The majority of our avalanche concerns revolve around weak faceted snow created in November. A recent increase in temperatures to above freezing in places will be stressing this faceted snow. In many places these facets have been capped by varying depths of hard wind slabs. This layering has made these facets more difficult to affect due to bridging, and signs of instability have decreased as well. Even though persistent slabs have become more stubborn to triggers a weak snowpack is still in place. Dangerous human triggered hard slab avalanches 1-3+ feet deep remain possible.
Persistent slab avalanches are difficult to predict and are notorious for catching even very experienced users off guard. Signs of instability will likely no longer exist and snowpits may give misleading results. For example, a person may dig a pit that shows poor structure yet they receive no results, ECTX. This person assumes the slope is safe. The reality may be that in close proximity, there could be areas near rocks (thin areas) or near the edge of a slab where trigger points exist that could release the entire slope if affected. The only effective practices for mitigating a persistent slab avalanche problem is conservative terrain choices and the use of safe protocols. Safe protocols include only exposing one person to an avalanche prone slope on the way up or down, having an escape route should a slope fail and maintaining good communication within your group.
The most likely places to trigger a persistent slab avalanche will be in thin rocky areas or anywhere the snowpack is thinner and the weak layer is closer to the surface. Other areas include places that have not been affected by recent wind events, and locations north of Thompson pass where colder temperatures has created an even weaker snowpack.
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′ 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
Detailed forecast for Thompson Pass (mid elevation 2000-4000′)
DATE TUESDAY 12/28 WEDNESDAY 12/29
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER FW BK OV OV FW FW SC SC FW
CLOUD COVER (%) 25 65 85 85 10 15 45 45 25
TEMPERATURE 32 34 23 18 18 24 17 13 12
MAX/MIN TEMP 34 16 25 11
WIND DIR NW NE S W NW W N NE E
WIND (MPH) 11 6 9 17 21 16 11 5 3
WIND GUST (MPH) 36 29 36 51 37 32 23
PRECIP PROB (%) 0 5 70 80 0 0 0 5 5
PRECIP TYPE S S
12 HOUR QPF 0.00 0.15 0.00 0.00
12 HOUR SNOW 0.0 1.9 0.0 0.0
SNOW LEVEL (KFT)1.9 1.9 1.9 1.6 0.0 0.0 0.0 0.0 0.0
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.
On 12/26 warm air moved in at elevation and caused rain to fall in certain locations. The distribution of this rain on snow event is unclear at present but was observed on 12/27 to have formed a thin crust up to 3700′ on Thompson Pass proper.
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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