Increasing temperatures and the potential for rain on snow will increase the potential for triggering wind slabs created during the last week. The latest wind event began on 12/21, and was able to trigger numerous natural wind slabs. In many places these were older/ deeper windslabs that were failing on facets created in November.
New wind slabs could be reactive up to 1 foot in depth, or could overload deeper persistent wind slabs creating much deeper and more dangerous hard slab avalanches. Slopes may fail once a person has travelled well out onto the slab. Look for signs of windslab such as pillowed smooth snow surfaces, areas that have a hollow drum-like feel and shooting cracks. Use caution, this is not our typical Thompson Pass snowpack, expect instability to linger longer after weather events than what is typical.
Temperatures will be increasing through the day as warm air overrides the cold air that is currently in place. This may cause temperatures to go above the freezing mark in our high elevation start zones and create our first rain on snow event since October. There will be the possibility for freezing rain during the day 12/26. There is not a lot of precipitation forecasted for our area, although our snowpack has repeatedly showed us that even small changes in weather can rapidly increase the chances for avalanches to occur. Expect for human triggered avalanches to become likely and natural avalanches possible during this change in weather.
A prolonged period of cold and dry weather beginning in November has created very weak faceted snow that is now buried in our snowpack. The depth of where this layer exists in our snowpack is highly variable due to wind re-distribution. These November facets can be found anywhere from at the surface in areas that have received heavy wind scouring to 3 feet deep in areas with thick wind deposits. In areas more protected from the wind this number lies somewhere in between.
We have not had a major precipitation event in 3 weeks and temperatures have remained cold. This has promoted faceting to continue in our snowpack. Even with our fairly benign weather regime, minor changes in weather have resulted in natural avalanche activity. This demonstrates our snowpacks’ inability to have stress applied without becoming reactive. Stress can be applied in various ways including loading from wind, new snow, cornice fall or the additional weight of a person or snowmachine. We have not seen precipitation or skiers/snowmachines testing slopes lately. We have seen our fair share of loading from wind and the result has been natural avalanche cycles.
Persistent slab avalanches will not always give the typical signs of instability like shooting cracks or collapsing, and may fail after many tracks have been laid down. Using safe protocols becomes even more important during this type of setup. Some of these include only exposing one person at a time to an avalanche prone slope, have an escape route should a slope fail and maintain good communication with your group.
Photo of developed facets found on Girls Mtn at 3000′ SE aspect (2mm grid). These were found 1 foot beneath the surface. This layer of facets has been found at varying depths ranging from at the surface to 3+ feet deep.
12/23- Berlin Wall north face ~5000′ N-HS-R3-D2. 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
Detailed forecast for Thompson Pass (mid elevation 2000-4000′)
DATE SUNDAY 12/26 MONDAY 12/27
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER OV OV OV SC SC BK BK FW SC
CLOUD COVER (%) 100 85 85 30 45 55 60 10 30
TEMPERATURE 14 26 25 24 25 30 26 25 25
MAX/MIN TEMP 27 24 30 23
WIND DIR SW SW SW W NE NE NE NE NE
WIND (MPH) 4 10 11 9 14 16 13 10 8
WIND GUST (MPH) 31 31 27
PRECIP PROB (%) 40 80 90 10 0 0 0 0 5
PRECIP TYPE S Z S
12 HOUR QPF 0.09 0.04 0.00 0.00
12 HOUR SNOW 0.5 0.4 0.0 0.0
SNOW LEVEL (KFT)2.1 2.5 2.2 2.5 2.8 1.5 1.4 1.4 1.2
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 as of 12/20
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.
Click the + Full Forecast button below for a list of current avalanche problems, travel advice, weather resources and more.
Help to improve your local avalanche center and contribute an observation to the website. You can also contact me directly at [email protected] (907)255-7690.
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