Record Water Vapor Values Hit the Northwest

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.  

Values are high in summer since warm air can hold more water vapor, among other reasons.  

As shown by the short-term forecast (valid 5 PM Friday) of the Total Precipitable Water Content, the plume of high values extends all the way back into the eastern Pacific.  More on that later.

Dew point is a measure of the water vapor content at one elevation (generally at 2-m above the surface). 

Here in Seattle, we experienced the highest dew point (65°F) since August 2022 (see plot below).  It was very "sticky" on Friday.

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.

Although the water vapor levels of this atmospheric river were record high, the atmospheric river was relatively run-of-the-mill.

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.

Red and orange colors indicate above-normal water temperatures.

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)

Atmospheric Moisture On Wedneday

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.