Colorado Snow discussion

[Robbie Crouse]

This is just a curiosity/academic discussion, but I've been reading the classic standard, [i]Mountaineering: Freedom of the Hills[/i], and there's a very interesting section on types and location of snow in mountaineering. I'd be interested to get your feedback.

The book points out that north-facing slopes often get more snow and less sun so the avalanche potential is greater in the winter and less in the spring. And vice-versa with south slopes. It also points out that snow is often greater on leeward slopes because of the drift from the windward side, so there is more avalanche risk on leeward slopes (at least in the winter). Since in Colorado the wind is almost entirely from the west, this would make the leeward the east-facing slopes and the windward the west-facing slopes.

But here's my question, I had always thought that west facing slopes in Colorado got more snow. At, least that's how it generally works for the ranges (e.g., the San Juans get a ton more snow than the Sangres). And again, in regard to rain, valleys to the west of the mountains get more rain than valleys on the east side. Is it the same with snow?

Any thoughts on this?

[Eli Boardman]

This seems to be a problem of scale.

The reason that western mountain ranges often get more snow than eastern ones is primarily due to large-scale "rain shadow" effects. For instance, an air mass moving west to east would generally be uplifted by the first range it encounters (let's say the San Juans), where it would drop most of its moisture, before continuing on to the more easterly (and thus drier) Sangres.

However, this only really matters on a hundred-mile or larger scale. On the couple-mile scale (different sides of the same peak), the windward side will be wind scoured (less snow) and the leeward side will be wind loaded (more snow).

So, you might say something like this in general, with ">" meaning "more snow than."

San Juans > Sangres
East-facing slopes in San Juans > West-facing slopes in San Juans
East-facing slopes in Sangres > West-facing slopes in Sangres

Comparison of slope aspect across ranges is not so meaningful, i.e. (West-facing slopes in San Juans =?= East-facing slopes in Sangres) could vary depending on the variables of the most recent storm.

[Robbie Crouse]

That's really helpful, thanks! I'm not much a skier, so I've not paid much attention to amounts/type of snow before (this seems like something the skiers know more).

Do you think the comparison of Elks to Sawatch/Mosquite ranges is similar to the San Juans to Sangres? It doesn't seem quite as start, but the Elks do seem to get more snow overall.

[Dave B]

Pretty much all permanent snow fields and "glaciers" in CO are E through N facing (regardless of range), which indicates the greatest deposition and/or least ablation is on those aspects. W facing slopes are typically scoured pretty dry by wind in winter, but hold more snow from wetter/bigger spring snow.

An additional component to greater avy risk on N facing slopes is colder snow surface temperature from lower incident radiation. Colder snow surface == greater faceting and, especially in CO, greater risk of persistent slabs.

[Robbie Crouse]

Thanks Dave! I wondered if the "type" of sunhit would influence east/west slopes: morning sun for the east slopes and afternoon sun for the west slopes. Since the afternoon sun is more intense / has more heat, I wondered if that would cause snow melt on western slopes at a greater rate. It sounds like it, at least in the spring.

Even if there isn't a direct application, I really enjoy learning all this stuff! Anybody have a read recommendation on Colorado topography and weather patterns?

[angry]

CAIC is hosting an awesome online snow and avalanche workshop with live Q&A session Oct 14th, 15th and 16th. www.eventbrite.com/e/colorado-snow-and- ... 3526954176

[thurs]

Here's a resource that may help you, which builds on what a previous poster said about the rain shadow effect. Overall, large-scale snow totals in the Colorado mountain ranges are defined by the type of upslope wind directions from the various storms, and this favorable upslope direction is dependent on how many other mountains are upwind of it, and as stated previously, on a smaller scale, there is generally more snow on the eastern sides of mountains due to wind transport. If it snowed regularly, without long breaks containing high winds, there probably would be a little more snow overall on western aspects, but any difference is quickly obliterated by the wind.



This is why, for example, areas like Wolf Creek (in the southern San Juans) do particularly well during an El Nino year -- more storms come with a southwesterly track. This is also why the foothills of Denver, despite being east of the front range, can do a lot better with these storm tracks than the front range mountains themselves -- upslope is from the east.

[Barnold41]

That's a great resource, thanks for sharing.

[Jorts]

But here's my question, I had always thought that west facing slopes in Colorado got more snow. At, least that's how it generally works for the ranges (e.g., the San Juans get a ton more snow than the Sangres). And again, in regard to rain, valleys to the west of the mountains get more snow than valleys on the east side.

Any thoughts on this?

In addition to the sun hitting the slope later in the day when it's warm (less snow coverage on west aspects), prevailing winds being westerly (less snow coverage west), the sun being further south in the sky during winter (less snow coverage south if the slope isn't wind loaded)... We also have the issue of mountain ranges blocking each other.

The SJs are heavily favored by SW flow; the northern mountains are favored by NW flow; Steamboat does well with both. Westerly flow hits the Vail zone more than NW, and NW flow hits Summit more than Vail.

As a result of these dynamics, Summit County mountains frequently has more storm/wind slabs development on SE aspects during the storm and the SJs often have it on NE aspects. Dry snow blows around more in general so with cross loading and ever shifting wind gusts, wind slabs can pop up all over the place.

Tangentially, but wrt to stability... There's a greater temperature gradient in thin snowpacks and on colder aspects. So that's why we have depth hoar and other persistent slab issues most seasons, all season, on nearly every snow covered slope until spring. That depth hoar layer becomes less reactive later in the season with a deeper snowpack bridging it. Until critical loading makes it reactive again (as with the early March cycle 2 seasons ago).

Snow science is fascinating.

[Robbie Crouse]

Fascinating indeed! It's pretty fun to geek out on.

[Squirrellysquirrel]

Freedom of the Hills is an excellent reference, glad to see discussion in relation to snow pack loads. Really getting into snow science this winter... guess it’s a way to appease my backcountry itch regardless of whatever COVID19 policies shape up for the winter 2021 hiking season.

Somewhat related...?
The article below details new research about the “direction” that snow drifts “point”, which generally indicate direction for water resources! First of it’s kind, not sure if someone will try and duplicate results for validation, etc. Neat concept, wonder if it’ll be integrated into avy coursework?

https://eos.org/research-spotlights/a-c ... ern-alaska