From Highs to Lows

As noted in my previous blog, the meteorology of much of the summer has been dominated by high pressure offshore, resulting in a warmer and drier-than-normal period.

But this week it all changed, with low pressure---also called troughing-- gaining the upper hand.

Consider the infrared satellite image earlier this morning (below).   A low center was positioned over southwest Washington, with a plume of moisture circulating around.

That moisture originated in the famous Southwest (or North American) monsoon.  The same monsoon that brought heavy rain and a dust storm to the Phoenix area.

But as this low moves out today, ANOTHER low is ready to move in!   The water vapor satellite imagery this morning shows the low lurking offshore.  A larger low.

A sequence of forecast upper-level (500 hPa, about 18,000 ft) weather maps provides the "lowdown" on the situation.

Tomorrow morning at 5 AM,  the low is off the southern Oregon Coast.

But by Sunday morning, it had moved northwestward to immediately west of the Olympic Peninsula.

A day later...still out there.

This is no "bomb cyclone".   The precipitation from this feature will stay offshore (see forecast totals through Tuesday morning below).

Temperatures won't impress either.... here are the forecasts for SeaTac. A bit cooler over the weekend with the low offshore.  No major heatwaves.

With a low offshore, the winds will be very modest over the region.  

Very bad for wind energy generation.  Very good for keeping wildfires down.

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Hot/Dry Summer and Minimal Wildfire over Washington State. WHY?

Wildfire acreage has been running far below normal over Washington State--- in fact, over the whole Pacific Northwest.

To demonstrate this, here are the latest WA DNR wildfire acreage statistics for Washington as of August 23.  2025 is well below normal.

All years show acres burned through August 23.

Or look at the map of large fires from the Northwest Fire Coordination Center (below).

Only three fires, and the two fires in Washington have been burning for a while and are only slowly growing.

In contrast to a lackluster wildfire season, we have had a warm, dry summer.  

Check the deviation from normal for temperature (left panel) and precipitation (right panel) for the last 90 days over the Northwest (below).

Most of the region has been warmer than normal (yellow, orange, and red colors).  It has been drier than normal over the region, except for the Olympic foothills and south-central Oregon. 

The media (e.g., the Seattle Times Climate Lab) and others have been suggesting that this would be a big wildfire year due to the warmth and dryness.  They were wrong. And yes, they suggested a major role for climate change in the predicted fires.

How did they get this wrong and why?  

It turns out that the connection between wildfires and meteorology is a bit more complicated than some media and climate advocacy groups suggest, and that simplistic arguments are simply inadequate.

Question 1:  Why so dry and warm this summer?  

The best science suggests that human-caused global warming has warmed the region by about 2°F and only caused slight drying. 

Why so warm/dry this summer?

Below is the difference of sea-level pressure from normal from June 1 through August 22.   Higher pressure than normal over the region, particularly offshore.

At 500 hPa (about 18000 ft, see below), the offshore high was very evident (red colors).  This pattern is a warm and dry one for our region, with sinking air aloft.   

No wonder we have been warmer and drier this summer.   There is no evidence that such a pattern has anything to do with global warming (I have studied and published on this exact issue).

Question 2: Why so little wildfire activity when it has been warm and dry?

First, with the dry conditions during the late spring and summer, seasonal grass growth, an important fuel for regional wildfires, has been less than normal, which works against wildfire activity.

Below is a graphic from the Forest Service FuelCast website, showing below-normal values (yellow) over most of the region.

Second, lightning activity has been less over most of Washington this summer due to the dry conditions.   Below is the difference in lightning between this summer and last summer over Washington, southern BC, and northern Oregon.

Most of the area has had less lightning this year, particularly the fire-prone eastern slopes of the Cascades. (blue colors indicate less lightning).

Image produced by Katrina Virts, Research Scientist, SNWG MO Deputy Project Scientist/Assessment LeadNASA MSFC Office of Data Science and Informatics (ODSI), The University of Alabama in Huntsville

Lightning is an important initiator of regional wildfires, particularly at higher elevations that are difficult for firefighters to access.    This is important.

Third, with high pressure offshore, the winds have been affected, since pressure controls the winds.  Strong downslope winds on the eastern side of the Cascades are often associated with cool air moving into western Washington behind upper-level troughs, something suppressed this year because of the high pressure.

Since the strong winds can contribute to the rapid spread of wildfire, lesser winds reduce the wildfire threat.  Take a look at the sustained winds at Wenatchee, WA, last summer (top) and this summer (bottom).  Fewer strong winds this summer (the red line is 23 kt)

Fourth,  the technology of wildfire firefighting has advanced dramatically during the past few years.  A series of high-resolution cameras, with AI interpretation, has been distributed over the region.

Increased use of high-resolution satellite data is also providing far better fire intelligence.  Drones are being used to discover and surveil fires.

