Is This Why the Spokane River is Running Dry?

There were a couple of interesting articles in the Spokane news recently.  First, residents along parts of the Spokane River have noted that the river is actually going dry in some spots between Post Falls and downtown Spokane.  Here's a link to the Spokesman Review article.  This article does a nice job of listing a number of factors, including water management, heat, evaporation, drought, and the increased use of the Spokane aquifer, in part due to population increase.

A few days later, there was another article pointing out that Spokane officially had it's driest summer (defined as June-August) ever (records began in 1881).  Here's a table of the 20 driest summers in Spokane.

There's an interesting fact in this table.  Did you spot it?  Notice how many of the driest summers have occurred since 2000?  8 of the 20 driest summers have occurred in the last 25 years.  

Here's a graph showing the summer precipitation for each year at Spokane.  The smooth green line is a running average which helps to show trends.  It's pretty easy to see that summers have been getting drier in Spokane since about the 1990s.

These two events would seem to be related, right?  Driest summer ever = lowest flow on the Spokane River.  But summertime precipitation doesn't mean much to our river systems.  In Coeur d'Alene, summer rainfall is less than 15% of the annual precipitation. It's the winter and spring that give us the vast majority of our annual precipitation.

The Spokane River basin covers most of the central Idaho Panhandle.  The map below (courtesy of Kmusser) does a nice job of showing this basin.  Any raindrop or snowflake that falls in the light brown area goes into the Spokane River. 

Let's take a look at how the central Idaho Panhandle precipitation has been faring for the past few years.  First, we'll look at the entire 48 states for the past 3 years.  Note that this is the actual precipitation deficit, not a percentage of normal like we usually see.

See that really dark red in the Northwest?  Those areas are more than 20" below normal.  Granted, these are also the wettest areas of the lower 48 states.  But still, that's quite a deficit.  Here's a zoomed in view.

So it's really confined to western Washington, the Cascades, and the Idaho Panhandle.  What's so special about these three areas?  The answer: they get the vast majority of their precipitation from westerly winds.  The Cascades and Panhandle mountains are essentially north-south barriers.  So a due westerly wind gives them the maximum precipitation.  Hold on to that thought.  We'll get back to it in a minute.

So the next natural question to ask is this: how do the most recent three years compare to the archives?  Have we ever seen such a deficit in 3 years?

The government has a number of sensors in the mountains of the West to measure snowpack.  They've been in place since the early 1980s, so more than 40 years of data.  Here is the annual precipitation for 3 SNOTEL sensors in the central Panhandle Mountains.

All three graphs are pretty similar.  But there's an important fact in each of them.  All three sites have had less than 40" of precipitation in each of the past 3 years.  While there have been some drier years (like 2000/2001), they were single year droughts.  Since 1980, none of these sites have had 3 consecutive dry years like the most recent 3 years.  

And if you look at the graphs above again, you can make the argument that those sites have really been dry 6 out of the past 7 years.  Nothing close to that in the remainder of the dataset going back to 1980.

So can we look any further back?  There is another data set that for some locations goes back into the late 1800s.  This is known as the Cooperative Observation Network.  These folks take daily weather observations for free.  The program was started by the Weather Bureau and state climatologists.  And the volunteer observers were often times incredible for their reliability and attention to accuracy.  Only in recent decades has it become much more difficult to find such folks.  We're a more mobile society now that in the late 1800s.  As such, there's actually more missing data in recent decades than there was 100 years ago.  This causes problems analyzing the data.  But I'll show you the annual precipitation from 4 locations in the central Panhandle: Wallace, Potlatch, Kellogg, and St Maries.

As I said, some of the data is suspect.  So we can't make as definitive analysis as we did with the SNOTEL data.  But the general shape of the 4 graphs is similar.  We're currently in a drier period.  There have been other dry periods (usually the 1920s and 30s) and some wetter periods.  

So what's causing this drought?  Well, there's actually 2 things going on.  First is an overall drier weather pattern in the Northwest.  Here's the percent-of-normal precipitation for the past 3 years.

We can see the the entire area of Washington, north Idaho, and western Montana have been dry for the past 3 years.  So there has been a general drought in this area.  But the mountains we talked about earlier have been extra-dry.  So there's something else going on besides just general drought.

As I said earlier, the Cascades and Panhandle mountains rely on winter storms that deliver westerly winds to maximize the precipitation.  This often happens after a cold front passes by.  It will often be very windy behind the front, with no rain or snow over the lower elevations.  But at the same time, the mountains get pounded with heavy rain and snow in this pattern.  So has something changed?

Below is an image that may be a little hard to interpret.  So I'll try my best to describe what it shows.  It's the westerly wind anomaly for the past 3 winters.  If the color is blue/purple, those areas have had weaker westerly winds than average.  Areas with red/orange/yellow have had stronger winds from the west than normal.

From the image above, the Northwest US has had weaker westerly winds over the past 3 winters, while to the south there have been stronger westerly winds than normal.  What this means is that the jet stream has been pointing more at California and Oregon, and less at Washington during the past 3 winters.  That's why our Cascade and Panhandle mountains are so much drier than normal.

We can see this if we look at the percent of normal precipitation for the entire western US for the past 3 years.

While Washington and surrounding areas have had a drought for the past 3 years, California, Nevada and southern Oregon have had 3 wet years.  Remember in the news how California was getting so much rain that their reservoirs were impressively full once again?  These charts explain why.  The jet stream has been bringing heavy rain into California, and Washington has been missing out.  

Will this trend continue?  It's impossible to know.  It's worth pointing out that the last 3 winters have had 1 La Nina, 1 El Nino and 1 Neutral pattern.  So our best predictive tool for winter precipitation can't explain our recent drought.