Back to Rain

After a remarkable spell of warm and dry weather to close out summer, rain returned to Fairbanks earlier this week, pushing August's rainfall total to 2.08"; and this means that all three summer months have been wetter than normal.  Perhaps surprisingly, even in the wet regime of recent years, only 2014 achieved that distinction, although 2017 was close (July 2017 just missed the mark).  Other years were 1930, 1932, 1948, 1955, 1962, 1990, and 2008, although we perhaps shouldn't compare earlier decades to the current standard "normal" of 1981-2010.

Before saying a few more words about rainfall trends, I'll just make a note on the recent warm spell: with 11 consecutive days (Aug 13-23) having a high temperature above 70°F in Fairbanks, this is actually the longest such spell since at least 1930 for so late in the season.  However, it's a long way from being the greatest late summer heat wave, as other years reached into the 80s with warm spells of similar duration.  For instance, 1950 saw highs in the 80s on 4 of 5 days from August 19-23, with most days above 70° for two and a half weeks.  August 2004 had 10 days of 80°F or above (the last on the 20th), and August 1977 saw an amazing 87°F and 86°F on the 21st and 22nd respectively.

Returning to rainfall, I got to wondering whether the persistently high rainfall of recent summers has a clear signature in the duration of rain: is it raining more of the time, or is the rain just heavier when it occurs?  Here's a quick analysis to address the question: the chart below (click to enlarge) shows the fraction of Fairbanks hourly (top of the hour) observations reporting rain since 1973.  I'm showing June-September here, because these are the months with by far the greatest increase in precipitation since 2014 (see this recent post); and here we're looking at observations of rain, not snow.  There's a clear signal for more frequent rain in each of the last 7 years, so we can say that yes, it has been raining more of the time in summer and early autumn since 2014.

 

Interestingly, however, the number of days with measurable precipitation has not increased as obviously; it's up this year, but 2018 and 2019 did not see an unusual number of wet days, and nor did 2014.

 

Here's the total June-September precipitation, illustrating again the sustained nature of the new wet regime.

 

So what do we have in summary?  Since 2014, it has been raining more of the time in summer and early autumn, and the frequency of heavy rain events has gone up considerably (as shown in the earlier post), but the number of days with rain hasn't changed much.  So when it rains, it rains for longer and comes down harder, but the frequency of dry versus wet days is not substantially different.