Season Snowpack Summary

Season Snowpack Summary (Live)

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November - December 11, 2018

December 12-19, 2018

December 20-31, 2018

January 1-13, 2019

November - December 11, 2018

Snow arrived to mountain locations in early October. While servicing a weather station at 9,200’ near Galena Summit on October 25th, SAC forecasters observed ~4” of compact melt-freeze layers and facets in shaded areas while sunnier slopes remained clear of snow. Since then, our snowpack has evolved through three notable snow events highlighted in Figure 1 below.

 

Figure 1: Snow depth and air temperature data from a representative Snotel site near Galena

Pass in the Smoky Mountains. The red highlighted areas show notable snow events, and the

labels correspond to the current character of these layers from recent observations.

Image 1: Shallow snow cover on east, and northeast aspects at middle and upper elevations

near Galena Summit. Photo taken on November 21st.

The Thanksgiving snow event (labeled “Soft slab” in Figure 1) spanned 2-days dropping anywhere from 6-16” of snow favoring the western Smoky Mountains, Galena Summit and the Sawtooths. Only a few avalanches were reported (see Image 2 below) but poor visibility during this prolonged event surely played a role. In the following two weeks, areas that harbored depth hoar near the ground have been extremely touchy and talkative. Observations from all forecast zones report poor snowpack test scores, widespread cracking and whumpfs that propagated up to one-hundred feet and shook snow out of trees.

Image 2: This slab avalanche was observed at ~9,600' on a ENE aspect of Titus Ridge near

Galena Summit. Photo: G. Tsuda

Image 3: This snow pit shows the early season melt-freeze layer at the base of the snowpack

and the layer of depth hoar responsible for widespread collapses and shooting cracks.

Image 4: A heavily faceted snowpack capped with 1-2 cm surface hoar. Photo taken in the

Soldier Mountains on December 7th.

The third and final snow event (labeled “Facets/Surface Hoar” in Figure 1) lasted 6-days, began on November 27th and brought an additional 6-18” of snow. As clouds cleared on December 2nd we entered a 5-day cold snap that grew surface hoar (Image 4) and heavily faceted the snow surface.

By December 11th, we’re left with a heavily faceted snowpack with a layer of depth hoar near the ground on many shaded middle and upper elevation slopes and a potentially weak interface of facets/surface hoar waiting to create a new persistent slab problem at the snow surface.

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December 12-19, 2018

This period began with a small storm that dropped 4-8” of new snow, burying the December 11th facets and sporadic surface hoar that topped the snowpack prior to the weather event. This small load, combined with wind, was sufficient to produce some natural avalanches - including several that stepped down to the depth hoar near the ground (Image 1) and some that ran in layers closer to the surface.

Image 1: Avalanche on Copper Mountain that ran on depth hoar near the ground. Photo taken on December 13th.

In upper elevation, shaded terrain, the depth hoar near the ground continued to show impressive signs of reactivity for several days following the December 11th storm. On December 15th, SAC forecasters in Beaver Creek received a large, rumbling collapse that propagated cracks at least 300 feet away (Image 2). Despite the obvious signs of instability, no human-triggered avalanches were reported.

Image 2: SAC forecasters collapsed the snowpack at the area labeled “collapse.” Cracks were observed over 300 feet from the collapse location. The angle of the slope was right around 30 degrees; in this instance, not quite steep enough to avalanche.

The December 11th storm was still insufficient to open up much terrain for backcountry recreation. The snowpack remained thin and we continued to publish bi-weekly General Snow and Weather Information updates as we awaited more snowfall. On the 15th, the weather forecast became more promising, and in anticipation of the expected snowfall, we began producing daily Avalanche Forecasts on December 18th, with the danger starting out at HIGH.

Two pulses of moisture impacted our area from December 17th through the 19th (Figure 1). During the 48-hour period, most of our forecast area picked up 8-14 inches of new snow - a decent amount of weight to add to our fragile snowpack with faceted weak layers near the ground and about 6 inches below the surface.

Figure 1: Snowfall at the Lower Titus weather station on Galena summit. The December 11th storm occurred early in the period, with the December 17-19 storm arriving towards the end.

Not surprisingly, the storm produced some significant natural avalanche activity. What was somewhat surprising, though, were the nature and distribution of the observed slides. On the one hand, in our deeper snowpack areas to the north where we were the most concerned about the depth hoar at the ground, we noted few natural avalanches. However, we observed a number of medium to large-sized avalanches in areas with a shallower snowpack - places that were nearly dry prior to the Thanksgiving snowfall. A very large (and fascinating!) avalanche occurred on Galena Peak at midday on December 19th (Images 3 and 4). While only 1-2 feet thick on average, the crown line extended for more than 4000 feet as it wrapped around the west bowl. It was truly impressive to see such a shallow, variable, and seemingly disconnected snowpack propagate an avalanche fracture for such a tremendous distance.

Image 3: A natural avalanche released in the west bowl of Galena Peak on December 19th. The crown line was over 4000 feet wide.

Image 4: Debris tongue from Galena Peak avalanche.

As we await our next snowfall, we are left with a complex snowpack with two very weak, well-developed facet layers found near the ground and mid-pack. But this much is clear: we are dealing with a dangerous snowpack that will likely get worse before it gets better.

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December 20-31, 2018

 We entered the final stretch of 2018 with some decent storms forecast to hit us but most of the moisture dodged us to either the north or the south. A small storm early in the morning of December 21st brought 2-6” of snow, favoring the Wood River Valley and Smoky/Boulder forecast zones. During the next 8 days, two minor storms brought a few more inches of snow. Check out the graphs below which show snow depth and precipitation at our Lower Titus weather station (Figure 1).

