Saturday, April 9, 2016

Snowpack Meltout

Despite a few hours of accumulating snow yesterday morning in Fairbanks, today's climate report shows only a "trace" of snow on the ground, which means the winter snowpack has officially melted out.  In practice it means that less than half of the ground area was snow-covered at midnight last night in the vicinity of the upper-air observation site at the airport.

The chart below shows the date of meltout for Fairbanks since 1930.  As Rick Thoman noted on this blog at meltout 5 years ago, there is no long-term trend.  There have been decadal-scale variations, such as periods of early meltout in the early decades and late meltout around the 1960s, but recent decades have not been unusually early on the whole.  The only possibly noteworthy aspect of the past 20 years is how few years have been very anomalous: only 2 years have seen meltout before April 16, and only 2013 was later than May 2.

The absence of a long-term trend is surprising in view of satellite indications that Northern Hemisphere spring snow extent has decreased in recent decades.  See, for example, the following articles:

It's also surprising that Fairbanks meltout dates have remained steady when we consider that spring temperatures increased substantially in the 20th century (with the main increase occurring at the 1976 PDO phase change).  The chart below illustrates this and also provides at least part of the answer for the meltout mystery: late winter snow depths have also increased over time.  It seems that increasing temperatures have been offset by greater snowpack depth, and so there's been no significant change in the timing of spring snowpack elimination.

1 comment:

  1. Despite the warm temperatures, you still need solar radiation to make a big dent in snow depth. If it's more cloudy, the snow will melt slower.

    Also, it seems like the minimum temperature recently has been dropping below freezing more often than normal. This winter has had very stable temperatures so this wouldn't be surprising. I think tracking the minimum temp (in relation to freezing) trend might give a small portion of the puzzle.