 15 and 35 mph indicate the potential for snow transport and formation of wind slabs. Look for various links under the weather menu above. Note that this station may get rimed during storms and report erroneously.
Give new snow time to bond with the old snow. Waiting at least 24 hours after new snow, and 48 hours after wind events will decrease your chances of finding unstable storm/wind slabs.
Weak faceted snow-layers under old and new wind/storm slabs exist at elevations above 10,000'. Column test in Beard Canyon on January 11th and near Alison Clay Bowl on February 4th indicating persistent slab problems. The Snowslide Canyon avalanche was triggered by wind loaded snow falilng on a persistent slab.
This problem has been found below, near and above treeline on northwest, north, northeast and east aspects. Reactive stability-test failures have occurred near and below treeline. The Snowslide Canyon avalanche crownline was above treeline approximately 12,000'
This problem is not widespread and predicting its location has been problematic.
The addition of new snow may transition our persistent slab problem into more of a deep persistent slab problem as slab thickness exceeds the threshold of one meter. Hopefully we will find evidence that this problem is healing and we can remove from future updates.
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Backcountry permits are required for travel in the Kachina Peaks Wilderness and available at local USFS locations, as well as, at the Agassiz Lodge on Saturday and Sunday 8:30 -11:30.
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. Access to the Kachina Peaks Wilderness is available from the lower lots at Snowbowl via the Humphreys Trail and Kachina Trail.
Weather updated February 15th.
Northern Arizona continues to be in a favorable flow pattern, with short wave troughs bringing modest amounts of precipitation followed by our customary snow stripping winds out of the north. On February 11th, Arizona Snowbowl (10,800') reported 9" of new snow. This was followed by nuking winds out of the north northwest 30-40 mph and gusting to above 60 mph. On windward aspects above treeline, the snowpack was scoured down to boulders.
At the time publication, we are in a pause between a series of disturbances. The formidable and warm St. Valentine storm has passed and the wind kicked up on Friday, which was evident from ridge top snow plumes. Thursday’s warm and wet Valentine storm resulted from a cold front tapping into significant subtropical moisture, resulting in high elevation rain and some very wet snow above 10 ,500 feet.
Our current storm cycle is expected to relax in intensity over the weekend, as colder post-frontal temperatures arrive. Lingering snowfall will persist on President's Day across much of northern Arizona. The snowline is expected to drop to as low as 3500 feet, with snow showers continuing into the workweek, but with little additional accumulation anticipated. On the distant horizon, prediction models are still too far out for precise determinations, but storm patterns appear to be setting up for more vigorous storm activity later in the upcoming week.
On Friday morning, Feb. 15th, the Inner Basin SNOTEL site (Snowslide) reported a snow depth of 53” (135 cm) at 9,730'. Arizona Snowbowl reported a settled base of 72” (183 cm) at 10,800'. So far this winter, 188" (477 cm) of snow have fallen at the mid-mountain study site. Since February 8th, SNOTEL temperatures have ranged between 5° F on February 12th, and 40° F on Feb. 12th. For the same period, ASBTP (11,555') reported temperatures between -4° F on Feb. 11th, and 34° F on Feb. 12th.