Increased coordination with aircraft used for suppression has allowed more rapid response to new wildfires.

Analagous to the  Ukraine war, new technology has radically changed the battle against wildfires.  

The bottom line is that the idea of warmer and drier means more wildfire acreage is simplistic.  Other factors can be equally or more important.

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3D Northwest Heat

 We are now in a short warm spell with temperatures getting into the mid 80s to lower 90s in western Washington.  Cooler weather (and some rain) will arrive next week.

 But for me, one of the most fascinating aspects of our warmth is how three-dimensional it is.  How spatially varying the heat can be.

Let me show you, by starting with yesterday's highs (below). Mid-80s around Seattle, 70s and low 80s over Northwest Washington, around 100F in the Willamette Valley, and upper 90s in the Columbia Basin.   An interesting feature is the near 100°F in the higher terrain between Olympia and the coast,.

In western Washington, the influence of the cool Pacific and inland waters results in cold air near the surface, but warmer temperatures right above.  This is particularly true when (like yesterday) there is easterly  (from the east) flow aloft moving warmer air from eastern Washington towards the west.

Yesterday (Friday) morning, this situation was so profound that there was an inversion (temperature increasing with height) above Seattle, something illustrated by the temperatures above SeaTac Airport at 8 AM (below).  Temperatures in the mid-60s near the surface, but 70s above 2200 ft.  

A profound inversion from 1000 to 2500 ft (more than 10 degrees).  I bet morning hikers noticed this warming with height!

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This morning at 7 AM, the inversion was even stronger  (see below), going from 67°F to 79°F above 1500 ft.

The inversion weakened during the day as solar heating warmed the surface.

A temperature inversion can act as an atmospheric lens for weather radar beams, causing  the radar beam to bend downward to the surface, which acts as a target.

This inversion bending of radar explains the huge number of false radar echoes off our coast right now (see below).  The radar beam hits the ocean surface and then is reflected back. 

Looks like a lot of rain out there...in reality there is nothing.

When we have inversion conditions, maps of temperature can get jarring, such as this morning at 7 AM in the Cascade foothills near North Bend.  Around 70°F on ridgetops but lower 50s and 40s in valleys.

According to the latest National Weather Service forecast, the warmth will continue over the weekend, with cooling by mid-week.  Possible rain next weekend.

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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.

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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.

What Season is This?

The visible satellite image this morning was stunning, reminiscent of an image from mid-winter (below).  The star shows the location of Seattle. 

 You can see the swirl of clouds around a deep low-pressure center west of British Columbia, a strong front offshore, and unstable air with convective clouds south and southeast of the low.

Impressive.

It is times like this that we should be glad that we have a coastal Doppler radar (thank you, Senator Cantwell), which shows the approaching frontal rain band offshore (see below).

This rain will come in later today, bringing another period of moderate precipitation.

Below is the latest high-resolution UW WRF model forecast of accumulated precipitation through 5 AM Saturday morning.

Very, very wet in the mountains, with large areas getting more than 3 inches.

Rivers are going to surge rapidly, as illustrated by the predicted discharge on the Snoqualmie River near Carnation (below).  If you go to Snoqualmie Falls on Sunday, you won't be disappointed. 

Another interesting aspect of this event is the high humidity today.  Dew points are reaching the mid-60s, making it feel sticky ....reminiscent of the East Coast.   

Note:  the dew point is the temperature to which air needs to be cooled to reach 100% relative humidity.  The higher the dew point, the more moisture the air has.

At SeaTac, the dewpoint today is the highest of the year!  See plot below

 There are some amateur YouTube sites and some media claiming that this is a highly unusual, record rainstorm.

This is not true. 

Having a strong event like this in mid-August has occurred numerous times before.    Below is a list of the record one-day totals for mid-August (Aug 4-21) at Seattle.

Yesterday's total of .22 inches was 44th on the list.  Today, SeaTac may get a half inch, which is certainly not without precedent.

Extraordinary Dual Wildfire Smoke Plumes

In ancient Rome, gladiators would face the crowd and say, "We who are about to die salute you".

Today, we have the meteorological version of this.

Look at the visible satellite image this morning.  There are two west-to-east smoke plumes, hundreds of miles long.

One originates on the Olympic Peninsula (the Bear Gulch Fire) and the other on Vancouver Island (Port Alberni fire) 

Neither fire was that large.....they are NOT megafires (sorry Seattle Times and NY Times).  Each is around 5000 acres.  

But they are smokers, burning in areas that have been logged with lots of  flammable debris on the ground (see example near the Port Alberni fire)

Fortunately, the days of these fires are numbered.  

As mentioned in my last blog, heavy rain is coming to the coastal mountains.    

Below is the current forecast precipitation total through Saturday at 5 PM.  Three to five inches of rain on west-facing slopes.    Substantial rain over eastern Washington.

A less-than-average wildfire season so far is going to end that way.