September 22, 2020

Heavy Rain and the Beginning of Autumn

Autumn began at 6:30 AM this morning and the atmosphere is clearly watching the calendar.

Tomorrow will bring the first of a series of wet weather systems,  each associated with a potent atmospheric river,  that will bring as much as TEN inches of precipitation to the northern Cascades and Olympics.   

As a result, wildfire season in Washington State will be coming to an abrupt close.

Precipitation will be heaviest during two periods, one tomorrow and the other on Saturday, when atmospheric rivers--narrow currents of large water vapor values--will make landfall on our coast.  

Here is the forecast atmospheric moisture content at 8 AM tomorrow (Wednesday), with the blue and white colors indicating the largest water vapor values.  When the water vapor current rises on our local terrain it is forced to condense, resulting in bountiful precipitation.

Even more impressive is the atmospheric river on Saturday is serious!  Fascinating how moisture is concentrated into such narrow "atmospheric rivers".  I will explain why in a future blog.

Get out your umbrella and rain jacket--you will need it.  To demonstrate that, I will show you the precipitation over the week in 24-h chunks.

For the 24h hours ending 5 AM Thursday, you can see the results of the first atmospheric river, with up to 5 inches in the Olympics and north Cascades.  Even eastern Washington gets substantial rain...which is very good.

The next 24 (through Friday morning), still substantial amounts!

The next day continues the rain.

And the 24h ending Sunday at 5 AM is even wetter.

Over the past six months, Seattle has had nearly normal precipitation (see below, blue is normal, purple is observed)

While the last three months has been drier than normal, by about 2 inches.  I suspect we will make up that deficit during the next week.  The good news is that watering season is over, with substantial savings in expensive-water Seattle.

And there is someone else that will be very happy:


My latest podcast:


  1. I know you are predominately WA focused, but What about Oregon, since they were hit harder by fires than we were.

  2. I hope that the rain shadowing effect with this front is not so strong as to prevent significant precip occuring on the east slopes of the Cascades,which badly need it.I notice that models are predicting upper heights at or above 590 Hpa by next Tues and Wed.Perhaps some record heat to close out the month?

  3. I am not a reincarnationist, however, I am putting in a request to come back as a mallard, just in case. Thanks for the offer to audit your class; I took a class in atmospheric science at the UW back in the early 1970s & have your book 'The Weather' which is very comprehensive so am standing pat for now.

  4. Finally! Hopefully there will be some left over for Oregon and California.

  5. This is spot-on! We're getting hammered in waves near Mt. Baker, with well over an inch since 7 am as of this writing (about midnight), and I expect others are getting even more.

  6. Looking at the forecast this morning, it looks like the atmospheric river is coming in sooner, on Friday rather than Saturday. This is good news for those looking to go mushrooming on Saturday in the Olympics.

  7. I am annoyed at the warm, dry spell coming next week. These storms are great and all, but I would trade them in a second for consistently cooler, wetter fall weather.

  8. Professor Mass, I'm curious to learn if this river of moisture is related to the tropical storm activity in the Western Pacific. Japan in particular seems to have been subjected to more than normal typhoon activity this cycle.

  9. It looks like the weekend is a bit drier now with the weather system being pushed north into BC in the latest models.

  10. It’s not the water that’s expensive; it’s the watersheds, the reservoirs, the salmon, the pipes, the pumps, the valves, the treatment plant, the inspections, the monitoring, the maintenance.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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