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

January 14-31, 2019

February 1-20, 2019

February 21 - March 7, 2019

March 8 - April 4, 2019

April 5-14, 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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January 14-31, 2019

The later half of January was typified by elevated avalanche danger as well as natural and human-triggered avalanches. To understand the full picture of these eventful two weeks we’ll need to look back to early January when the storm track turned back toward Idaho. Between January 5th and January 22nd the forecast area picked up anywhere from 30-45” of snow and 3-5” of SWE. For most of the forecast zone this accounted for more than a third of the winter’s total precipitation and roughly doubled the snowpack in the shallower areas of the Wood River Valley.

Table 1: Snow and snow water equivalent values since January 6th. This cumulative load was

a key player in the slab avalanche cycle that occurred on January 20th.

On January 17th we issued HIGH danger and an avalanche warning in the Wood River Valley and Soldier Mountain zones. The warning covered the brunt of a 36-hour storm that brought 11” of snow (1.11” SWE) to Baldy and 17” of snow to the Lower Soldier weather station in the Soldier Mountains. The storm arrived from the south, which meant lots of precipitation for the Wood River Valley and southern Smokys and less to the north of Galena Pass.

Figure 1: Persistent slab avalanches that failed sometime around January 17th in the East

Fork Drainage of the Wood River Valley Zone.

A brief break in weather allowed Wood River Valley residents to dig out just before a second storm arrived from the west. This system brought a broader blast of warm moisture that proved to be too much for the deeply buried weak layers at the base of our snowpack. A widespread slab avalanche cycle began sometime in the afternoon of the 20th and continued into the morning hours of the 21st. Some of these slides were absolutely massive, leaving crown lines up to 20’ deep and thousands of feet across. See video here.

Figure 2: This impressive persistent slab avalanche was triggered on January 21st from a

nearby ridge. The slide failed on weak snow near the ground on a northeast-facing slope at

8,800' in the Northern Sawtooths. Photo: A. Wirth

Figure 3: These large avalanches occurred late on January 20th or early on the 21st. This

image was taken from Baldy and looks north into the western Smokys.

Figure 4: This deep slab avalanche released at 9200' on a northeast aspect at the head of

Meadow Creek in the northern Sawtooths. It broke 5-20 feet deep, 400 feet wide, and ran 1200

vertical feet. It failed on deeply buried faceted snow near the ground. T. Haskins photo.

Accompanying the natural avalanche activity was a string of six straight days with reports of human-triggered slides beginning on the 17th and ending on the 22nd. You can view details and pictures of all of these slides on the human-triggered avalanches page of our website. Most notable of the six was a slide that caught, carried and buried a skier and snowboarder in the Warm Springs Creek drainage of the Wood River Valley zone. The skier was fully buried but was able to dig himself out of the debris in approximately 25 minutes. The snowboarder was partially buried with his head and torso beneath the snow. He also extricated himself. They were riding in the "sidecountry" or "out-of-bounds" terrain on Bald Mountain, outside the ski area boundary. Neither rider was seriously injured. Click here to view a video summary at the crown of the avalanche.

Figure 5: A skier and snowboarder were caught, carried 100-150 yards, and buried in an

avalanche they triggered in the Warm Springs Creek drainage on Sunday, January 20th. They

were riding in the "sidecountry" or "out-of-bounds" terrain on Bald Mountain, outside the ski area

boundary.

The final week of January was quite mellow by comparison as a ridge of high-pressure moved overhead to bring fair weather and impressive temperature inversions. The final avalanche activity of this period occurred on the 26th as unseasonably warm temperatures, direct sun and calm wind at upper elevations peeled off a couple of interesting slab avalanches that demonstrated their ability to step down into weaker snow at the ground.

Figure 6: This slab avalanche released near Paradise Creek in the Western Smokys on a

southwest aspect at 9700 feet. Initially releasing as a shallow slab, it stepped down to deeper

weak layers near the ground as it ran into steeper, rockier terrain. Photo: Sun Valley Heli Ski

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February 1-20, 2019

Early February was a stretch not soon to be forgotten. A series of powerful winter storms gave new life to a below average mid-winter snowpack, pumping it up above seasonal norms (see Table 1). As you’d expect, such a dramatic increase in overall snow and water amount produced some impressive avalanches. There were 13 reported human-triggered slides during this period, several of which were triggered remotely. Three structures were impacted, including a slide that severely damaged a house near Ketchum. Several slides crossed roads in and near towns in the Wood River Valley, and avalanches near Hailey temporarily blocked the Big Wood River. The best way to get a handle on all the action is to break it up into periods: February 2nd-6th and February 8th-20th.

