September 15, 2021

Heavy Rain and Rapidly Rising Rivers: The Details on the Friday/Saturday Inundation

If you were planning to take a hike on Friday, forget it.  Thinking about painting your house?  No way.

It is going to be very, very wet on Friday and early Saturday, and I suspect some daily precipitation records will be broken.

But let me give you the details.

Let's start by showing you the latest accumulated precipitation totals forecast by the European Center model for the period ending 5 PM Sunday.  Virtually all of the mountainous areas of the region will get more than 2 inches, with some being hit by 5 or more (particularly the Olympics and North Cascades).

The UW forecast model, driven by the U.S. GFS global model,  for the period ending 5 PM Saturday, has a similar distribution. 3-5 inches in the mountains will be extensive, with some "favored" areas being wetted by 5-7 inches.

How much confidence should you have in this forecast?  An important question.

We are close enough in time, that the models should be relatively locked on to the correct solution, but as I have discussed many times, the best way to get at uncertainty is to look at ensembles of many forecasts, each slightly different.

So take a look at the fancy presentation of the ensemble precipitation forecast from the top-of-the-line European Center ensemble system (below).  You are looking at accumulated precipitation at SeaTac Airport over time (time increasing to the right).

The average of all the ensemble members is shown by the green line and the extreme range of the forecasts (the highest and lowest forecasts) is shown by the brackets.  The blue boxes show you the forecasts that encompass the forecasts relatively close to the median (the median is the value in which half the forecasts are greater, half are less).

Wow... an amazing forecast, with the ensemble going for around three inches over the entire period.   

Considering that the average MONTHLY rainfall at Seattle is about 1.6 inches, that is a lot.   The spread of the forecasts is relatively small for the big wet day (Friday) there is great confidence that Seattle is going to be very, very moist.

The rain shield will arrive in the western interior around sunrise Friday and the heavy rain will extend into Saturday morning.  Cool and showery over the weekend.

Now let me emphasize that there will be considerable horizontal variation in the rainfall.  To show you this better, here are the high-resolution forecasts by the UW model of total precipitation for the 48-h ending at 5 AM Saturday.

Lighter amounts (less than an inch) to the northeast of the Olympics..that is the rainshadow effect to the lee (downstream) of the high terrain of that barrier. More rain south of Seattle than north of the city.  Big increases on the windward side of the Cascades...and, of course, less rain to the east of the Cascade crest.

So head to Port Townsend or southern Whidbey Island if you want to escape the worst of it.

The moisture that will hit on Friday is already streaming towards us from the western Pacific (see arrows in the latest water vapor satellite imagery below).

Two more days, it will be here.

The impacts of the precipitation on our rivers will be profound. 

 Right now most of the west-side rivers are below normal to normal.   But they all will be above normal by Sunday.  

To illustrate, here are the current and predicted flows from the NOAA/NWS River Forecast Center for the Snoqualmie River at Snoqualmie Falls.   HUGE, rapid rise to near-record levels.  The falls will be quite a sight on Sunday.  And you can get a nice breakfast there as well.

Finally, some historic context.  Below is a plot of the record daily precipitation amounts for SeaTac Airport.  Middle to late September has had some big events, reaching about 1.5 inches.  And I suspect most are like this one.....tapping significant moisture from the western Pacific.

Enjoy the change....and make sure your umbrellas are handy.  And I didn't even get to the issue of strong winds.   Next blog.


  1. Hi Cliff. Love your forecasts and perspective. I point everyone to you as a great source for weather information. Heads up that the graph you included for the river forecast is the NF Snoq, not the Snoq @ Snoq, which is a better representation for what will be going over the falls. It's currently forecast for just under 6,000 cfs, which is big for September but not big compared to later in the Fall. I'm a WW kayaker, so very attuned to the river forecasts, as they're our indicator for "fun". Cheers!

  2. Hi Cliff,
    You seem like a fair-minded scientist. I disagree with some of your takes, but I appreciate that you clearly have integrity. In today's climate (almost all wet!) there is a corporate-government backed orthodoxy and those who are off of the 3 x5 card of allowable opinion are smeared.
    Given that you are already a pariah, I think you might as well further pariah-cize yourself by pointing out that the same bigotry which seeks to silence you is at work against early (ambulatory) treatment of covid, and any opinions that are not "GETTHEVAXYESTERDAYALREADYITISSAFEANDEFFECTIVE!!"
    Like you, people such as Robert Malone, Jay Bhattacharya, Martin Kulldorff, Pierre Kory, Peter McCullough, have stood up, against interest, and have been censored by goliath bullies. Right or wrong, they have the courage of their convictions and they are suffering endless attacks for questioning the orthodoxy.
    The environment is toxic. A nephrologist in Tacoma noticed that his vaccinated patients were doing worse for their kidney function markers. He recommended against the booster to these patients. One patient told a higher-up and now this doctor is threatened with loss of job.
    This isn't a right/left thing, it is a deadly serious truth thing. Many of us expect Prof. Mass to stand up for unhindered scientific dialogue, and perhaps some will broaden their perspective if you use your credibility to fight the rigid corporate/legacy narrative.

