From the Cliff Mass Weather Blog

Cliff Mass

Several of the rivers in and around Yellowstone Park experienced record-breaking flows this week and my podcast today tells the story.   And I also provide the forecast for the Northwest over the next week

As I discuss in my podcast, there are two main elements behind the Yellowstone flooding:  (1) the melting of an extreme snowpack and (2) heavy localized rain.   And to make this happen in such an extreme way, a lot of moving pieces had to come together.

The snowpack on June 11th, right before the flooding, was extreme, with the regional terrain having snowpack water percentages of hundreds of percent above normal (see below).  That means LOTS of water ready to melt.  This snowpack was the result of our cool, wet spring.

June 11th snowpack percent of normalWarming temperatures before and during last weekend resulted in substantial melting.
And then there was the rainfall.   A plume of moisture moved in from the southwest–an atmospheric river–and was directed right into the Yellowstone region, where a small area of enhanced precipitation occurred (see forecast precipitation for the 72 h ending Tuesday at 5 AM PDT).  Forecasts were excellent by the way.

This rain not only contributed to the river rise but helped melt the snow as well. Take a look at a few of the regional precipitation totals below:  impressive, with values as high as 3-4 inches over the weekend.

Another way to view the combined effects of precipitation and melting is to examine the observations at a USDA Snotel site, where both snowpack and precipitation are measured.

Consider the nearby Monument Peak, Montana location.  The accumulated precipitation is black and the amount of water in the snowpack is shown by blue. Early this week the black line went up quickly (lots of rain) and the snowpack plummeted.  A LOT of water became available for local rivers.

I end the podcast talking about whether global warming played a significant role in the flooding, something claimed by several major media outlets and some climate activists.

The evidence clearly suggests a minor role for global warming.   The huge snowpack was a major contributor and global warming generally REDUCES snowpack.  The position of key features, like the atmospheric river and a developing low over land, have no evident global warming connections.  

The planet has warmed by roughly 1 C during the past century, and let’s assume that humans are the sole cause.   1C increased would increase atmospheric water vapor by 6-7%, perhaps enough to slightly enhance the flooding, but nothing more.

The devastating event would have happened anyway.

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