D3 hard slab avalanche on Billy Mitchell ~4000’/ North aspect that occured on 2/18
Weak faceted snow exists at the base of our snowpack that has shown the potential to produce full depth hard slab avalanches. The last reported avalanche that failed on this layer occurred on 2/22. Although, propagation at the ground in stability tests has been observed as recently as 2/28 in the continental zone.
As the recent weather has become benign, our snowpack has been allowed to settle and adjust. Triggering one of these deep hard slabs has become less likely due to its depth, although the possibility still exists. The most likely place to initiate one of these hard slabs would be in our continental region, and/or in steep rocky terrain where the snowpack is thinner and trigger points are closer to the surface. This weak layer is believed to be less of an issue near the coast.
A big problem with the current situation is that this deep layer is very difficult to properly assess and the consequences would be severe if triggered. If one of these hard slabs was initiated, it most likely would not be survivable.
Patience is recommended. Unfortunately these problems do not heal themselves quickly like a storm snow instability can. Deep slab problems are complicated and difficult to predict even for the most experienced user. Signs of instability at the surface will likely not exist, which can trick users into thinking that conditions are safe. Tracks are not a sign of stability with this problem, avalanches can occur even after many tracks have been laid down.
Generally stable surface conditions have recently been observed. Human triggered avalanches that involve the surface snow may still occur in isolated locations such as steep convex and or unsupported terrain. Winds have been light lately, but moderate southeast wind have kicked up to moderate and may be forming new wind slabs on the lee side of high elevation ridges. Hand pits are a good way to quickly asses how well surface snow is bonded. Watch for signs of instability like collapsing and shooting cracks in the areas that you choose to travel.
The possibility also remains to trigger a hard slab avalanche that involves the entire snowpack. This is especially true in areas north of Thompson Pass. Avalanches near the surface will have the potential to step down to layers deep within the snowpack creating deadly avalanches.
2/28- Numerous wet loose running at low elevations with some reported gauging to the ground above town.
2/23- Full depth hard slab avalanche in steep rocky terrain. East facing side of RFS draining into Cascade Creek.
HS-N-R2-D2-O/G Steep Camp Couloir 2
2/17-18- A Significant avalanche cycle occurred during this time period. Most of the activity failed on deep persistent weak layers. These were the 1/24 facet-crust combo and the November facets. Numerous hard slab avalanches were observed that failed near the ground.
Deep hard slab activity included: Billy Mitchell, Happiness, 40.5 Mile Peak, NE Crudbusters Multiple spots around the Tonsina Glacier, West side of Iguana backs (really wrong place), Multiple spots in the Tsaina valley, Girl Mountain, Cracked Ice, Python, Buttress west of Nicks (this is has slid multiple times this year), North Oddessey shoulder. This is only a portion of the activity that occured.
Most of the activity occured around 4000′, although there were outliers at upper elevations.
Iguana Backs (Really Wrong Place)
North Oddessey Shoulder
2/17 – Keystone Canyon was reported to have many size 1-2 loose avalanches on the steep canyon walls. Crowns with depths of 10-15″ have been reported as well on the ice climbs Simple twist, Bridal Veil and Green Steps. Photo approach to Simple Twist
2/15- A large deep persistent avalanche occurred on “Back It In” near Mt Dimond. The trigger is unknown, but was most likely triggered remotely by a helicopter landing on a recon mission. It appears that a cornice fall triggered a shallow slab over rocks which then stepped down creating a deep slab ~500 feet wide that entrained the entire snowpack. Several other deep slabs failed sympathetically below.
2/13 – Meteorite ramp was reported to have released full path failing at the bedrock.
2/9 & 2/10- Numerous natural avalanches occurred during this time period.
Mt tiekel (50 mile) and 38 mile ran naturally, with 38 mile depositing a small amount of debris on the road.
The entire Python Buttress failed naturally close to the ground with a crown line approximately 1/4 mile in length. Numerous avalanches in the gully to the east occured as well. Debris from both areas extended ~200 meters across flat terrain.
Gully 2 and Nicks had significant slides run into their fans.
Snowslide Gulch ran naturally and hit the road.
DOT mitigation efforts produced activity at Three Pigs and Snowslide Gulch.
Natural activity was more extensive than noted. Snowfall continued after the event refilling crowns and preventing a good look at the extent of natural activity.
1/30-Several natural persistent slab avalanches were observed in the intermountain and continental zone.
Natural D3 Persistent slab avalanche/ Shovel/ northeast aspect/~5000′.
South aspect hogback ridge/~3000′
1/29- Observed numerous D2 avalanches on Billy Mitchell (cry babies area) and one D3 on Happiness. These were all northerly aspects. On Billy, slides originated between 3-4000 feet and on Happiness ~5000′.
1/28- During brief break in the clouds, observed a Natural D3 avalanche on Catchers Mitt. It is likely this slide failed on a persistent weak layer, as the crown was deep (3-4) feet and ran a considerable distance.
