The Patterns of Warmth

I have always been fascinated by the complex temperature patterns of the region, controlled by our substantial terrain and land-water contrasts.

The warming period over the next few days presents a great opportunity to view such temperature structures, so let's take advantage of this period by viewing the ultra-high-resolution forecasts run at the University of Washington.

Starting with the surface (2 meters above the ground) air temperatures for 5 PM today, you find the warmest temperatures in eastern Washington (away from the cooling of the Pacific) and in the lowest elevation of the Columbia Basin and river valleys.

Far cooler at higher elevations in the Cascades and Olympics and near the waters of the Pacific and Strait.   Northwest Washington (e.g., San Juans and Bellingham) is cooler than the South Sound,

The Willamette Valley (including Portland) is the warmest area west of the Cascades due to its isolation from the cool Pacific.  Why? The coastal mountains are an effective barrier, and the Willamette Valley doesn't have sea level conduits to the Pacific like the Puget Sound region.

On Thursday, high pressure will build overhead and temperatures will warm, particularly over the Willamette Valley and southwest Washington, with a cool zone remaining on the coast.  Portland will be toasty, but Seattle will only peak out around 80°F.

 

Note that eastern Washington is almost unchanged.

Friday will be different:  MUCH warmer, as a ridge of high pressure aloft builds over the  West Coast.  

Upper-Level (500 hPa pressure, ~18000 ft)Friday afternoon)

Here are the predicted temperatures.... wow.  Portland is warmer (upper 90s) than the Columbia Basin, and Puget Sound (away from the water) is in the 80s. Still some relief on the coast and Northwest Washington.

Why is western Washington warming?  

Because easterly, offshore-directed flow reduces the marine influence and provides some compressional heating on the western side of the Cascades as air from eastern Washington descends the slopes.

The nice thing about living here is that it is almost always possible to escape the heat: you can go up in elevation, head to the water, or check out northwest Washington.  A Washington State Ferry ride is usually a good, cool bet, with the warmest period often producing interesting mirages.

__________

Get outside and enjoy the weather with Play Tennis Seattle. Find nearby hitting partners, coaches, classes, and join the Seattle tennis community. 

https://www.joinplaytennis.com/seattle

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.