September 14, 2021

The First Heavy Rain Event of the Season

Everything is going to change on Friday.

You know change is in the air when the National Weather Service starts warning of clogged drains and gutters, and localized debris flows.

Consider the European Center model forecast for accumulated rainfall through Sunday at 5 PM (see below).  

Mama Mia!  2-4 inches in the mountains and .5-1 inch in the lowlands of Oregon and Washington.  Even eastern Washington gets some decent rain.


The UW model precipitation is similar. Note how regional extensive the rain is, including east of the Cascade crest. This is the storm we have been waiting for.



The heavy rain will occur mainly on Friday and early Saturday.  If you were planning on putting in some plants or planting grass seed.....this is the time to do it!

This rain will mark the end of the wildfire season in the Northwest, the remaining fires will rapidly decline with the massive moisture, high relative humidity, and MUCH lower temperatures, with highs dropping into the lower 60s.

Snow, YES SNOW, above 6000 ft.

The reason for this change is the development of a strong trough of low pressure over the northeast Pacific, with a powerful jet stream of strong winds to its south.  

Here is the forecast for roughly 30,000 ft at 11 AM on Friday.  The yellow colors show you the strong jet stream winds.... coming right into the Northwest.  Some as strong as 150 mph.   The solid lines are heights (think of them as pressure).  The low center is due west of British Columbia. 

This is a very favorable pattern for heavy rain in our region.


This jet stream of strong winds is associated with a very moist plume of moisture that will drive into our region and rise over the mountains, releasing huge quantities of water. 

To illustrate, here is a graphic for 11 AM Thursday showing something called Integrated Water Vapor Transport (IVT), a measure of how much water vapor is being moved horizontally by the winds.  Blue indicates very high values.  Just drop a mention of IVT when talking to your friends...they will be impressed.


And the latest forecast model runs have a lot more coming after this.  If you were Noah, you would know what to do (see below).    Ironically, with all the talk of drought, our region could end up with near-normal summer and annual rainfall total before the month is over.  The water is well-timed for the upcoming salmon spawning season.




19 comments:

  1. Good morning Cliff. Just a quick note to say I really enjoy your blog. I would be considered a progressive or liberal and certainly believe in climate change due to human caused carbon emissions, but I respect your science based approaches to regional climate activities. Many times we all look for affirmations to our beliefs, but it is important I think to read things that challenge us and our way of thinking. Just wanted to say thank you!

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  2. I knew this was going to happen - the first big rain was bound to hit before I have a chance to finish stacking and covering my firewood. I hope we get another good stretch of sun in before the fall fog and drizzle makes itself at home. A couple days of rain won't soak in very deep, so the firewood can dry out again. A month of it would significantly change my heating plans for this winter.

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  3. And now it's time to give credit where it's due, since Cliff predicted this event days before the other weather services even had it on their radars.

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    1. Bless your pea picking heart... The European model predicted this event and had it on their radar a week+ ago; surely the same on this side of the pond too. Cliff, using his scientific know how and how to put the data into words. He used the fancy radar mix and max machine and relayed the information to us.

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  4. Professor Mass, this circulation pattern induces me to wonder if the moisture in the atmosphere arriving this weekend is entrained remnants of the series of tropical storm events sweeping through or near Japan over the last several weeks. Is my thinking unfounded and faulty or is that a phenomenon that meteorologists (you) monitor?

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  5. Professor Mass, this circulation pattern induces me to wonder if the moisture in the atmosphere arriving this weekend is entrained remnants of the series of tropical storm events sweeping through or near Japan over the last several weeks. Is my thinking unfounded and faulty or is that a phenomenon that meteorologists (you) monitor?

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  6. Cliff, isn't this heavy rain unusual for September?, this looks like a November storm.

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  7. Ohhhhh! You going to do a blog on the upcoming moisture soon? Can't wait! So done over this summer. Never been one for summer, my motto "let it snow and be ten below." I can hole up this way!

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    1. Budd, thanks for sharing. Let me guess… You, too, have pickled herring in the ‘fridge.

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  8. I would wait until after the rain to seed a lawn. Rain will scatter the seed all over.

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  9. I would argue "summer" being somewhat of a poorly defined thing. I would define it as the hottest quarter of the year for a specific location. From a solar perspective, about May 7 to Aug 7 in the northern hemisphere. From a temperature perspective May 21 to Aug 21, maybe a two week lag for most locations?

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    1. Here in the NW it's about a five week lag or slightly more, from a temperature perspective, in summer- but not in winter! This puts July and august almost equally warm, and June and September almost equally warm. But May is warmer than October and the temperature decline in the fall is faster than the warm-up in spring. The odd thing about it is that the maximum and minimum are not exactly 6 months apart- the minimum is about the first week of January (here the lag IS only about 2 weeks), even though the maximum is at about the end of July.

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    2. it is interesting that it can be just as warm in september as in june, although the days are so much shorter and the sun much weaker.

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    3. Thermal inertia. It tends to be greater near the oceans than inland.

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  10. Cliff: I think we should get some glory out of this summer. Surely the three months from June 15 or 16 to September 15 or 16 have been the dryest three months in Seattle history?

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  11. strangely enough, it is not the driest summer....

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  12. Thanks Cliff. My wife and I use your shared content to make informed decision on the weather. And when hiking we now appreciate weather phenomena we see and use science to help theorize the reason of weather. Thanks for your passion and inspiration and humility.

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  13. This storm may very well end the fire season on both sides of the Cascades, especially if it is followed by continued cooler and moist weather. However, it may be too soon to completely write off the fire season west of the Cascades since if an extended east wind period returns, the upcoming moisture could dry out pretty fast. As mentioned in an earlier posting, I was on a project fire on the Olympic Peninsula in 1972 only two weeks after about 10 inches of rain fell, followed by a dry east wind episode, and this was in October. After such a dry period as we have gone thru, this upcoming rain may not be enough to end the fire season if a prolonged east wind period sets in again.

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