Connection between atmospheric rivers and avalanche deaths

[Tricia]

The Desert Research Institute released a study on July 14th concerning the connection between atmospheric rivers and avalanche deaths in the Western United States
From the article:

Atmospheric Rivers, as described by the National Oceanic and Atmospheric Administration (NOAA), are “relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics.”

When ARs make landfall on the West Coast of the US they release water vapor as rain or snow, supplying 30 to 50 percent of annual precipitation in the West and contributing to cool season (November to April) extreme weather events and flooding.
Researchers conclude that the intense precipitation associated with AR events is paralleled by an increase in avalanche fatalities. Coastal regions experience the highest percentage of avalanche fatalities during AR conditions; however, the ratio of avalanche deaths during AR conditions to the total number of AR days is actually higher further inland, in states like Colorado and Utah

 

[Posaune]

Not surprising.

 

[Magi]

When ARs make landfall on the West Coast of the US they release water vapor as rain or snow

So the news is that avalanche deaths go up after large snow events...?

 

[Tricia]

I think what they're saying is that the type of snow that comes in with an atmospheric river is prone to avalanche danger.

 

[Magi]

I think what they're saying is that the type of snow that comes in with an atmospheric river is prone to avalanche danger.

You're right! Thanks for prompting me to read the whole press release.

Money quote that supports you: “Although ARs are less frequent in inland locations, they have relatively more important roles in intermountain and continental regions where snowpacks are characteristically weaker and less capable of supporting heavy rain or snowfall,”

I'd be interested in seeing the relative danger of large snow events pulling folks to the backcountry vs the relative instability of the snowpack causing more deaths per skier.

 

[markojp]

West coast was full of rivers this past season, yet avy deaths dropped. Training and risk awareness count.

 

[fatbob]

Not sure it's saying anything new. When you look at the comments on West coast v Colorado and Utah hasn't that always been true of avy risk if not deaths. Wetter West coast snow is more stable than drier interior?

 

[pais alto]

West coast was full of rivers this past season, yet avy deaths dropped. Training and risk awareness count.

^This. Currently, avy fatality rates seem to be level or dropping in the face of increasing winter backcountry use.

 

[Tricia]

West coast was full of rivers this past season, yet avy deaths dropped. Training and risk awareness count.

I don't know the actual numbers for the Sierra, which I can probably look up, but I felt like we had a lot of avalanche deaths around these parts.

 

[SBrown]

I don't know the actual numbers for the Sierra, which I can probably look up, but I felt like we had a lot of avalanche deaths around these parts.

https://snowbrains.com/12-avalanche...-averages-27-asked-3-experts-years-total-low/

Good article.

eta: One year could just be a statistical anomaly, of course. CO seemed to have a better snowpack, the way things played out. No snow early, which helps lessen the weak layer at the bottom, and then when it came, it came hard and fast. Usually we get all those little snows and then it gets sunny. I was in SW CO in late Feb/early March, and my brother said the snowpack had been incredibly stable (until the day I got there, of course). January saw very little sun.

 

[TonyC]

The driving force here is not the atmospheric river itself but the underlying snowpack which tends to be progressively more dangerous as you move inland from coastal to intermountain to continental regions.

So it's not surprising to see this:

Money quote that supports you: “Although ARs are less frequent in inland locations, they have relatively more important roles in intermountain and continental regions where snowpacks are characteristically weaker and less capable of supporting heavy rain or snowfall,”

Coastal region avalanches are typically during or within several days after storms. Weak layers in continental snowpacks can stay there for months. That's why we are still skiing steeps in August at Mammoth but A-Basin has to shut Pali down in mid-May.

The level of coastal avalanche activity is thus likely to be directly proportional to the amount of storm activity regardless of snow type. Inland region avalanches will be more sensitive to other factors, incidence and type of snow, but most particularly how severely the weak layers set up in the early season.

No snow early, which helps lessen the weak layer at the bottom, and then when it came, it came hard and fast. Usually we get all those little snows and then it gets sunny.

2016-17 was exactly the most desirable snow safety scenario for Colorado.

“Avalanche danger is the opposite of what most people think. Years with lots of snow are actually more stable. Thin snow means weak snow because of temperature gradient metamorphism, which forms faceted snow, the cause of most avalanche deaths. ” – Bruce Tremper, former director of the Utah Avalanche Center and author of “Staying Alive in Avalanche Terrain” told SnowBrains via Email

This is generally true, though 2014-15 was also low in avalanche deaths. In a really horrible season like that, there are long periods with not enough snow for people to be interested.

 

[TonyC]

I found this in Epic archives:

"The avalanche research community here classifies snowpack as coastal, intermountain or continental. They are focusing on avalanche hazard but water content correlates well to these categories: Coastal snow averages around 13%, intermountain 8-10%, and continental 6-8%. The source material for these classifications is here: http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477(2000)081<2367:SACOTW>2.3.CO;2 A flowchart for classifying can be found on p. 9 of that paper and the North American sites studied on pp. 6-7. "