Turnagain Pass Avalanche Forecast

It’s a day to carefully evaluate terrain and consequences if an avalanche does release. We need to give the mountains time to adjust to the rapid loading. Low angle slopes without steep slopes above are great ways to enjoy the new snow without worry. Yesterday’s powerful storm brought feet of snow to the mid and upper elevation terrain, inches of rain to sea level and multiple hours of sustained winds at ridgetops blowing 60-90 mph (with gusts as high as 129 mph on Maxs and 133 mph on Sunburst). The rain/snow line rose to around 1,000′ and then dropped back to close sea level overnight. Not surprisingly there was a widespread avalanche cycle observed from the highway.

Storm totals (beginning Monday afternoon through 6 am Wednesday morning)

• Turnagain Pass at 1,880′:  3.4″ of water equivalent, roughly 2-3′ of snow above treeline.
• Girdwood Valley at 1,700′:  3.3″ of water equivalent, roughly 2-3′ of snow above treeline.
• Portage Valley at sea level : 3.1″ of water equivalent, roughly 2-3′ of snow above treeline. This is probably low for that region. Water numbers are from AKRR wx station, Bear Valley RWIS totals were not available. The Tunnel wx station near Grandview had 4.33″ of water equivalent.
• Summit Lake at 1,400′: 0.8″ of water equivalent, roughly 8-12″ of snow above treeline.

Today’s partly cloudy skies may allow for easier travel above treeline and with that, the danger that a person could trigger a large avalanche if venturing into avalanche terrain. Two to three feet of new snow has fallen in the mountains. There are a variety of storm snow related issues to pay attention to and view as likely today starting with wind slabs. Very strong winds can have unusual loading patterns. Sometimes wind slabs are farther down onto slopes than expected and terrain channeling of the wind can significantly cross-load terrain features. In sheltered areas (if were any) storm slabs may be present as heavy warm snow fell on light colder snow. Avoid travel on or under cornices as they may be quite tender today and often break farther back than expected. At low elevations that received heavy rain, wet loose avalanches may still be initiated before the next crust forms.

Red flags to watch for:
–  Recent avalanches, from today or avalanche activity from during the storm?
–  Whumpfing (collapsing) of the snowpack, sure sign to avoid avalanche terrain period.
–  Shooting cracks, likely to be seen near ridgelines and on slopes where the wind has formed wind slabs.

The good news is the new snow should bond relatively quickly. From what we know, the pre-existing snow surface lacked any persistent type of snow grain and was mainly wind slab and settled powder. However as always, the snowpack is guilty until proven innocent and there are always surprises out there – it’s the first day after a storm and we can’t get caught being over confident.

Deep Persistent Slabs:
Above 2,500′ there is a layer of facets (weak snow) at the base of the snowpack, now 5-8′ deep. There is a chance this storm may have brought it back to life in places – meaning the rapid loading of new snow and wind may have overloaded the layer causing it to collapse and initiate very large avalanches to the ground. However, that has yet to be determined and is something we will be looking for as we get eyes on the forecast area and south of it towards Summit Lake.

One hour precipitation graph from the base of Alyeska. Courtesy of Mesowest/MADIS. This really illustrates precipitation rate and the rapid loading that was occurring. At mid and upper elevations snowfall was 1-2″ per hour. 

Sunburst wind rose shows the sustained winds from the northeast for the 24 hr time period. The max gust was 133 mph which may be a record. Thankfully the wind sensor did not blow away!