Under the Weather: Fall storms get off to an early start with an atmospheric river


Autumn has only just begun, but we’re already expecting the stormy weather that we’re accustomed to seeing this time of year.

On Wednesday, an atmospheric river is expected to take aim directly at the Pacific Northwest, bringing with it some significant rainfall.

What is an atmospheric river? I wrote an article about this phenomenon and you can read it on my blog here. Make sure to come back to this page to read more about this week’s particular event!

The photo above is a visible satellite image for late Tuesday morning. You see the swirl of clouds just offshore? That’s the first system headed our way.


Above is the “column-integrated water vapor” product from the UW Weather Research and Forecasting (WRF) model, run by the University of Washington Department of Atmospheric Sciences. The model shows the amount of water vapor (the gaseous form of water) in a vertical column of the atmosphere if all that water were condensed into liquid water. This image is valid for late Wednesday morning. You see that train of reds, whites and purples aimed directly at the Pacific Northwest? There’s the atmospheric river. And it’s bringing some significant rainfall into the region.

Below you can see the total amount of rainfall that the American Global Forecast System (GFS) model is predicting through 11 p.m. Wednesday night over Western Washington, with the arrow showing our approximate location. This specific model is suggesting that between now and that time, we could receive about 1.5 inches! That’s quite significant. Considering other model outputs, we should expect about 1-1.5 inches on Wednesday alone.

However, the amount we receive will depend on the exact direction that this plume of moisture comes from. In the image above, do you see that area of blue? That indicates reduced precipitation due to the Olympic rain shadow. (Read more about that here.

This atmospheric river event is coming at us from the southwest, which puts areas such as Port Townsend, Oak Harbor, Coupeville and the San Juan Islands in the rain shadow. If this direction changed slightly to come from the west, it’s possible that our region would be included in this rain shadow, which would reduce our total amount of rainfall.

The main atmospheric river event occurs on Wednesday, but rain will still be falling for much of the week. Through Friday, we could exceed 2 inches of total rainfall.

But that’s not all. Models are showing that another atmospheric river is heading our way this weekend, but this one is coming more from the west, as shown below.

As I mentioned before, this would likely put our area in the rain shadow, which would reduce the amount of rainfall we receive. I’ll give an update on this second atmospheric river on Friday.

To get an idea of how much rain could fall over the next week or so, take a look at the GFS model output below over Western Washington through 11 p.m. Sunday. Some areas on the coast and in the mountains could exceed 9 inches of rain! That’s insane.

Fall is really making a dramatic entrance, isn’t it? After all the heat and smoke that we’ve had recently, I’m welcoming the wet weather with open arms. Are you happy that fall has arrived? Let me know in the comments below!

Now, here’s the forecast for the next few days. Along with the rain, we’re expecting gusty conditions. These winds could blow weaker branches and leaves off the trees. Those leaves can fall into storm drains and gutters, which would prevent the rain from properly draining. Make sure to keep an eye on storm drains and gutters and clear any blocked ones ASAP. Behind Wednesday’s system, there is a chance for some isolated thunderstorms on Thursday. When thunder roars, go indoors!


— By Kelsie Knowles

Kelsie Knowles is a meteorologist and recent University of Washington graduate who lives in north Lynnwood. After writing weather blogs as a KOMO News intern, she discovered a passion for writing about weather. You can learn more in her blog www.wxnoggin.com and you can also follow her on Twitter at @kels_wx3.


Leave a Reply

Your email address will not be published. Required fields are marked *

Real first and last names — as well as city of residence — are required for all commenters.
This is so we can verify your identity before approving your comment.

By commenting here you agree to abide by our Code of Conduct. Please read our code at the bottom of this page before commenting.