Tuesday, May 8, 2018

Snowpack Season

The winter snow cover melted out on Friday in Fairbanks, according to measurements by the National Weather Service at the airport.  This is somewhat later than normal, which is not surprising in view of the healthy snowpack at the end of winter and the rather cool temperatures of late.

The chart below shows the 88-year history of dates when the continuous winter snowpack began and ended in Fairbanks.  The variance of dates is greater for snowpack onset, but that's nearly all because of two major outliers in 1934 (the great December chinook) and 1992 (early arrival of winter).

The dashed lines show long-term linear trend lines for the two series of dates, but instead of least squares regression I've used quantile regression, with each trend line showing the best estimate of how the median has changed over time.  The reason to go with quantile regression is to avoid an undue influence from outliers, which seem likely to be a problem here, especially for the snowpack onset dates.

It's interesting to see that the regression estimates indicate that snowpack onset has become earlier by nearly a week, and meltout has become slightly (about 3 days) later, over the 88 years.  But it's important to note the degree of uncertainty: the 90% confidence interval includes a zero trend for both series, so neither trend is statistically significant; we can't confidently rule out the possibility that the trends are just a reflection of random chance.

One thing we can say with confidence, however, is that snowpack meltout has not become significantly earlier over time, and this is intriguing.  We know that April - when most of the melting occurs - has become significantly warmer, so how is the snow not disappearing more quickly?  There might be a variety of explanations involving changes to sunshine or precipitation during melt season, but the simplest explanation may be the best: it seems there is just more snow on the ground these days.  The chart below shows the snow depth in Fairbanks on April 1st each year, and there's an upward trend that is significant at p~0.05.

More investigation will be required to determine whether and how this trend is related to precipitation changes during winter; my impression from previous work is that Fairbanks winter precipitation has NOT increased over the long haul, so it seems this is a rather interesting question.


  1. This year (and perhaps others with a later than Median snow melt) we experienced recent cold spells, associated clouds and a resulting lack of sun (less insolation), and periodic light snow cover that may have influenced albedo and air/surface heat exchange into the frozen snow pack.

    The lingering snow column this year in Fairbanks was wet from intermittent winter rains that later froze into a possibly sealing layer, and dense in places from winter winds compacting the accumulated mass.