The Autumn Weather Transition is Occurring

Autumn doesn't wait until September 22 in the Pacific Northwest.   

During most years, there is a transition to cooler temperatures and the return of rain during late August, and this year will not be different.

A plot of average temperatures at SeaTac Airport (below) illustrates my point.  Today (indicated by the blue line) is near the transition point.

This change is not only forced by changes in solar radiation intensity, but also by changes in the large-scale pressure systems that control our weather.

Let me illustrate these large-scale changes by showing you a series of average upper-air (500 hPa pressure, about 18,000 ft) monthly weather charts.

During mid-summer (July) there is a big ridge (dark red) over the Southwest and a weak trough centered to the north of us.  The jet stream (located where the colors change rapidly) is north of Washington State.   

By September, the situation is very different, with the southwest high pressure weakening and moving inland, and a trough (lower pressure) moving towards the west coast.  The weather implications of these changes are profound.

Consider what is going to happen during the next 15 days using the best forecast available (the European Center AI forecast), looking at the same type of upper level chart.

During the next 5 days, the upper-level ridge over the Southwest is apparent.

During the next 5 days, the SW ridge weakens and starts to shift eastward, and a strong trough moves into our region

And the map for the subsequent five days is downright scary with a very deep trough of low pressure (purple color) offshore:  cool and wet for us!

The implications for our weather are profound.   The European Center temperature forecast for Seattle shows highs falling into the lower 70s and 60s:   

Rain will return as shown by the forecast below for two 7-day blocks of time into the future.

No major heatwaves are predicted in these model runs, and it is rapidly getting too late in the season to get one.

Enjoy the "fall transition" in August.  One of the treats of living in the Pacific Northwest.

PS: The latest radar shows showers moving in!

Climate Change and the Little Giant Fire: The Real Facts

Perhaps the most impactful Washington State wildfire of this summer is the Little Giant Fire near Lake Chelan, ignited on July 15 by a lightning storm and now extending over 135,985 acres (see map below)

The fire is still active today, with plenty of smoke and flames (see satellite image yesterday afternoon)

There are many claims that this fire resulted from climate change, drought, and extreme heat (see below).   

As I will demonstrate in this blog, such claims are demonstrably false.  

One local newspaper has been particularly over the top with such claims (see below).

Those blaming the Little Giant Fire on global warming make several unfounded claims:

(1) That the fire was unusual, representing something new and unprecedented

(2)  That lightning, which initiated the fire, is increasing due to global warming

(3)  That the low snowpack this year (driven by global warming) was of central importance,  resulting in far earlier drying of the surface.  That in a "normal" year before global warming, the fire origin location would have been snow-covered.

(4)  That drought contributed to the surface fuels being unusually dry at the time of the lightning strikes.

All these claims can be easily shown to be false.

Location of the Fire Initiation

There are several claims that the fire started at 5000 ft or above.   

Satellite-based fire detection shows the fire initiation was down on the slopes at around 3900 ft (the red dot shows the start location).

It was claimed that the fire started in the forest; in reality, it began in a debris field of previous burns with downed logs, grass, and bushes (red dot shows the initiation location).

Below is a view of the fire-start area from below before it started.

A picture from below looking at the same location during the early days is shown below.  The fire was burning down the hill towards lower elevations.

This is a sample of what burned.  

The Truth About the Snowpack

As noted above, lightning initiated the fire around 3900 ft, after which it then burned towards lower elevation.

There is a nearby USDA Snotel site (Pope Ridge) at 3540 ft.  Just perfect for this fire.  The snowpack (snow water equivalent) at this site is shown below (black line is observed this year, green line is the long-term average).

Certainly, the snowpack was below normal this year.  It melted out on April 7 when the normal meltout is May 7.   A month early!

But since the fire did not start for more than two months into the future (July 15), the early melt-off was not relevant to the fire. 

 Even in a "normal" year, the snowpack would have been long gone and the moistening of the surface gone.

So low snowpack was not an element in this fire.

Lightning and Global Warming

Then there are the claims that the massive lightning "storm" that initiated this and many other local fires that day was the result of global warming/climate change.  And these claims cite some academic papers:

The problem is that lightning observations do not agree with or confirm these predicted trends.  

In fact, lightning strike numbers appear to be steady or declining over the U.S., including the Pacific Northwest (see below, trend from the NLDN network).  

Other networks (e.g., the UW WWLLN network) show the same thing.  I am working on a paper on this topic, by the way.

Bottom line:  Can't blame the lightning that started these fires on climate change!

Previous Fires

The region of the Little Giant Fire has burned many times before, including the historical period before the current global warming.

To demonstrate this, below are the regional fire areas from 1973 until 2025.  Much of the Little Giant Fire's footprint burned during that period.

For example, in 1994, the Tyee Creek fire also started in mid-July and burned a similar area. It was also started by lightning. 

The 1970 Entiat Fire burned more than 175,000 acres and consumed over 80% of the entire Entiat Valley. Lightning initiated.

The region around Lake Chelan is very vulnerable to wildfire for many reasons. 

 It is on the dry side of the Cascades, but gets frequent lightning. It is vulnerable to strong westerly winds descending the eastern side of the barrier. Fire suppression allowed unnatural tree densities that support massive wildfires. Highly flammable invasive grasses have moved in (eg., cheatgrass). And each fire leaves more debris to burn in the next configuration.

Fuel Moisture

Many of the online and media claims of global warming and drought contributions to the Chelan wildfire claim that the fuel moistures were low.

The data shows otherwise. 

Below are the estimated fuel moisture levels over the eastern slopes of the Cascade for bush (100-hr) and log-size (1000 hr) fuels.

They were within normal levels (gray shading).  Fuels are normally dry after mid-July on the Cascade slopes. Dry enough to burn without global warming.

Regarding claims of drought, here is the water-year precipitation (October 1 through July 15) for Chelan.  This year was a bit ABOVE NORMAL.   So much for drought.

What about the summer temperatures at Chelan (see plot for the past 50 years for June 1 to July 15)?  Nothing unusual or particularly warm.

  

Summary

When you examine the actual data, it is apparent that the Little Giant Fire has little to do with global warming and climate change.  

Science and facts matter.  Or at least it should.