Jan 31st

Feb 19th

Banner Summit

71%

97%

Galena Summit

73%

103%

Chocolate Gulch

64%

104%

Soldier Ranger Station

92%

145%

Table 1: Snow water equivalent (percent of average, 30-year dataset).

February 2nd-6th

During this period, SAC staff rated the danger HIGH on three of the five days and issued an AVALANCHE WARNING for the Soldier and Wood River Valley Zones on February 5th. Avalanche activity was concentrated in the Wood River Valley and to a lesser extent, Galena Pass. The majority of the slides involved weak faceted snow near the ground. Remotely triggered slides and widespread collapsing were commonplace as shallower snowpacks struggled to bear the weight of the storm’s snowfall.

Following two feet of snow in three days near Galena Summit, a skier triggered a slab avalanche on a wind-loaded, north-facing, slope on February 3rd. The slide carried the skier a short distance and continued down the slope to knock over their partner as they fled the debris. On the same day just to the north, a skier remotely triggered a slab avalanche on a steep, northwest-facing, convex roll. Both avalanches failed on or stepped down to weak snow near the ground.

February 4th and 5th brought three more remotely-triggered avalanches, including a close call in the Baldy sidecountry (more here). In an awesome display of crack propagation, one 2-4’ thick persistent slab was triggered from nearly a half mile away (Figure 2). You can watch video footage of the aftermath of this slide by clicking here.

Figure 1: This remote skier-triggered slab on February 4th was the first of many avalanches to fail at the ground during the month of February. The slide broke 800’ wide and 2-8’ deep and sent refrigerator-sized blocks rumbling 2,000’ into the trees below. Photo: J. Preuss

Figure 2: On February 5th, a collapse on the ridge produced a crack that traveled 1,000' to the near side of the slide and a half mile in total releasing two separate avalanches. Both were ~700' wide and 2-4' deep. One stopped on a bench and the other ran ~900 vertical to the valley floor.

February 8th-20th

The 9-day storm beginning on February 8th was the biggest of the season (Table 2). February 8-13 saw unseasonably cold temperatures and low-density snowfall. Sustained moderate to heavy snowfall prompted 6 HIGH danger days. Four of these days were AVALANCHE WARNING days in the Soldiers and two were AVALANCHE WARNING days in the Wood River Valley. One relentless 24-hr period dropped 28” of snow and 1.3” of SWE at the Sun Valley Ski Resort on Bald Mountain.


Table 2: Snow and snow water equivalent from February 8th through February 16th.

This larger storm first set its sights on the southern mountains, then shifted to the northern mountains and back to the southern mountains for the grand finale. Overall snow totals reached a broad 3-5’ across the forecast area. Countless storm and wind slab avalanches failed during the storm. However, as with the first storm, the bulk of the persistent and deep persistent slab activity occurred in the Wood River Valley mountains and in areas with shallower snowpacks in the Smoky and Boulder Zone (Figure 3).

Figure 3: On February 10th, this avalanche was triggered remotely from 400' away. It failed on weak snow near the ground on a north aspect at 8,200' in the Trail Creek drainage.

At the tail end of the storm on February 14th, temperatures spiked and the rain line rose to ~400-700' above the valley floor near Hailey. Many small to medium sized slides (involving recent snow) released at elevations near and just above the rain line. A series of avalanches on the east face of Della Mountain temporarily blocked the Big Wood River, prompting an emergency flood warning. Farther north near Ketchum, the little bit of additional snowfall on February 15th was enough to tip the scales (Figure 4). Avalanche activity in the Lake Creek drainage north of Ketchum was particularly widespread (Figure 5). Several D2’s and at least one D3 ran full track, impacting two homes (Figure 6). Details of Wood River Valley activity on the 14th and 15th can be found here.

Figure 4: On February 15th, this slide along Warm Springs Rd. displaced water from Penny Lake, sculpted the snow on the far side, and destroyed the dock.

Figure 5: A combination of large quantities of new snow and strong winds produced this large persistent slab avalanche on Sun Peak in the Lake Creek Drainage.

