Monday, August 31, 2015

Does this mean summer is over?

Our hot summer came to a screeching halt on Sunday with a Pacific cold front bringing our first widespread rain in what seems like months.  The forecast for this week is decidedly cool, with daytime highs in the 70s and lower 80s.  And one look at the calendar begs the question:  Did summer just end without any warning?  Kinda felt like autumn on Sunday didn't it?  And we often say that on average, summer in the Inland Northwest is between Independence Day and Labor Day.

It's hard to define when one season ends and another begins.  But of all 4 seasonal transitions, the summer-to-fall transition is typically the most marked.  A marked transition doesn't happen every year.  Some are more noticeable than others.  The most impressive "autumn arrival" was probably 1959.  Here's a plot of the temperatures for that summer and fall in Spokane:

On September 12, the mercury reached 90F in Spokane.  A cold front moved in, dropping the temperature to 83F on the 13th, 65F on the 14th, and only 51F on the 15th.  But what's most impressive is that after that, the temperature never warmed above 67F for the rest of the autumn.  In other words, Spokane said good bye to the 90s, 80s, and 70s all with one cold front.

At this point we're not expecting a dramatic repeat of that event for 2015.  Spokane averages one day in the 90s in September and 7 days in the 80s.  The record is 10 days in the 90s, back in 1938.  And no, Spokane has never hit 100F in September.

Looking at the warmest 8 summers (based on June, July, and August average temperatures), only one (1958) had a normal September.  The other 7 were all much warmer than normal in September.  

Year Jun Jul Aug Sep
1922 +7.6 +3.2 +2.2 +3.1
1938 +4.5 +5.0 -0.9 +8.2
1940 +6.0 +2.8 +1.9 +5.1
1958 +3.3 +3.0 +4.9 -0.9
1967 +1.2 +0.6 +5.7 +5.7
1998 +0.2 +5.3 +3.0 +5.5
2013 -0.6 +3.9 +3.5 +3.4
2014 -0.6 +5.7 +3.5 +3.5
2015 +9.1 +4.2 +4.2 ???

The folks at the Climate Prediction Center are also favoring a warm and dry September.  Here's the outlook they produced today:

It calls for below-normal precipitation with above-normal temperatures for the month of September.

We typically look at a model called the GFS for forecasts of the weather for the next 7-14 days.  But when we want to look farther into the future, we look at the CFS (Climate Forecast System).  So let's see what it says for September.  First, we'll look at the CFS that was run back on August 10th.

The yellow and orange colors indicate areas where the model thinks temperatures will be above normal in September.  Large area of abnormal warmth over Alaska and Canada, spreading into most of the western US.

Now let's see the CFS from August 20th:

Notice the change.  The model still expects warmth in the West, just not nearly as strong or widespread.

Now here's the latest run of the model, which was fed the data from August 20-30:

We're back to a large area of abnormal warmth over Alaska and Canada, but only a minor area in the northern Rockies.  

The folks at CPC look at this (and a lot of other data) and concluded a warm September for the West.  So don't put away your summer clothes just yet.


  1. Well, this year we need a cold September. After all, the fires are still ragins all over the states of Washington, Idaho and Oregon. I would love to see the temps drop straight down to zero!