On Friday afternoon, my housemate asked if I could dig up the weather records for precipitation for 2006 to 2007. Surely! I did, and stumbled upon a treasure trove of data—highs, lows, precip, snow and snow depth for every day back to the 1890s. Except for 1913 and the early 2000s, the data are pretty much complete.
This got me to thinking: what is a normal snow year in the Twin Cities? A lot of memories go back to the 1970s when there was a lot of snow, but what was the skiing like before then? Were winters always snowier than now? Or should we stop complaining?
Well, several hours and a 17 megabyte Excel file later (I'd be willing to share it if anyone is interested), I can provide some sorts of answers. During the average winter:
85 days have at least 1 inch of snow on the ground.
71 days have at least 2 inches,
61 days have at least 3 inches,
53 days have at least 4 inches,
37 days have at least 6 inches,
20 days have at least 9 inches,
12 days have at least 12 inches,
3 days have at least 18 inches and
0 (well, 0.3) days have at least two feet accumulated.
Another way to break it out is by the number of days with at least three inches of snow (which generally equates to good skiing). I have all these data by year (and by all, I mean all) but it'd be too long to post. But the decades break out as follows:
| Decade | 0-39 | 40-69 | 70-99 | 100+ |
| 1900s | 5 | 5 | 0 | 0 |
| 1910s* | 4 | 1 | 4 | 0 |
| 1920s | 3 | 5 | 1 | 1 |
| 1930s | 4 | 3 | 3 | 0 |
| 1940s | 4 | 4 | 1 | 1 |
| 1950s | 4 | 0 | 2 | 4 |
| 1960s | 3 | 3 | 2 | 2 |
| 1970s | 0 | 1 | 4 | 5 |
| 1980s | 2 | 3 | 2 | 3 |
| 1990s | 2 | 4 | 2 | 2 |
| 2000s* | 0 | 3 | 2 | 0 |
(* data not complete for 1912-3 or 2000-1 to 2003-4)
Now to some pretty pictures. First, from Charles Fisk's climatology site, we have a great chart of the mean and maximum snow depth, by calendar date (click to expand):
Fantastic. Here's another: percent of days, by calendar day, with at least three inches of snow on the ground (click to expand):
A few interesting things here. First, note the similarity to the trail-report derived chart from the past seven winters. If anything, winter seems to be coming later and staying later. Also just for some data points, there's a better than 50/50 chance of skiing on any given day from about Christmas to Birkie, and it's a pretty sharp drop on either side, except for some weird blip in early March, where there's a better chance of skiing on March 10 than March 3. The best stretch is January 12 to February 4, with a better than 70% chance of three-plus inches of snow. The single best days are February 2 and 3, which have each been skiable 75% of the time (with at least three inches of snow). There's probably a reason City of Lakes picked that weekend. And if we get to ski this week, we'll be bucking history: there's only a one-in-three chance of skiing on any given day in this week of December.
We can also look at different levels of snow depth. For instance, the year with the most days with at least one inch of snow on the ground was 1964-5, with 137 days.
With at least 2 inches, the winners were 1964-5 and 1981-2, with 131 days (this was the year when the record depth of 38 inches was set after storms dropping 17 and 21 inches two days apart in mid-January)
With at least 3 inches, the winner was 1981-2, with 130 days
With at least 4 inches, the winner was 1978-9, with 124 days
With at least 6 inches, the winner was 1978-9, with 121 days
With at least 9 inches, the winner was 1978-9, with 103 days
With at least 12 inches, the winner was 1968-9, with 84 days (amazingly, there were only 4 other days with at least six inches on the ground)
With at least 18 inches, the winner was 1966-7, with 45 days
With at least 24 inches, the winner was 1966-7, with 11 days
What's striking here is that all of these extreme years occurred from 1964 to 1982. And the data go for 105 years! So there was definitely a snow peak in the mid 1960s to early 1980s.
For low years, 1930-1 had just 19 days with an inch of snow, 1958-9 had but a dozen days with two inches, 1943-4 had only one day with three inches, and 1901-2 and 1930-1 were the only two winters in which four inches of snow depth was not recorded. The data are less telling here, and recent drought winters (2000-1 and 2002-3 come to mind, as does 2004-5) for which the data are not complete may compete, but if the bare ground is getting you down, just repeat to yourself: it could be worse.
These data, too, we can view as charts. First, we have a chart of the number of days with at least so many inches of snow on the ground (click to expand).
Next, we have the same data, but in 11-year rolling averages for each depth, by number of days with at least that amount on the ground (click to expand).
What's interesting here are the peaks and valleys in snowfall, smoothed over the years, as well as the overall, long-term upwards trend (although the 2000-2004 years might pull recent data down, even as 2001-2 would pull it up). There was a peak in the late 1940s and early 1950s, and then a major drop-off before huge winters from the mid 1960s through early 1980s pushed the average number of days with at least two inches on the ground over 100, from below 60. And look at the 6 inch contour—from under 20 to nearly 80, a nearly fourfold difference! The long term variation is really quite amazing.
In any case, there's no telling what the current winter will bring. We're a bit late on setting the one-inch-or-more record (although we do have October 13 in our back pocket) but, with a snowstorm in the offing, we hopefully won't fall in to the wall of shame. We'll take a look back in the spring when we revisit the climate series and look back on the winter that was. Until then, let the trail reports fly!
(And the usual disclaimer: IANAM—I am not a meteorologist. Or climatologist. Oh, and the Excel file is up to 23 MB.)
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