Are Our Dry Seasons Getting Drier?

Back in July during our hot stretch of days, I looked to see if there was any trend in how hot our summers were.  You can view that blog here.

In that article, I also took a quick look at precipitation trends for the the summer, that is, for June, July and August.  But given our long wait for autumn rains this year, I wanted to take a little closer look at the warm season rains.

As we've discussed before, June is really a springtime month in the Inland Northwest, although there have been some recent exceptions.  Our dry season is definitely the three months of July, August and September.   See the green bars in the graph below.

And each year it's a common question: when will the fall rains arrive, ending the wildfire and dust seasons?  Sometimes that's a widespread event. But most years are like this year.  Some areas received rain, while others are still waiting.  Here's the map of yesterday's rainfall.

So has there been any trend with precipitation in our dry season?  The answer is most definitely: yes.  Here's the July, August and September rainfall for Spokane since 1881.

Again, the blue line is the rainfall for each year, while the green line is a smoothed average.  Of course there are wet years and dry years, but the smoothed average was pretty consistent from 1881 until about 1990.  The average for those dry season months was right around 2" of rain.  But since around 1990, there has been a sizeable downward trend in dry season rainfall.  The current smoothed average is just shy of 1" for those three months.  That's less than half of what it was for over the first 100 years.

Of course, this has obvious implications for wild fire as well as blowing dust.  But for farming, it could be a game changer depending on your crop and location.  Our forefathers saw multiple years with over 3" of rain during these months.  The last time we saw that at Spokane was thirty years ago in 1995.

Taking a look at Moscow, ID, the trend is even more stark.

After a fairly wet start in the early 1900s, there was a marked dry period in the 1920s and 30s.  But then things started to gradually get wetter.  By the 1970s and 80s, the dry season in Moscow was wetter than ever.  But once again around 1990, things started to change.  Now, the average rainfall for the dry months is Moscow is about 1.5", compared to 3.7" back in the 1980s.

Digging in a little deeper, I took a look at each month separately.  While all three months showed the trend, the most dramatic was the month of July.  Here's the July rainfall for Spokane.

During the 1980s, you could expect an inch or two of rainfall in most July's.  Now?  Don't count on more than a quarter inch.

Climatological trends like these are interesting because we don't always understand their causes.  Clearly the Inland Northwest was dry during the 1920s and 30s, while the 1970s and 80s were rather wet.  But todays dry season is drier than anything we've seen since measurements began in the 1880s.

As for our upcoming weather next week, looks like we return to the warm and sunny 70s.  By the end of the week there will be a renewed chance of rain.  The main storm track will once again be from eastern Oregon, across the southern ID panhandle and into western MT (as it was for most of September).  Below is the chance of rain on Friday.