Figure 6: This persistent slab avalanche at the mouth of Lake Creek released on the morning of February 15th and hit the home in the photo. It released at 6,200’ on a northwest aspect. Photo: W. Everitt.

As skies cleared, observers noted several other persistent and deep persistent slides. A monsterous deep slab avalanche on Titus Peak reached its historic trim line (Figure 7). On February 17th, a SAC forecaster remotely triggered another beast of a deep slab in the northern Boulders (Figure 8). Not to be outdone, the Soldiers joined the party with a deep slab of its own (Figure 9). On February 20th, four days after the brunt of the storm, skiers remotely triggered a persistent slab avalanche that blocked HWY 75 at Galena Summit (Figure 10).

Figure 7: This deep persistent slab avalanche failed naturally on Titus Peak on a NE-N aspect at 9,700-9,900'. It was likely triggered by a large piece of cornice breaking off and tumbling down the slope. The debris reached the historic trim line.

Figure 8: On February 17th, this deep persistent slab avalanche was remotely triggered from the ridgetop in the upper-right portion of the frame. It failed on a SW-W-NW aspect at 10,000' and ran over 2,000 vertical feet.

Figure 9: This deep persistent slab avalanche released on a wind-loaded NE-facing slope at 9,400' on Smoky Dome. It's an estimated 600' wide, up to 15' deep, and ran 500 vertical feet. Riders in the area estimate debris piles >20’ deep. Photo: S. Frost

Figure 10: This persistent slab avalanche was triggered remotely by a group of skiers descending Titus Ridge near Galena Summit on February 20th. The slide occurred on a SE-facing slope at 8800', covering both lanes of HWY 75 with up to 10' of snow.

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February 21 - March 7, 2019

Spoiler alert: this period experienced the biggest storm so far this season—and the largest avalanches. It’ll be a unique winter indeed if we top either this storm or the avalanche cycle that ensued.

February 23rd began a 5-day storm cycle that produced an average of 5 inches of SWE throughout the forecast area. The Soldiers, Western Smokys, Sawtooths, and Banner Summit were clobbered with 6-7 inches of water, while the Wood River Valley and Galena Summit received 3-5 inches of SWE. In addition to the large quantity of snowfall, the other defining feature of this weather event was an increase in temperature during the storm (Figure 1). The storm began with mountain temps in the teens F and warmed to nearly freezing by the storm’s end. In addition to creating inverted, upside-down storm snow at mid and upper elevations, the warming produced rain in the Wood River Valley and Soldiers to around 7000 feet. The Sawtooth Valley remained cooler and nearly all of the precip fell as snow.

Figure 1: Precipitation and temperature at the Lower Titus weather station from February 23 to March 1.

Given the prevalence of weak layers in our snowpack, it should come as no surprise that the avalanche danger quickly increased, reaching HIGH on February 23rd and peaking at EXTREME on February 27th (Figure 2). We issued an Avalanche Warning for four consecutive days, from February 25th through the 28th. Avalanche danger wasn’t the only concern as the storm cycle raged on; rapid snowfall rates and wind drifting quickly overwhelmed snow removal efforts and overloaded roofs. Galena Summit closed for a six day period, and all three routes out of Stanley closed for a short spell. Highway 21 west of Stanley remains closed as of this writing (March 7th).

Figure 2: Avalanche danger from February 24 through March 1. An Avalanche Warning was in effect for a 4-day period from February 25-28.

While road closures and difficult travel hampered backcountry observations during the storm, there was no shortage of avalanches observed in the urban interfaces of the Wood River Valley and in the foothills near Stanley. Numerous small to medium-sized avalanches occurred on the slopes adjacent to Hailey and Ketchum. On the night of February 25th, avalanches hit two homes near the Greenhorn Fire Station mid-valley. The avalanches crossed the Big Wood River and jumped the bank on the far side, sending debris onto the roof of one home and the second story balcony of another (Figure 3). Both homes suffered broken windows. One of these slides also damaged the railroad bridge and sent debris within 100' of Highway 75. A few other homes were hit, but no major damage was reported. As the storm tapered off on February 28th, a significant avalanche cycle was revealed in the foothills surrounding Stanley. What the slides lacked in size (most were D2 or smaller), they made up for in quantity. In an area with little history of slides hitting homes, two avalanches came within 100 feet of reaching cabins (Figure 3).