    1. Professor Mass, what might your response be to this? I, as the rest of the NW especially those who are adamant readers of this blog await your response, if any.

    2. I have been vaccinated and support vaccination, if that is what you are asking.

    3. "if that is what you are asking."
      But that is clearly not what I'm asking. Are you concerned with the censoring of speech and scientific dialogue that we see, which seems to serve a push to get everyone vaccinated?
      Or, are you concerned that a year ago it was censorable to suggest that keeping your vitamin D levels high might help?
      Or are you concerned that the legacy media seem monolithic in their: --unwillingness to permit anyone to air concerns about mask mandates?
      --unwillingness to discuss any cost benefit analysis for lockdowns
      --unwillingness to honestly discuss the safety of ivermectin
      --unwillingness to question the wisdom of sacrificing the rule of law for the goal of following
      These men (Robert Malone, Jay Bhattacharya, Martin Kulldorff, Pierre Kory, Peter McCullough) are scientists and doctors who think there are questions such as these and many more which are piling up, and the suppression of open scientific dialogue is Lysenko-like.
      You could say, "I'm not in that field, and so I can't weigh the claims of opposing sides," and you could say, "These men are in a minority" But at least one fair question is this: does it seem to Prof. Mass like there is a fair and open scientific dialogue happening on these critical questions and others like them? Does it seem that the scientific climate (there's your field!) has become corrupted or derailed?

  3. Hate to think this will be our big fall storm then dead the rest of the fall\winter season if so then we better enjoy it.

    1. reason to expect that. In fact, the longer term forecasts are quite wet...cliff

    2. Cliff detailed what a typical La Nina year would mean for the PNW, and although last year's La Nina year wasn't anything special, this season could be quite a bit wetter and colder than normal.

  4. Cliff,
    Any sense for snow levels for this event? Trying to determine if a thru hike of the Enchantments will be off the table for next Tuesday due to snow accumulations (elev ~ 7500-8000)

  5. Saying that we might have almost normal rain fall for the summer because it all came in 6 days out of 100 is ludicrous, that's just a numbers game, tell all the wild life and plants that all is normal, I think they would disagree.

    1. JimmyT...there is nothing ludicrous about it. And did you check the Seattle Times, which had a silly story about low summer totals of precipitation BEFORE a major rain event. I am going to do a blog putting this summers rainfall in perspective. I hope you read it.

    2. Cliff, I think the question should be, how often do we get basically all of our normal summer rain in a single 24 hour period? Is that normal or is it usually spread out throughout the summer?

      "Normal rainfall totals" can definitely be 'not normal' if the pattern of rainfall is way outside the normal...which seems like the case this time. But I'm curious about the history of summer rainfalls and if/how they are spread out throughout the summer.

    3. Scott...a perceptive question. It turns out that typically much of summer rains come in intense periods, often during the beginning or end of summers. Not a general increase. It is rare to get "normal" or average rainfall on a day...more likes to be zero with a few cases of heavier rain...cliff

    4. Cliff, thanks for the reply. I don't have records to back this up, but having lived here my whole life I also remember usually having very sunny/warm stretches of summer days and then an occasional rainy/stormy day, then right back to sun again. So more as you described with a few heavier rainy days, but otherwise just sun! :)

  6. Darn, I was going to pour concrete Saturday morning. Perhaps next weekend will be drier.

    1. Latest reports are that we will dry out, after Monday--at least until the weekend...better pour that cement in the middle of the week!

  7. Cliff, we are participating in a sailboat race from Shilshole to Port Ludlow on Saturday and racing back on Sunday. How wet are we going to get? Based on the various models I’m planning for light to moderate breeze of ~10-12 knots. Your input would be appreciated.

    1. If you leave late enough the heavy rain will be over...but it will be windy!

  8. I live in the convergence zone and with predictions like this it is often hit or miss. A shift of the airflow from one side or the other could put us into a rain shadow or in a line for a heavy shot of rain. So, I will expect both. That way it is correct either way!


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