(Warning: Long post of limited general interest. You should probably turn back now.)
I have mentioned elsewhere that my family and I are coming to explore Glacier in the second half of June, despite a known risk that the Sun Road may not be open when we’d like it to be, or even by the end of our trip. It has been volunteered that this has been a relatively heavy snow year so far which may not mean much but certainly doesn’t seem like it can be a sign of an early opening.
It got me wondering if the opening date could be predicted based on something like recorded snowfall, without any announcement or guidance from the park service. If so, how early and how accurately can such a prediction be made?
(This is a good time to point out that we will no doubt have an excellent trip whether or not we get to stand on Logan Pass. This post is driven by intellectual curiosity and my penchant for numbers geekery.)
I found an April 2015 article in the Flathead Beacon which at least implied that a correlation indeed exists between snowfall and the road opening. Further, it pointed me at a wonderful, publicly-available statistic: the Snow Water Equivalent (SWE) measurements from the Flattop Mountain SNOTEL site. SWE, as I understand it, is a calculation based on both the depth and the density of the current snowpack. A little further digging assured me that there is no more appropriate SNOTEL site than Flattop Mountain from which to gather data.
So I gathered data: SWE numbers for the first day of each month from January to June for the last 15 years. I already had the Sun Road opening dates for those years. I looked at the data from several angles and certainly saw a general correlation between SWEs and opening dates but nothing real definitive.
To make a long story medium, I got better results once I decided to make my question much more specific (and much more Paul-centric). My new question was this: Based on SWE measurements, can I predict whether this will be a “good” year (Logan Pass opens by June 21), a “bad” year (Logan Pass opens after June 26), or a “so-so” year (Logan Pass opens some time in between)?
Looking at both discrete SWE numbers and the change in SWE over certain periods, I found some pretty decent correlations and created some matrices that support making predictions on March 1, April 1 and May 1.
(Tagging back to the first paragraph, is there no correlation between January snowfall and the subsequent road opening? Yeah, there is a little, but just a little. In fact, neither of the two highest January SWEs in my data occurred in “bad” years and neither of the two lowest January SWEs occurred in “good” years. The 2017 measurement of 19.9 inches is the third highest of the last 16 years.)
Anyway, below are my matrices, in which I tallied the timing of the opening against various SWE measurements for 15 years. I was trying to find SWE values for which my tallies would end up in the upper-left or lower-right corners of each matrix.
What does this mean? Let’s look at how this would have worked in 2016 as an example. (The 2016 data would not have been in the matrices then, but that doesn’t change the story much.)
Here’s how 2016 would have played out:
• January 1: SWE of 17.7, slightly below average but we know that doesn’t mean much.
• March 1: SWE of 31.7. This is less than 35 so we’re already feeling pretty good that it won’t be a bad year and we have about a two-thirds chance of seeing a good year.
• April 1: SWE of 41.8, an increase of 24.1 from January 1. This increase is less than 30 so we have roughly the same conclusions as we did a month ago. There is a slightly stronger indication of a good (as opposed to a so-so) year.
• May 1: SWE of 38.3, a decrease of 3.5 from April 1. The discrete measurement is less than 50, so again we think it’s most likely going to be a good year and almost certainly not a bad one. The change from April is less than a 2 inch increase, so that table tells us to expect roughly the same thing.
The Sun Road opened on June 19, 2016 – a good year on the Paul-centric scale, as the matrices predicted.
As would any self-respecting numbers geek, I will certainly update this thread as 2017 unfolds.