1/22-1/24- A D3 natural avalanche was observed on 1/22 in snow slide gulch that ran to just below the summer trail.
On 1/26 2-D3 naturals were seen on the east face of Mt Tiekel (beyond forecast zone) that ran half way through their aprons. These likely occured between the 1/22-1/24 time period.
Clouds prevented observation of where these slides originated
1/22- Clouds made observing avalanche activity difficult, although numerous large wet loose slides were observed on south aspects of Town Mountain in the Port of Valdez.
1/13- Multiple large natural avalanches were noted following the snowfall on 1/13. Most were near high elevation ridge lines, although mid elevation storm slabs were noted on north aspect of Catchers Mitt and south aspect of Mile high. Other avalanche not shown in photos include Goodwills north aspect and Oddeyssey north aspect.
1/1-1/4- The new years day wind event created an avalanche cycle that was difficult to document due to crowns being rapidly reloaded by 80 mph winds. Below are photos of a couple very large slides that were still visible in the Hippie ridge area. Naturals were also noted on Three Pigs, 40.5 Mile, Crudbusters, Python Buttress.
12/29- Multiple natural wet loose D1-D2’s were observed in the Port of Valdez with no step downs noted.
12/23- Berlin Wall north face ~5000′ HS-N-R3-D2-O. It is possible this occurred on 12/21, although it was not observed until 12/24.
12/21- Numerous natural avalanches observed all along the north side of Thompson Pass, as a result of strong NE wind event along with a couple inches of new snow and rising temperatures. Observed naturals on all aspects except windward slopes with crowns originating from 1000 feet to 5500 feet in elevation. Most of these were hard slab avalanches. Crown depths were difficult to discern due to reloading, although some crowns looked to be up to 2 meters in depth.
12/19- D 2.5 natural avalanches were observed on the north facing buttress west of Gully 1 and Schoolbus.
12/14- Several natural avalanches were observed although poor visibility prevented a full view of the action. The most notable natural was observed in Nicks Happy Valley on a NW aspect ~4000′. Crown depth was not visible. Debris ran down the valley and piled up at the typical snowmachine pickup.
12/8- Large remote trigger/ sympathetic avalanche event occurred 12/8 with avalanches extending from Gully 1 to Nicks. Avalanches were soft slabs that ranged in size from D1-D3. Over 10 separate avalanches were counted with crown depths averaging 2-3′. One avalanche had a crown length of half a mile while another was triggered over a mile away from the point of collapse. See observation section for full report and more photos.
12/7- Only a few natural avalanches were noted during the last storm. It is likely there were more during the storm, but crowns may have been filled in by subsequent wind and snow.
D2’s on Town mountain was observed ~3000′
A couple of D2’s were noted in N. Oddessey gully and Big Oddessey.
D2 on 40.5 mile peak ~5500′.
12/2-12/3- Several natural D2 avalanches were noted on south aspects of Three pigs, Hippie Ridge and Averys. These windslab avalanches originated between 4000-5500 feet elevation.
NWS Watches and Warnings
Point forecast for Thompson Pass
DATE THURSDAY 03/03 FRIDAY 03/04
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER OV OV OV OV OV OV OV OV OV
CLOUD COVER (%) 100 90 80 75 95 90 80 95 100
TEMPERATURE 22 26 23 22 24 28 26 24 26
MAX/MIN TEMP 26 21 28 24
WIND DIR E SE SE SE S S SE E SE
WIND (MPH) 10 10 9 8 10 13 9 13 18
WIND GUST (MPH)
PRECIP PROB (%) 90 70 70 50 70 80 60 90 100
PRECIP TYPE S S S S S S S S S
12 HOUR QPF 0.14 0.06 0.14 0.21
12 HOUR SNOW 1.8 0.2 1.7 2.5
SNOW LEVEL (KFT)0.0 0.0 0.0 0.0 0.0 0.2 0.1 0.2 0.4
Snow and Temperature Measurements
All snowfall measurements are expressed in inches and temperature in Fahrenheit. 24 hour sample period is from 6am-6am.
* 24 hour snow water equivalent/ SWE.
** Season total snowfall measurements for 46 mile began December 1st.
Season history graphs for Thompson Pass
Click on links below to see a clear and expanded view of above Season history graphs
TP WX Nov 21
TP wx Dec 21
TP Jan 22
TP as of 2/20
Winter weather began early this season, with valley locations receiving their first snowfall on the last day of Summer (September 21st). Following this storm, above average temperatures and wet weather occurred from late September through early November. During this time period Thompson Pass received 96 inches of snowfall by November 7th and Valdez recorded 7.73″ of rain.
After the 7th of November our region experienced a sharp weather pattern change. Temperatures dropped below seasonal norms and snowfall became infrequent. Between the time frame of November 7th- November 28th Thompson Pass only reported 19″ of snow with 1.1″ of Snow water equivalent (SWE). Temperatures remained below 0° F for most of the period. This cold/dry weather caused significant faceting of the snowpack, with poor structure the result.
