Saturday, February 14, 2015

Late Winter Snow Depth

A few days ago the Iditarod's board of directors voted to move the (re)start of the race from Anchorage to Fairbanks, as in 2003, because of a lack of snow along some rugged parts of the trail in the Alaska Range.  I couldn't help noticing a comment by race director Stan Hooley, reported by the News-Miner as "I think we’re all worried about the weather [...] There’s a pattern that’s developed. And that’s a concern to all of us."  In view of this, it's of interest to look at the snow depth data from several first-order observing stations to see if there is any evidence of changing snow conditions in recent years.

The charts below show the mean snow depth in February and March for several locations with a long history of reliable data.  The first chart shows the data for Anchorage, McGrath, and Nome (along the traditional Iditarod route), and the second chart pertains to Kotzebue, Bethel, and King Salmon (for comparison purposes).

The first thing that jumps out is that there's no evidence of systematically lower snow depth in recent years at these locations, and in fact quite the opposite is true; on average, snow depth has been higher in the last 20-25 years in Anchorage, Nome, and Kotzebue, and slightly higher in McGrath.  The averages for two consecutive 30-year periods and for the most recent decade are shown below.  It's worth noting that at Nome, where the increase has been rather dramatic, the difference between the two 30-year periods is not statistically significant; however, the difference between 1955-1984 and 2005-2014 is very statistically significant, according to a Mann-Whitney U-test.

The second interesting aspect is the marked increase in variability of the snow depth at McGrath, Nome, and Kotzebue (especially the latter).  See below for a summary chart.  At McGrath, despite the fact that snow depth has increased on average, 4 of the lowest 9 snow depths have occurred since 2003, along with 3 of the top 9 snow depths.  The situation is more extreme in Kotzebue: of the past 11 years, only 1 (2007) did not fall in the top or bottom 20% of the overall historical distribution.

Although this is only a cursory look at historical snow data, the results suggest that perhaps the problem for sled dog racing in Alaska may not be a lack of snow in the mean, but too much variability from year to year.  Typically one thinks of climate changes primarily in terms of shifting the mean (or median); but this appears to be a rather clear example in which changes of variance have been very large - and possibly more significant in terms of impacts for human and animal activity.

[Update February 24: reader Andy mentioned the snow shadow on the northwest side of the Alaska Range and asked about snow depth data from Farewell Lake.  The chart below shows the distribution of daily snow depth for 17 winters at that location; the median is between 5 and 10 inches for much of the winter, and very low snow is not at all uncommon in February and March.]


  1. It's worth noting that the start in Anchorage is ceremonial. The race stops shortly after the start. The race actually begins (except for this year and in 2003) in Willow about 50 miles north. And since this race is starting in Fairbanks most of the trail isn't the same as other years.

  2. Sorry I'm late to this post, but as an Iditarod veteran and close watcher of race trail conditions, I think it's also worth noting that the problem areas of the trail (the leeward side of the Alaska Range) are always relatively snowless and rough. There is a distinct snow shadow in the winter extending from Rainy Pass most of the way out through the Farewell Burn toward Nikolai. What's most discouraging to me is that the Fairbanks option is realistically "safer" every year, and now the Board will have to figure out which years in the snow shadow are worse than others. There used to be a weather reporting station at Farewell Lake... I'm curious if they had any sort of snow depth data over the long term.

    1. Andy, thanks for the comment and for pointing me to the Farewell Lake data. That station has essentially complete snow depth data in February and March from 1986 through 2000 and then for 2004 and 2007. In those 17 years, the mean Feb-Mar snow depth was 7.7". However, the mean snow depth was less than 4" in 8 of 17 years. So this very much supports the idea of a snow shadow and indicates that poor race conditions are nothing new in that area.

      I'll see if I can post a chart later today.

      By the way, congratulations on your Iditarod experience!

    2. I posted a chart of the distribution of snow depth through the winter at Farewell Lake.

  3. Thanks Richard! Interesting to see how there is a dearth of snow in February. I think the race has lucked out a few times on that stretch of trail, which they usually cover the first week of March.