On Friday, substantial rain hit the Northwest, with some stations receiving their typical rainfall for all of August in one day. No annual daily precipitation records were broken, but several stations exceeded records for that date.
The forecast model predictions (below) verified very well, with 3-6 inches in favored locations on the western sides of local terrain.
But there was something really unusual about the situation on Friday.
The moisture content of the air.
Something called the Total Precipitable Water Content (TPWC), the amount of liquid water that would result if ALL the water vapor in a vertical column of air were precipitated out.
At Salem, Oregon, the amount reached 1.82 inches, the highest value EVER observed at that site (see below). In the plot below, the red line shows previous daily record values.
The heavy precipitation from this moisture plume (which some folks call an atmospheric river) resulted in amazingly rapid rises of actual rivers in the localized zones of heavy upslope precipitation (such as the NW Olympic Peninsula and the western slopes of the central and northern Cascades).
Consider the South Fork of the Sultan River in the foothills of the central Cascades (below). The black line shows the river discharge, brown shows the record lows, and blue shows the record highs.
Wow. A stunning rise from a near record low to exceeding the record high for that date.
High river levels closed several roads, including the entry to the Hoh River Valley.
How can this be?
The key measure of atmospheric river strength is how much water vapor is moving in over a unit of time. That depends on BOTH the water vapor content AND the wind speed.
The water vapor content of this event was very large, but the wind speed was modest. Think of a river with a very high level, but a weak current.
Finally, why was this moisture plume/atmospheric river so moist?
We start with the optimal time of the year (late summer), when the atmosphere is most moist. I checked it out: the vast majority of the intense moisture plumes in our region occur in July to September.
Second, the North Pacific is now unusually warm (see below), mainly due to an anomalous atmospheric circulation during the last year.
Third, the large-scale atmospheric circulation took unusually high values of water vapor over China and moved it across the Pacific. (orange and reds are high values)
And this moisture was concentrated by a strong low in the Gulf of Alaska
And finally, what about global warming? Yes, a warming planet results in more evaporation from the oceans and higher atmospheric water vapor content.
So the roughly 2°F global warming would have helped, but probably was not the dominant cause.
How can I say this? I plotted the years of the top 25 events at Salem, Oregon. If global warming were dominant, there would be a clear upward trend.
A weak increase over time, but clearly not the dominant story.
Very informative, thank you Cliff. Emeritus state climatologist Nick Bond has given a guest lecture in my UW course for many years. He says that warming in western Washington is mainly in the form of warmer summer nights, which he says is mostly driven by higher humidity. He showed a graph entitled "WA State Low-Level Humidity in Summer (Jun-Aug)" and the data run from 1950 to 2023. A clear upward trend is visible. This seems to tell a different story from your "Top 25 Events" graph, which is confined to Salem, OR. Your thoughts? I can share a link if you'd like.
ReplyDeleteJerry..yes, send me the link..cliff
DeleteSlide 10.
Deletehttps://www.cbdl.org/wp-content/uploads/15.-Bond.pdf
PS he did say that increased irrigation is a driver of increased humidity, along with ocean warming.
Deletethere are other reasons for nights being preferentially warmed.
DeleteI propose a LARGE dehumidifier
ReplyDeleteWSDOT said that the Amanda Park area near Lake Quinault had something on the order of 4.54 inches of rain on Friday. Thats a lot at any time of year, but in August, its downright unheard of! US 101 was actually closed for about 24 hours west of Amanda Park due to flooding, which is also quite unheard of!
ReplyDeleteCliff which meteorologist first used the term “atmospheric river”? Such a wonderful turn of phrase should be inducted into the Clickbait Hall of Fame.
ReplyDeleteHarry..the first reference I know about is: Zhu, Y., and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers. Mon. Wea. Rev., 126, 725–735
DeleteOn Jan 15, 2022, a very large underwater volcano, Hunga Tongo, erupted sending 150 million tons of water vapor into the stratephere
ReplyDeleteInconsequential.
DeleteJHK..be careful... it may have caused temporary warming..cliff
DeleteOn 15 Jan, 2022, a very large underwater volcano (Hunga Tonga), erupted, sending 150 Million tons of water vapor into the stratosphere. It increased the global atmospheric water vapor content by 10%. Since that time we have seen "atmospheric rivers", rain in the desserts and of coarse, increased water vapor means increase atmospheric temps as water vapor is a tremendous green house gas. You can verify all I have written.
ReplyDeleteHey, do you know if there can be an effect between active periods of solar storms (like the northern lights visible a lot in the past year) and more water vapor? I heard that the atmosphere can be thicker during active solar periods because water is created when hydrogen particles come from the sun and interact with carbonite rocks to make H2O. There is a balance between water particles escaping into space at the very outer edges of the atmosphere, while new water is being created from the solar flares.
ReplyDeleteGPS Degraded By Atmospheric Water Vapor. I went to roam some Forest Service roads around Greenwater early Saturday morning. To my surprise, my GPS device could not connect with the satellites, even in an open clearing without obscuring vegetation. Guessing the dense clouds were degrading the GPS signal to the point of uselessness. Would love to hear your perspective on the phenomenon... TIA, and thanks for all you do!
ReplyDeleteI drove the Mountain Loop from G Falls up to the pass that afternoon, and it was remarkable to watch the South Fork of the Stillaguamish respond to the rainfall. It stayed in its banks, but some of the folks in the riverside campgrounds (there are probably a dozen NF sites along that stretch) were clearly nervous about spending the night within a few yards of the river. It's no mystery what they all had on their minds.
ReplyDeleteI actually went swimming in the rain- a rare experience in the NW since it is unusual to have warm rain falling on a warm lake (Martha Lake). It was nice variety.
ReplyDelete