Moderate snowfall returned to our area the last day of November and deposited 6-12 inches of new snow. The amount varied depending upon the locations’ proximity to the coast. As the storm exited on the 2nd of December it was quickly replaced by moderate to strong northeast winds.
On 12/5-12/6 Valdez received 2 feet of new snow with Thompson Pass reporting 16″. Blaring red flags like collapsing, shooting cracks and propagation in stability tests were immediately present. On 12/8 a significant remote/ sympathetic avalanche event occurred from Gully 1 through Nick’s Happy Valley.
Strong outflow winds began on 12/11 with periods of light snowfall. This has caused slab thicknesses to become variable in areas exposed to NE winds.
A fair amount of natural avalanche activity occurred during the 12/11 wind event mostly on southerly aspects. The week following this wind event fairly benign weather occurred which allowed the snowpack to adjust and for stability to improve although snowpack structure has remained poor.
On 12/21 our area received a couple inches of snow along with temperatures rising and strong outflow winds. This combination of weather kicked off a fairly significant natural avalanche cycle. Many of the slabs appeared to be deeper wind slabs that were created from the 12/11 wind event. These failed on faceted snow created in November. The event is yet another indicator of our poor snowpack structure and its inability to receive any major change in weather without the avalanche hazard rising in conjunction.
On 12/26-28 warm air moved in at elevation and caused light rain to fall up to ~4000′. A very thin rain crust was formed in many locations that was unable to support a persons weight.
A prolonged period of strong north winds began on new years day with wind speeds reaching 80 mph. As winds tapered to 30-40 mph on the 5th temperatures plummeted with lows exceeding -30 F in the Tsaina valley.
Snowfall returned to our area on 1/13 with a foot of snow reported on Thompson Pass. An additional ~6 inches of snow were received on 1/15 with settled storm totals of 2.5 feet above 5000′.
Moderate outflow winds kicked up on 1/16, but were short-lived and not wide spread. This was followed by two days of calm and mild weather.
Stormy weather returned to our area on 1/19 with 10 consecutive days of measurable precipitation. The Initial change in weather brought ~1 foot of dry snow which was quickly followed by a big warmup from 1/21-1/24, with snow line rising to 3000′. This caused the snowpack to go upside down at the surface which created a decent amount of natural wet/loose activity near sea level and a couple D3 slab avalanches on Snowslide Gulch and Mt.Tiekel. Observation of avalanche activity was limited due to continued storms and crowns being filled back in.
After 1/24, precipitation continued with temperatures slowly dropping and snow line returning to sea level. Precipitation ended on 1/29 with moderate northeast winds building wind slabs as skies cleared.
Snow water equivalent storm totals for 1/19-1/28 are as follows:
Thompson Pass: 4.3″ (1/19-1/26)
46 Mile: 1.93″
A period of moderate outflow winds directly followed as the late January storms cleared out.
1/30-2/3- A period of mild weather occured with clearing skies dropping temperatures and light to moderate northerly winds. A fair number of large natural persistent avalanches were observed that occured during the previous storm and as a result of outflow winds at the tail end of the storm. First human triggered persistent slab avalanche since 12/8 reported on 1/30. This was remotely triggered.
A parade of lows began on 2/4 with precipitation measured in Valdez for 9 consecutive days. On 2/9-10 south winds ramped up to 50 mph and triggered a natural avalanche cycle with debris covering and closing the highway in a couple spots. Numerous deep persistent slab avalanches were observed in the mid elevation band. Storms continued after the natural cycle preventing a thorough record of the activity.
Following the 2/9-10 natural cycle storms continued to roll through our area with moderate to heavy accumulations occurring each day. On the 17th a more significant system moved through bringing ~20 inches of snow to Valdez and Thompson Pass with 2″ of SWE. This storm sparked a widespread natural avalanche cycle creating very large hard slab avalanches on all aspects, with many failing near the ground. Even small 30° glacial rolls were reported failing down to black ice. The November facets have been reactive this year every time there has been a major change in weather and this storm was no exception.
Our area received a quick break in the weather from 2/19-20 allowing the snowpack to settle some. This reprieve was followed by a foot of snow at sea level and 2 feet at upper elevations, that arrived on the night of 1/21 accompanied by strong winds. Very little natural activity was noted when skies cleared the 23rd. The exception to this was a full depth hard slab that failed in steep rocky terrain above cascade creek (see avalanche activity).
2/24-26 saw light to moderate precipitation with temperatures going above freezing at valley locations region wide.
The avalanche hazard is Moderate at all elevations. The upper snowpack has been showing signs of strength as of late with no major flaws within the top 3 feet. Our main concern remains faceted snow at the bottom of our snowpack that could produce very large avalanches that would involve the entire snowpack.
Posted 03/03 8:15 am.
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The Valdez Avalanche Center produces forecasts Wednesday- Sunday.
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