Strong north winds occurred between 11/26-11/29 stripping snow surfaces down to old crusts and forming pencil hard wind slabs on lee aspects.
Wind slabs have since gained a lot of strength. on 12/2 wind slabs were found to be very hard (pencil) and unreactive. We can not yet rule out the possibility of a person or machine triggering a wind slab avalanche in isolated areas in a cross loaded gully, or the lee side of a high elevation ridge line (SE-NW). Triggering a wind slab at this point would result in a hard slab avalanche which could carry consequences, therefore we are maintaining a conservative forecast. The most likely areas to encounter wind slabs that remain reactive will be in the Continental zone, where slabs overlay a faceted snowpack. In addition, protected areas where wind slabs are found to be softer (4 finger), slabs may still be reactive. Shooting cracks or collapsing are a sign that windslabs remain reactive. The hazard for this avalanche problem is decreasing.
There are multiple rain crusts from October that are faceting in the mid and lower snowpack. Recent winds have created very strong snow at the surface making it difficult for a person or machine to affect these deeper layers.
At the present time this layer is not active. There will be an inversion in place over the weekend (higher temperatures at upper elevation). At this point our area is not forecasted to be strongly affected by this. If temps get higher than forecasted and go above freezing for a day or more there is a small possibility that weak layers in our mid and lower snowpack could reactivate.
There is a higher possibility of affecting these layers in the continental zone where a more faceted snowpack exists.
11/14- Debris from a D3 natural avalanche at snow slide gulch ended 100 vertical feet above the Lowe river.
Large avalanches (D2-2.5)also occurred in multiple other locations including Berlin Wall, Catchers Mitt, South Three Pigs and Billy Mitchell. The activity extends beyond this list, and mostly occurred during the peak of warming and precipitation on 11/13.
Multiple natural D1-1.5 avalanches were observed on multiple aspects at low elevation. No step downs noted.
12/1- 2 D2.5 natural avalanches were noted on Three Pigs (Beaver slide, Pig Leg). Pig leg ran into the top 1/3 of the fan and Beaver Slide stopped near the end of its track. These both likely occurred during the outflow wind event 11/26-11/29.
D2 natural wind slab was observed on 40.5 mile peak on a west aspect ~3000′. Crown depth range estimated 1-2 feet and 200′ long
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NWS Watches and warnings
NWS Point forecast for Thompson Pass
Date Saturday 12/03/22 Sunday 12/04/22
Time (LT) 06 12 18 00 06 12 18 00 06
Cloud Cover SC SC BK OV SC SC FW CL FW
Cloud Cover (%) 50 40 55 70 45 30 5 5 5
Temperature 17 24 28 25 22 27 26 26 26
Max/Min Temp 29 21 27 25
Wind Dir N NW NE NE NE SW SE W W
Wind (mph) 4 3 4 3 2 3 2 3 3
Wind Gust (mph) 16
Precip Prob (%) 0 0 0 0 0 0 0 0 0
12 Hour QPF 0.00 0.00 0.00 0.00
12 Hour Snow 0.0 0.0 0.0 0.0
Snow Level (kft) 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Click on link below for Thompson Pass weather history graph:
*HN24W- 24 hour Snow water equivalent in inches
*SWE– Snow water equivalent
**46 mile seasonal snowfall total begins December 1st.
Our snow season began with above average precipitation and temperatures. Beginning in September, snow lines generally hung around 4500′ until 10/12. At that point our area received the first snow down to sea level with 12-16 inches on the north side of Thompson Pass.
On 10/15 wet conditions continued with the freezing line rising to 5000′ or higher. As skies finally cleared on 10/22 we were left with a thin rain saturated snowpack capped by a stout rain crust up to 4500′. Above 4500′ much deeper snowpacks existed due to significant early season snowfall at upper elevations.
Dry and cold conditions along with moderate outflow winds finished out the month of October.
On 11/1 precipitation returned with 18 inches of snow and ~1″ of SWE on Thompson Pass. This new snow was initially reactive with several natural D2 avalanches reported on Thompson Pass. These slides were running on a firm bed surface consisting of old rain crusts and old wind slabs from October.
On 11/4 a strong north wind event kicked up with 65 mph+ winds on Thompson Pass. Our snowpack received significant damage as already thin snow below 4500′ was stripped down to old wind slabs, rain crusts and the ground.
Precipitation returned on 11/8 and became heavy on 11/11. Storm totals of around 50 inches were recorded at Thompson Pass DOT between 11/8-11/13. Snow lines rose to ~3000′ near the tail end of the storm with heavy rain occurring in low lying areas.
Skies cleared on 11/14 through 11/18 with a strong temperature inversion setting up. Valley temperatures north of Thompson Pass fell to 0° F with above freezing temperatures existing above 4000 feet. Valdez temps remained mild. This weather allowed for widespread surface hoar up to 1 cm to develop in low lying areas.
Precipitation returned on 11/19, with incremental snowfall on Thompson Pass and areas north. The Valdez area received rain during this period. 12 inches have been recorded at TP DOT between 11/9-11/23.
11/26-11/29- Strong outflow (N) wind event. Many areas below 3000′ were stripped to the 11/13 rain crust, destroying the 11/19 BSH layer.
The avalanche hazard is Moderate above 2000 feet in the Intermountain and Continental zones. Human triggered wind slab avalanches 1-2 feet deep remain a possibility in isolated locations. Cross loaded gullies, as well as the lee side of terrain features and high elevation ridge lines will be the areas to look out for. This avalanche problem is decreasing as wind slabs gain strength with time.
Posted by Gareth Brown 12/3 8:00am.
For a description of avalanche problems, weather information, season history and more click the (+ full forecast) button. Avalanche forecasts will be issued Wednesday-Sunday.
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