Last week in the upper Lynn canal, there was evidence of surprising warmth at altitude. The Haines avalanche center’s weather stations registered temperatures of up to 40 degrees at mountain top level. A 33-mile resident reportedly observed snow melting in the high country above his home and water cascading down the mountain when it was a cold 5 degrees on the valley floor. This is indicative of an inversion phenomenon, also called an overrun. We contacted Rick Fritsch, Forecaster at NOAA’s Juneau office, for comments.
Fritsch “In most of South East Alaska we saw temperatures that were aloft, anywhere from 2000 to 500 up into the atmosphere, much, much warmer than the surface temperature. Cold, dry arctic air that came at us from the Yukon and British Columbia settled down low, and kept us very cold at the surface. Meanwhile, we had air coming in from the South West, at altitude, that was significantly warmer than the surface
Fritsch says the warm air layer likely extended from 2500 to 5000 feet, meaning last week one could have been comfortable up mount Ripinsky wearing only a sweater.
These air layers are eventually mixed up by wind. He describes how drastic the differences can be between his workplace and downtown.
Fritsch “We could be in the upper teens, maybe lower twenties, and no wind. And then ten miles away downtown Juneau there is a lot of wind, making all the air mix vertically, it will be 20 to 30 degrees warmer downtown than it is here in the valley. So the wind sheltered areas are really where we see the inversions most of the time in the winter.”
This high altitude warm air melts the top layer of the snow, which freezes again when the temperature drops. This increases the risk of avalanche. Here is Haines Avalanche Center Director Eric Stevens explaining the process.
Stevens “The ice layer forms in the snow pack, and it act like a barrier, and it keeps temperatures from moving within the snowpack, and it keeps moisture from moving within the snowpack, and it builds energy right around that crust, and causes crystals on either side of that crust to get weaker, and those form a weak layer over time, and then avalanches can fail on that weak layer.”
It isn’t a rare phenomena, and the avalanche center keeps track of those ice forming events and takes them into account in their risk assessment.
Stevens “So the inversion created a new crust this last weekend, but it’s definitely not the first crust in the snowpack, and there were other more significant warm ups over the last month or two that have caused much more significant crust even than the one this weekend. So we are much more worried currently about other crusts that are deeper in the snowpack, in fact from November especially.”
Stevens warns of a particularly high risk of avalanches for at least the first part of the winter.
Stevens “Currently we have a pretty dangerous persistence slab on top, that can be tricky, it can be hard to predict, and that can definitely lead to deadly avalanches, much more so than in previous years. So we really want to express caution and make sure people check the avalanche forecast, and generally just being conservative with their decisions in the backcountry. And because these are persistent weak layers this danger is going to last for a little bit. That’s a matter of weeks, could be months depending on how the winter progresses. Hopefully by late winter early spring things will be kind of healing a little bit, but it’s way too early to say. For now it’s pretty sketchy out there.”
Global temperature rise and the more extreme weather variations associated with it increase the risk for avalanches everywhere. Here is a link to the Haines Avalanche Center website.
