Turnagain Pass

Turnagain Pass:

Several large natural avalanches released sometime during or after the storm on Sunday/Monday. Yesterday we observed 4 large avalanches along the E and SE aspects of Seattle Ridge. The furthest north was approximately across from the Sunburst parking lot and the other three were in gully features on the SE aspect of Peak 4940′.  At 2200′ on the NW aspect of Magnum we saw another recent avalanche but it was hard to tell if it started at higher elevation. All these avalanches were roughly size D2 to D2.5 and likely released at the interface with the old snow surface or on a deeper weak layer.

Natural avalanche at 3200′ on SE aspect of Seattle Ridge that ran down into treeline elevations. You can see some brown color in the bed surface of this avalanche which could indicate it released on a deeper weak layer but it is hard to tell from afar. Photo 12.28.22

One of the avalanches off Peak 4940′ that released at about 3000′. This area is heavily wind loaded and it is difficult to see the exact extent of the crown, but the debris runs down into the small evergreens in the gully. Two more very similar avalanches were just N of this one along the SE face of Peak 4940′. Photo 12.28.22

Natural avalanche at 2200′ on NW face of Magnum. Due to the visibility it is hard to tell if this avalanche released up high and ran into lower elevations or just released on the steep lower roll on the NW ridge of Magnum. Photo 12.28.22

Winds have picked up overnight coming out of the E and SE with averages of 15-25 mph and gusts to 45 mph at upper elevations. Above 2000′ there is 6″ or more of dry new snow on the surface which could easily be transported into fresh wind slabs up to 2′ deep. At higher elevations the new snow should be lighter and colder and will be easier for the wind to transport. To identify wind loaded features keep an eye out for shooting cracks, hollow feeling snow, and active wind loading. It is important to get off the beaten path to feel the snow surface around you and get a sense for areas that are more wind affected. Small test slopes can be a great way to determine if wind slabs are a concern in the area you are travelling. Wind slabs are most likely to be found along upper elevation ridgelines and cross loaded gullies.

At Turnagain Pass yesterday we found a breakable melt freeze crust 1-2″ thick up to about 2000′. There should be another couple inches of fresh snow on top of that now, but it definitely made for challenging travel at lower elevations. Above 2000′ the crust became very thick (6″) and supportable with about 6″ of new snow on top. Travel conditions were much improved once that melt freeze layer became supportable!

Below 1000′ the snowpack looked pretty saturated and wet loose avalanches are possible on steeper terrain features.

With the addition of 2″ of water since last weekend the structure of our snowpack is looking quite different. This is especially true at elevations below 3000′ where some of the recent precipitation fell as rain or wet snow. As the wet snow and melt freeze layers continue to freeze, the upper snowpack will become very strong and initiating an avalanche on a deeper buried weak layer will become much more difficult.

Above 3000′ we still have limited recent information about avalanche activity and how weak layers are reacting after the new snow load. Our stability tests yesterday indicated that the interface with the old snow surface which was about 1.5′ deep is the most concerning weak layer. We were not able to get any results on the layer of facets above the Thanksgiving melt freeze crust (3′ deep), but it may be possible to get a failure on this deeply buried weak layer at upper elevations or in parts of the forecast area with a thinner overall snowpack. Those typically thinner areas include Crow Creek in Girdwood and the southern end of Turnagain Pass near the Johnson Pass trail head.

We recommend a conservative approach to terrain selection and decision-making until we learn more about how the buried weak layers are reacting to the recent storm, especially at upper elevations where we have no information.

Snowpack structure at 2200′ on Tincan on a N aspect. The stand out features are the super thick melt freeze crust buried about 6″ deep and the interface with the old snow surface about 1.5′ deep. Photo 12.28.22