Above 3,500ft Considerable
2,500 to 3,500ft Moderate
Below 2,500ft Low
Degrees of Avalanche Danger ?
A Considerable Avalanche Hazard exists for Wind Slabs on leeward aspects, mostly West to North, at mid and upper elevation on slopes above 35º.
A Moderate Avalanche Hazard exists on all aspects and elevations for Persistent Slab Avalanches on slopes above 35º.
It will be possible for wind slabs to step down into the old persistent slabs today. Take note: We have seen 3 human triggered avalanches in the past 3 weeks, all caught and carried, 2 buried, and no injuries. The persistent weak layer is the culprit for all these unintentional avalanches.
WIND SLAB AVALANCHE PROBLEM
A Considerable Avalanche Hazard exists for Wind Slab avalanches today at mid and upper elevations, on West through North aspects, on slopes 35º and steeper. 10″+ new snow this week combined with increasing winds, gusting E/ESE 26-38 mph on 3/8 and 3/9 have formed slabs 6-12″ thick. Expect mostly small avalanches, D1 to D2 in size.
Several natural wind slabs were observed on 3/9 on west and northwest aspects around 3500-4000′. Instability exists in portions of terrain. Expect wind slabs to be touchy, near and below ridgelines and on gully sidewalls. Visual surface clues like sastrugi or wind sculpted terrain, and poles tests that identify upside down snow, or shooting cracks- will be bulls-eye clues of instability. It will be possible for wind slabs to step down into the deeper, weaker persistent grains under the Valentines Day layer. This could be 2-3 feet deep in some locations.
Stability tests on recent wind slabs show easy propagation on isolation. The good news: wind slabs tend to heal relatively quickly once the wind stops blowing. As the peak of Friday’s wind activity begins to taper this morning, natural avalanche activity will begin to subside.
PERSISTENT SLAB AVALANCHE PROBLEM
A Moderate Avalanche Hazard exists for Persistent Slab avalanches at all elevations on all aspects, on slopes 35º and steeper. Expect avalanches to be up to D2 in size, and 2-3 feet deep. Evidence of this layer exists in portions of terrain, but is not always obvious. This problem will most likely be stubborn to trigger.
The persistent slab problem we persistently talk about is not going away. However, it is getting buried deeper in the snowpack. The good news: it will be harder to trigger because of its depth. The bad news: if you trigger it, the consequences could be huge. The reason we are not seeing larger avalanches in this PWL is likely due to spatial variability in the snowpack.
This problem continues to be a low probability/high consequence risk. Stability tests require a lot of force to get this persistent weak layer to fail, however, poor structure still exists. Larger loads, such as snow machines, may have better chances for triggering a persistent slab. Triggering a persistent slab is more likely where the slab is thinner, which is difficult to identify and avoid. Many people may successfully ride a slope before someone finds a trigger point.
Are we good, or just lucky? This type of avalanche problem is difficult to assess and will require a certain amount of luck to safely mitigate. If you are already going to tangle with this beast, maximize your safety with safe travel protocol, choose more conservative terrain, and carry and be proficient with avalanche rescue gear.
As friend and forecaster, Drew Hardesty always says, “Terrain is your friend.” Use it to your advantage, choose terrain wisely. Ride one person on the slope at a time. Get out of the way at the bottom of the slope. Use communication devices and agree on safe zones ahead of time. Route choice is your best choice. Hard slab persistent avalanches may run farther than you anticipate.
Recent Avalanche Activity
3/6-3/7 Numerous loose snow avalanches (sluffs) were observed earlier in the week after 6″ of new snow on 3/6. Sluffs were observed on all aspects at mostly mid and upper elevation on slopes above 40º. Sluffs ran on old sun crusts on southerly aspects and old wind crusts on leeward aspects and were D1-D1.5 in size.
3/9 Natural wind slabs failed on new snow/old snow and also stepped down to old persistent grains. Wind slabs were observed on Marmot W/NW aspects and NW Arkose Ridge line and were D1-D2 in size.
This week’s weather at 3550′:
Temps averaged 17ºF, with a low of 8ºF and a high of 25ºF.
IM reported 10″ of new snow this week with 0.6″ of water (SWE) on 3/6 and 3/8.
Winds at IM reported gusting ENE/NE 17-31 mph on 3/9.
Overnight at 3550′:
Temps averaged 15° F.
No new snow overnight.
This week’s weather at 4500′:
Temps averaged 14ºF, with a low of 7ºF and a high of 21ºF.
Winds averaged SE 7 mph, max SE 23 mph . Gusts averaged SE 13 mph, max gusts E 38 mph.
Overnight at 4500′:
Temps averaged 12ºF overnight, with a Low of 11ºF .
Winds averaged SE 12 mph overnight, with a max gust SE/SSE 32 mph.
NWS recreational forecast for Hatcher Pass here
NWS point forecast here
State Parks snow report here
Additional Info & Media
Expect the avalanche hazard to decrease to Moderate as winds diminish and wind slabs start to bond to old snow surfaces on Sunday.
NWS is calling for a trace of snow at Hatcher Pass today, temps 15-23º at 3000′, and winds E 4-11 mph.
Sun and warming temps Sunday could increase avalanche hazard on southerly aspects Sunday.