Saturday, October 10, 2015

Precipitation Trends

The recent wet September prompted me to look again at long-term precipitation trends in Fairbanks, especially in light of the hypothesis that increasing precipitation could be responsible for the trend towards earlier autumn snowfall.  In a post earlier this year, I showed that Fairbanks has become slightly drier over time (1930-present), but the only calendar month with a statistically significant trend is August, which has become drier at the 99% level of significance.

To look more closely at the variation in trends through the year, I calculated 30-day running precipitation totals and obtained a linear trend (1930-2014) for each day of the year.  I did this both for the actual precipitation amounts and also for the ranks of the values within the 85-year distribution.  The reason for this is that a single outlier year could throw off the least-squares trend calculation for the raw precipitation amounts, but the ranks won't suffer from this problem.  The chart below shows the results.

We see that most of the year (about 70% of it) has become drier over the 85-year history, regardless of whether we use the precipitation values or ranks.  The drying trend in August is very noticeable, as is a trend towards wetter conditions in July.  The rank trends show some other interesting peaks and troughs at various times of the year, some of which are no doubt caused by random sampling variability.

It's interesting to note that most of the autumn has become slightly drier, so the slight moistening trend in the month of September is an anomaly for the season as a whole.  The chart below shows the September precipitation totals since 1930.  We see that the early years (about 1930-1960) were relatively wet, then there was a drier spell in the 1960s and 1970s, and it has been wetter again since about 1990.  Thus when reader Gary comments that "late August to late September used to be the Golden Month", he might be recalling the drier decades in the middle part of the history.  The overall trend over the entire period is quite small, although it's definitely positive; note how few Septembers have been very dry in recent decades.

The chart below shows the number of days each September with daily precipitation amounts at or above 3 different thresholds.  As I noted in the earlier post, the frequency of 0.25" daily precipitation has increased quite a bit since 1990.  Smaller precipitation amounts have actually become slightly less common over time.  It's interesting to note that in the first 30 years (1930-1959), only 9% of September precipitation days produced 0.25" or more, whereas in the past 26 years (1990-2015), this ratio is up to 17%.  Changing Pacific sea surface temperature patterns almost certainly bear some of the blame for this, but an examination of the details will have to wait for another time.


  1. My recall of 'The Golden Month" noted above dates from 1965-on, but oral history of friends, and Chart 2, suggest it was an interlude preceded (and now followed) by wetter more inclement periods at least for Interior Alaska.

    It now seems like October is the new September with relative warmth, sun, and pleasant weather conditions. The large relatively deep ponds I've seen around Fairbanks are as of yet unfrozen.

    However observations today at the Fairbanks International float pond indicate some freezing in near-shore margins and surface water temperatures slipping into the mid-30's. A few cool, clear, windless nights and the ice will soon form.

    Large lakes in the drainage will remain uncovered with ice until November if this current weather pattern continues.


    1. Thanks for the additional detail and local observations, Gary. Indeed temperatures have become much warmer than normal and appear likely to continue that way for a while.

      The first 10 days of October were warmer than the last 10 days of September in Fairbanks this year; this has happened 7 times since 1992 (7 of 24 years), but only 5 times from 1930-1991. Not once in the 1960s and 1970s!

    2. Here's an online or in person Webinar 10/16/15 regarding this coming winter is available here:

      The CPC is painting their Climate Outlooks maps a red hue for Alaska:


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