Figure 3: (Left) Avalanche debris piled up against a home near Greenhorn after crossing the Big Wood River. Note the avalanche debris on the second floor balcony. (Right) An avalanche near the Williams Creek Trailhead south of Stanley came within 100 feet of reaching a cabin.

On February 28th, Ben was able to drive Highway 75 to Galena Lodge and observed that a massive deep slab avalanche had run in the NW bowl of Galena Peak. The crown was around two miles wide and the avalanche debris traveled around 3000 vertical feet, snapping mature timber, flagging a tree 40-50 feet off the ground, and running to the maximum extent of the historic runout zone (Figure 4).

Figure 4: Looking up the track of a historic deep slab avalanche on Galena Peak. The avalanche started in the NW bowl (the upper left of the photo) and the debris snapped mature timber and piled up more than 50 feet deep in places.

As visibility and backcountry access improved at the start of March, observations began to trickle in and the full extent of the avalanche cycle became apparent (Figure 5). Three distinct cycles took place. The first involved small to medium avalanches releasing in wind-loaded areas or breaking within the storm snow. Some of these events occurred at or just above fluctuating rain lines. The second produced many medium to large avalanches that broke 2-6 feet deep on a mid-pack persistent weak layer. These were most prevalent on southerly aspects and occurred throughout the forecast area - leading us to suspect a crust/facet layer from early February or facets at the old snow surface. The third was an extensive cycle of very large to historic deep slab avalanches that also occurred during the storm. These monstrous events were most common in the Smoky and Boulder Mountains. These avalanches ran on basal facets that formed early in the season and broke 6-10+ feet deep, taking out the entire season’s snowpack. The size and quantity of these slides exceeded anything in recent memory.

Figure 5: The February 23rd storm produced an extensive avalanche cycle of large avalanches running on mid-pack weaknesses or on basal facets. (Upper left) Deep slab avalanche on Big Peak in the Western Smokys. (Upper right) Deep slabs in the popular ski terrain on Durrance. (Lower right) Avalanches in Eagle Creek running on a mid-pack persistent weak layer. (Lower left) Deep slab on Shaw Mountain near Dollarhide Summit. All photos courtesy of Sun Valley Heli Ski.

A remotely-triggered or naturally-releasing avalanche was reported in the Soldiers on March 1st; otherwise benign weather during the first week of March allowed the snowpack to quiet down. No human-triggered avalanches were reported during or following the storm. We continued to warn about the possibility of triggering a persistent slab, but the avalanche danger gradually dropped to MODERATE by March 7th.

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March 8 - April 4, 2019

The headlines for the month of March and the early part of April couldn’t be more opposite than those in February. A quick hitting storm in early March, brought 9-12” of new snow to the forecast area as well as a few interesting but isolated slab avalanches that failed on a finicky layer of near surface facets (Figure 1). What may have developed into a bigger problem, was quickly forgotten however as storms moved north, precipitation ended and the long range forecast predicted nothing but sun and gradually warming temperatures (Figure 3).

Table 1: Most mountain locations received less than two feet of snow between March 8th and April 4th. The majority of this snow fell in two small storms, one at the beginning of this stretch and one at the end.

Figure 1: These small avalanches failed in the southern Boulders on a thin layer of near surface facets. Similar slides occurred near Galena Summit.

Figure 2: A brief storm on the night of March 12th brought a few inches of snow and more importantly, steady moderate N wind to the Boulder and Smoky Mountains.

Between March 14th and March 22nd a weather pattern shift pushed low pressure to our east and allowed a persistent ridge of high pressure to dominate the inland northwest. During this stretch clouds and precipitation were pushed to our north and gradually warming temperatures and intense spring sunshine pushed meltwater into the snowpack. Several wet loose avalanches were recorded. Wet slab avalanches were also a concern but the ingredients for a widespread wet slab avalanche cycle never quite came together.

Figure 3: This graph shows the temperature near Galena Summit. The 8-day warming trend is highlighted by the red trend arrow.

Figure 4: These wet slab avalanches occurred on March 17th, a few days into our 8-day warming trend. These slides failed on SE-facing slopes at 9200’ near Easley Peak in the Boulder Mountains.

A series of small, short-duration storms punctuated the latter half of March and early April bumping up the danger during and shortly after each storm. In classic spring fashion, snow accumulations were highly localized and rain lines fluctuated dramatically between valley floor and up to nearly 8,000’. When skies cleared, warm temperature and hot sun wasted no time stripping new snow from very steep, rocky terrain. On March 27th, after a couple of nights without a freeze, a wet loose avalanche blocked one lane of highway 75 near Hulen Meadows north of Ketchum (Figure 5).

