Strong northerly winds have been building windslabs 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. On 12/24 new windslabs were found to be very stiff and stubborn to triggers. Even though the surface snow has become less sensitive, human triggered avalanches will remain possible today on lee aspects and cross loaded gullies.
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.
There is a 1-3+ foot deep slab sitting on the November facets. These facets have continued to show propagation results in stability tests. Red flags such as shooting cracks, large collapses and natural avalanches have all been present in the Thompson Pass area lately. This has been observed to a lesser degree in the maritime forecast zone.
Very strong northeast winds have recently loaded lee aspects, further stressing facets in these areas. In areas exposed to wind, the slab overlying the November facets has variable depths. These range from scoured down to the facets (little to no slab) on windward aspects, to 2-4 feet deep on lee aspects (dense wind slabs). In areas exposed to recent winds the persistent avalanche problem is confined to specific areas (lee aspects). Persistent wind slab avalanches were naturally active as recently as 12/21. Triggering a persistent slab avalanche will be likely, directly following major changes in weather, such as strong winds. The hazard will slowly decrease as we move further away from wx events, but will remain a dangerous possibility.
Persistent slab avalanche problems are tricky and require a lot of patience, as well as a conservative approach in choosing terrain. Good protocols are very important during this type of avalanche problem ie: only exposing one person at a time to an avalanche prone slope, good communication and having an escape route should a slope fail. It is possible to trigger avalanches remotely from flat terrain, be aware of the terrain around you and the consequences of triggering a slope above you.
Photo of facets found on Girls Mtn at 3000′ SE aspect. These were found 1 foot beneath the surface.
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 SATURDAY 12/25 SUNDAY 12/26
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER BK OV BK BK OV OV OV BK FW
CLOUD COVER (%) 65 70 60 65 95 90 85 55 15
TEMPERATURE 10 15 15 15 17 24 21 20 20
MAX/MIN TEMP 16 15 25 18
WIND DIR NE N NW NW S SW SW W N
WIND (MPH) 8 3 4 4 3 10 10 8 10
WIND GUST (MPH) 27
PRECIP PROB (%) 20 30 20 5 40 60 80 30 5
PRECIP TYPE S S S S Z S S
12 HOUR QPF 0.03 0.01 0.07 0.07
12 HOUR SNOW 0.0 0.0 0.4 0.7
SNOW LEVEL (KFT)0.0 0.2 0.5 1.1 1.5 1.3 0.6 0.7 1.6
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.
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