Figure 1: Snowfall and snow depth at our Lower Titus weather station, located at 9,200’ near Galena Summit.

In spite of the lack of precipitation, the snowpack showed obvious signs of instability during this period. Widespread cracking and collapsing of the snow was reported by many different observers traveling throughout our forecast zones. A bit of new snow and wind on the 21st was enough to set off another cycle of activity, including the avalanches shown below (Figure 2).  

Figure 2: Avalanche activity in the North Fork of the Big Wood River, Wood River Valley Zone. Photo taken on 12/21/2018.

In the final week of 2018, reports of cracking and collapsing diminished as the weak snowpack ever so slowly adjusted to its load. However, thoughts of stability were dashed with the arrival of the final storm of the year, starting late in the evening of the 29th. This storm delivered 3-12” of snow to the area, heavily favoring the northern Sawtooths and Banner Summit areas. This new snow produced an additional round of avalanche activity, including dozens of avalanches large enough to bury a person (Figure 3).  

Figure 3: This large, persistent slab avalanche occurred at the end of the last storm of the season, likely during the day on December 30th. This slope is near the head of Cabin Creek in the southern portion of the Sawtooth advisory zone on a SE facing slope at 9,500’. It appears to have started as a wind slab and then step down to a deeper weak layer in the snowpack.

The theme of the season thus far has been small storms and big avalanches - not a great sign. Looking forward into 2019 it appears that we’re set up for more of the same. Bitterly cold temperatures and clear skies dominated the final day of 2018 and the first day of 2019. These conditions are rapidly weakening the snow surface and will likely produce more avalanche activity once buried and loaded.  With depth hoar on the ground, facets, crusts, and surface hoar in the middle of the pack, and a weakening snow surface, we may be in for a wild ride when we get our next big storm.

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January 1-13, 2019

After the loading event at the end of 2018, the first days of the New Year brought dry, settled weather that helped to stabilize the snowpack. By January 5th, the avalanche danger decreased to MODERATE at upper elevations and dropped to LOW elsewhere. During this dry spell, we began to see a divergence of snowpack conditions across the forecast area. Where the snow depth remained shallow - less than about three feet - the entire snowpack weakened and faceted. This was observed on Galena Summit, but was especially pronounced further south towards Baker Creek and into the Wood River Valley Zone. Meanwhile, in the western Smokys, Salmon Headwaters, and Sawtooths, the deeper snowpack retained strength - although the faceted snow near the ground remained a concern. Across our area, a new crop of facets and surface hoar formed at the snowpack surface.

Figure 1: Snowfall at the Lower Titus weather station from January 1-13. The early part of the new year was dry, but the biggest storm system of the season began on January 5th and lasted through the 7th. Lower Titus reported 19 inches of snowfall during this period amounting to 1.4 inches of SWE.

Two back-to-back storm systems arrived on January 5th and provided 48 hours of snowfall (Figure 1). The first round targeted Galena Summit and the Salmon Headwaters, but our entire area received a decent shot of snowfall. The second pulse again hit the Galena Summit/Salmon Headwaters area but also delivered significant snowfall to the Soldiers and Banner Summit. All told, the forecast area received 12-24 inches of snowfall amounting to 1-2 inches of SWE (see storm total sheet). The two weather systems were accompanied by moderate to strong winds blowing from nearly all corners of the compass. Although not “huge” by any stretch, in a season of below-average snowfall, it was our biggest storm cycle to date.

Given the complex but widespread pattern of weak layers across our area, we issued an avalanche warning on the morning of January 7th. There were no shortage of natural avalanches (read observation) - large persistent slabs were observed in the all of our forecast zones. A few had crowns over a quarter mile wide (Figure 2).  Numerous smaller wind slab avalanches occurred as well.

Figure 2: Avalanche activity associated with the January 7th Avalanche Warning. Clockwise from upper left: Gladiator Peak in the Boulders ran nearly full path with extensive propagation; a persistent slab avalanche in sheltered, mid-elevation terrain in Coyote Creek in the Smokys; a large avalanche in Grand Prize in the Boulders clearly broke down into deeper weak layers; fewer persistent slab avalanches occurred in the Sawtooths, but this one released on Mount Cramer on a rocky, shallow, wind-loaded slope.

A few more inches of new snow accompanied by strong southerly winds arrived on January 9th. The next day, local ski guides doing recon on Butterfield near Baker Creek remotely triggered the east bowl (Figure 3). The crown was about 80 feet wide and 1.5-3 feet in depth and released on a 37-38 degree, wind-loaded slope. The avalanche likely broke on facets and depth hoar near the ground. Prior to the January 5th storm, a forecaster in the vicinity reported the snowpack had weakened considerably throughout its 2-foot depth and was barely supportable on skis.

Figure 3: Persistent slab avalanche on Butterfield, January 10th. The slide was remotely triggered from lower angled terrain above the crown. The avalanche failed on faceted snow near the ground.

Other observations during this time period also pointed to dangerous conditions in certain portions of our area. The greatest concentration of obs reporting instability came from the vicinity of Baker Creek, where on January 12th a snowmobiler at Baker Lake compared a collapse to a faint sonic boom!

High pressure arrived on January 11th and strong temperature inversions brought balmy temps to the upper elevations. Wet loose activity was added to the avalanche problem list, and the likelihood of persistent slabs declined. The forecast continues to warn of the possibility of triggering a persistent slab on very steep, wind-affected slopes where the snowpack remains less than about 3 feet deep.

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