Figure 5: This wet loose avalanche started small and gouged into slushy snow near the ground before spreading rocky debris across the northbound lane of highway 75 north of Ketchum. Photo: Amy David

Figure 6: On March 27th, wet, heavy snow and warm daytime high temperatures initiated

dozens of wet loose avalanches in the Sawtooth Mountains.

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April 5-14, 2019

This period began with wet and especially warm weather from April 5th through 10th. Several nights without a freeze at low and middle elevations (Figure 1), combined with light rain, led to increasing concerns for wet slab avalanches. Especially in the Wood River Valley, mid-elevation shady aspects had a dry snowpack with weak basal snow near the ground. As melt and rainwater introduced at the surface trickled through the snowpack and wetted the weak facets, an impressive cycle of wet slabs ensued - primarily in the Wood River Valley and in the southern Smoky-Boulder zones. An Avalanche Warning was issued the evening of April 8th and continued through the 9th.

Figure 1: Temperature (F) at the Ketchum Ranger Station. The freezing point is marked with the dashed red line. The period of more than 48 hours above freezing is circled in red.

The Warm Springs drainage west of Ketchum was ground zero for destructive wet slabs. At 2 PM on April 8th, two large wet slab avalanches released in the “Burn” on the backside of Baldy (Figure 2a) and ran down into the Lower Board Ranch, destroying two homes (Figure 2b) and brushing a third (watch video). This slide broke mature timber up to 18 inches in diameter. Additional wet slabs released beyond the pavement further up Warm Springs canyon, piling over 20 feet of avalanche debris and broken trees on the road (Figure 2c). On April 10th, another large wet slab released in the Burn. The flow was deflected by the debris from the April 8th avalanche and was thus prevented from reaching the Lower Board Ranch yet again.

Figure 2: Natural wet slab avalanches from April 8-10. a) Looking up the path of the April 8th avalanche that destroyed two homes in Lower Board Ranch. The debris broke mature timber and piled up 40 feet deep. b) A Lower Board Ranch home was moved off of its foundation. c) Debris from several slides blocked the Warm Springs Road beyond the end of the pavement for several days. d) Wet slabs on Griffin Butte above Hulen Meadows.

Other significant wet slab avalanches were noted on Griffin Butte above Hulen Meadows (Figure 2d), in Wilson Creek in the Pioneer Mountains, and just south of Cathedral Pines before Baker Creek. A handful of smaller wet slabs occurred further north in the Smoky-Boulder zone. Almost all of the observed wet slabs occurred on shady, mid-elevation sheltered slopes. No wet slabs were reported in the southern portion of the Wood River Valley near Hailey and Bellevue.

During this same time period, precipitation - both snow and rain - was the bigger player in the northern portion of the forecast area. From April 6th through 9th, the northern reaches of the Smoky-Boulder zone received 1.5-2” of SWE and a whopping 2-4” of SWE fell in the Sawtooths and on Banner Summit. Stanley received 1.5” of rain, and the rain line rose to over 9000’ during the storm. An Avalanche Warning was issued for the Sawtooth zone on the morning of April 8th. This complex weather system wrapped up with a cold front bringing 24 hours of N winds that raked the alpine. The combination of rain, wet snow, warm temps, and wind produced a complicated cycle of avalanche activity at upper elevations. The largest slab avalanches broke down to older weak layers buried in early March. Many smaller slabs released as a result of snowfall, rain, wind, or a combination of these (Figure 3). Wet loose avalanches were commonplace and rain-induced “roller balls” could be seen up to 9500’ in elevation.

Figure 3: Photo of upper elevation terrain in the Smoky-Boulder zone illustrating the complex pattern of loose snow and slab avalanche activity following the April 6-9 storm.

Unsettled weather continued for the remainder of the period, bringing a slow trickle of snowfall to mid and upper elevations with light to moderate winds. With temps dropping below freezing, the likelihood of wet avalanches diminished. The danger was rated as MODERATE and the forecast continued to discuss the possibility of upper elevation wind slabs and wet avalanches of both the slab and loose snow varieties. The last Daily Avalanche Forecast was issued on Sunday, April 14th, and we wrap up the 2018-19 season with a snowpack that’s currently 115-130% of average.

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