May 13, 2012

Snow in the Cascades, continued

Filed under: Hiking, North Cascades, Weather — geezerwriter @ 12:01 pm

I started examining the snow depth situation in the North Cascades last week, and just couldn’t let it go. I had noticed that the snotels gave the water-equivalent of the snow on the ground, but my first thought was that we wanted to know the actual depth of the snow and that the water equivalent, while more useful for agriculture and Seattle City Light’s summer planning, would be kind of abstract and even a little precious for our purposes.

But I got about halfway into patching all the missing data in the snow-depth table when I noticed that the water-equivalent data didn’t have all those gaps. This makes sense to me since it seems that measuring snow-depth remotely would be a harder problem than measuring water-equivalent, just the opposite of the situation that would obtain if we were present on the site. How would you locate the top surface of the snow? A very tall pole with some sort of automated range finder looking down on the snow? On the other hand, for the water-equivalent you would just need to get the weight of the snow (OK, it’s the mass, but I’m not a physicist) above a certain area and do a little calibration and arithmetic – a suspended plate and a pressure sensor would do the trick.

It also makes sense that the WE would be more stable – a large, powdery snowfall can bump the SD way up suddenly, but it would settle back down in a few days. By spring we are looking at well-compacted snow.

The WE data may indeed be of higher quality than SD, but it not what we are looking for! [I’m somehow reminded of the old story of the drunk crawling on the ground under a streetlight searching for his car keys which he had dropped while fumbling to open the door. When a passerby pointed out that the car was half a block away, the drunk said, “Yeah, but the light’s better here.”]

So I looked more closely at a couple of years’ data. First I did a quick check of the ratio between SD and WE and found that it generally started the snow year at 10 or 12 to one (consistent with what I’d always heard that an inch of rain falling as snow will pile up a foot or more deep) and tapers down irregularly to about 2 to one by the spring (as the air is pressed out by the weight of the overlying snow). And we don’t care much about the absolute values, just how fast the snow is disappearing, so a ratio is OK.

The one value we do care about is zero – when is the snow all gone? Everywhere I checked both numbers hit zero at about the same time and that pretty much settled the issue. So here is the graph of all ten years of operation of the Middle Fork Nooksack snotel:

Note, if you will, how all the curves but two crash into horizontal axis in early July (actually June 26 to July 12). (May 31, 2005 and August 12, 2011 are the two outliers.) As before, I took the average of all the years except this year, last year and 2005 and got this picture:

On the snow-depth graph on the other posting, this year’s blue line had already crossed below last year’s green one and seemed like it might join the average group. This water-equivalent graph is less encouraging: this year peaked higher and earlier but looks like it is trying to cross 2011 – will it drop quickly or hang up there for another month? The current warm weather is a good sign (it reached over 60°F at the snotel yesterday).

My best guess is that unless we get some really toasty weather (or, even better, some nice warm rain) it will just about split the difference between last year and the average. This is also not inconsistent with the fact that the North Cascades Highway opened Thursday, about two weeks earlier than last year.


I poked around and found some info about the sensors used on the snotels. I understood hardly anything but there was a reference to a “200 inch Transducer” , suggesting that perhaps it was not a coincidence that this year’s snow fall seemed to max out right at 200 inches, but in fact overshot the sensor’s capacity.


1 Comment »

  1. Okaaaayyy, looking at this it seems like we will be on track to be about two weeks earlier than last year getting into the High Country, but not as early as in a “normal” year. Am I right? We can still go up there and traipse around on the snow, which I suspect we will be doing anyway, being bored with the Chuckanuts… exercise is exercise, and I’m all for whatever we get, whenever…

    Comment by DJan — May 13, 2012 @ 6:02 pm

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