Avalanche Advisory
Advisory: Provo Area Mountains Issued by Mark Staples for Saturday - March 17, 2018 - 7:40am
bottom line

Today the avalanche danger is CONSIDERABLE at upper elevations. Layers of weak facets lurking in the snowpack 2-5 feet deep can produce large avalanches today. Also, with snow and strong southerly winds, many fresh wind slabs will be found that will be easy to trigger. Safer conditions exist on southerly facing slopes without wind drifted snow. In these places assess bonding and stability within the new snow layers.
special announcement

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current conditions

Overnight the Provo area mountains received a trace to 2 inches of snow. More fell south of Provo Canyon. Just a little north the upper Cottonwoods received 7 inches of snow, and I suspect upper areas in American Fork Canyon may have received similar amounts. Temperatures this morning are in the mid to upper 20's F at ridgetops and near 32 degrees at many trailhead locations. Winds increased since yesterday and are blowing 10-20 mph gusting 30-50 mph from the S and SW at ridgetops. At lower elevations winds are averaging about 5 mph but gusting 15-30 mph.

When assessing avalanche danger, we look at snow water equivalent (SWE) not snow amounts because it tells us how much weight was added to the snowpack. Buried weak layers don’t care how much snow or rain fell, only how much weight was added. Total water amounts since Wednesday morning are:
recent activity

Soft wind slabs were triggered by both backcountry skiers and ski patrols. Many were small; however one much larger one occurred in Cardiff Fork under Cardiac Ridge that was about 200 feet wide. Photo of the Cardiff slide below (M. White).

One deep slab was triggered by my partner yesterday on a north facing slope at 10,000 feet. This slide broke on a layer of weak facets just under the dust layer about 2-3 feet deep. While deep, this was a small slide because it was a relatively small slope. It broke in a rocky area which is where facets were weaker and the snowpack was shallower.
Avalanche Problem 1
type aspect/elevation characteristics
LIKELIHOOD
LIKELY
UNLIKELY
SIZE
LARGE
SMALL
TREND
INCREASING DANGER
SAME
DECREASING DANGER
over the next 24 hours
description

With more snow and strong S and SW winds this morning, expect to see more wind slabs that can be easily triggered today. They should be larger and much more widespread than they were yesterday. Look for fresh deposits of wind-blown snow from top-loading just under ridgetops and from cross loading along gullies. See images below of top and cross loading.

Also with more snow falling today, watch for instabilities within the new snow even on slopes without wind-blown snow. Look for cracking in the new snow. Test bonding of the new snow on small slopes where the consequences of triggering an avalanche are small.
Avalanche Problem 2
type aspect/elevation characteristics
LIKELIHOOD
LIKELY
UNLIKELY
SIZE
LARGE
SMALL
TREND
INCREASING DANGER
SAME
DECREASING DANGER
over the next 24 hours
description

Yesterday, my partner triggered a deep slab avalanche on a small slope. Even though the avalanche broke 2-3 feet deep, it was relatively small because the slope was small. Bigger slopes can produce bigger avalanches (video). Also, it was triggered in a rocky area which is typically a good place to trigger these types of avalanches. Buried rocks make good trigger points, but you don’t always know where they exist. See the rock in my snowpit wall in the photo below as a good example.

Here’s what we know about this avalanche problem

  • Layers of weak faceted snow exist near the ground and just under an obvious dust layer.
  • These layers are buried 2-5 feet deep with a strong and supportable slab on top of them.
  • With continued loading since Wednesday morning, these faceted layers are stressed by the weight of new snow and more likely to fracture and produce a large avalanche.
  • This avalanche problem mostly exists at upper elevations on N, NE and E aspects. It is lurking on adjacent aspects but is not as common. South facing slopes do not have this problem.
weather

A cold front in Nevada this morning is approaching northern Utah. It should bring an additional 5-9 inches of snow which should fall mostly this morning. Light snow showers should continue through the afternoon. Strong south winds will continue until his front passes, and they should start to diminish this afternoon. Temperatures shouldn’t change much from where they are this morning. They may warm a bit at lower elevations with normal daytime warming but could drop some at higher elevations once the cold front arrives.
general announcements

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This information does not apply to developed ski areas or highways where avalanche control is normally done. This advisory is from the U.S.D.A. Forest Service, which is solely responsible for its content. This advisory describes general avalanche conditions and local variations always occur.