July 25, 2021

The Southwest Monsoon Brings Thunderstorms and Wildfires to the Northwest: Will This Change Under Global Warming?

Every summer starting around mid-June and ending by the beginning of September, moisture streams northward out of the Gulf of California into Arizona, Nevada, and the eastern portions of the Pacific Northwest.

Known as the Southwest Monsoon or the North American Monsoon, this phenomenon is associated with the river of moist unstable air that brings thunderstorms into the southwest U.S. and occasionally into eastern Oregon and Washington.

This moist current, with an origin over the warm Gulf of Mexico, is the result of high pressure that develops during the summer over the Southwest U.S. When this high shifts to near the Four Corners area of the SW (where AZ, NM, CO, and UT meet), the moisture can surge into the eastern portion of the Pacific Northwest.  This should happen later this week.

To see an example, lightning moved into eastern Oregon on July 20th (see below)

This was associated with the Southwest Monsoon and the Four Corners high, bringing moisture in from the south and southwest ( see upper level (500 hPa, 18,000 ft) weather map below for mid-day on July 29th.:

A plume of moisture associated with the Southwest Monsoon will move northward later this week out of the desert southwest.  To illustrate, this image shows you the atmospheric moisture pattern on Saturday, with blue, white, and red being the highest values (in that order.)

The 24-total precipitation ending late Sunday shows a typical SW Monsoon distribution, with precipitation extending northward into southeastern Oregon.

More on the weekend precipitation (which will include lots of thunderstorms) will be found in a future blog.

The importance of the mid-summer thunderstorms from the Southwest Monsoon is evident in the climatology of many stations in the southwest U.S.  Here are record daily precipitation totals in Phoenix, AZ...the biggest amounts are in July through September...that is the Monsoon.

You can see a weakened, but similar, effect at Las Vegas.

Thunderstorms associated with the Southwest Monsoons are potent sources of wildfires east of the Cascade crest.   Some of these thunderstorms are high-based and little precipitation reaches the surface.  Thus, a lightning stroke can start a fire if the vegetation is dry, as it usually is by midsummer.  Even if there is substantial rain, the lightning stroke can initiate a smoldering fire, that bursts out when the weather subsequently improves.

What about global warming?  Will it bring more or less Southwest Monsoon precipitation to our region?

I am, in fact, working on this problem with Professor Eric Salathe of UW Bothell, using very high-resolution regional climate simulations.

The answer appears to be that there will be more summer precipitation from the monsoon reaching eastern Washington (see a map showing you the difference between the end of the 20th and 21st centuries assuming very rapid increases in greenhouse gases).  Green indicates a wetter summer under global warming.  There is a number of reasons why global warming might stoke eastside thunderstorms, such as greater availability of water vapor.

But there is a dark side to a wetter summer:  more lightning, which can ignite wildfires.   
Stay tuned....we have a lot more work to do on it.


  1. Thank you for the education Mr. Mass. I was always in awe of the summer lightning while spending vacations at my grandparents in Spokane. Now I know.

  2. some Europe/China levels of rain is sounding pretty good right now

    1. I believe Germany got much of the rain that the West coast normally gets. David EEJASP@aol.com

    2. Spread out over a few weeks, maybe. ;-)

  3. Thanks for the article. I lived in Phoenix for several years and I really liked monsoon season. It's good to see Arizona getting plenty of water this year compared to what I thought was pretty dry monsoon last year. I like checking the GOES East Band 11 Southwest Rockies loop in the afternoon to watch the storms develop.

  4. Speaking of rain... When was the last day we had any measurable precipitation around the SW Puget Sound? We must be approaching something around 45 days at this point?

  5. Corresponding/coincident decreases in precipitation west of the Cascade Crest, and particularly the mountains seems pretty noteworthy as well, especially considering the projected decrease in snowpack.

  6. There is huge natural variation in the high desert climate. Anyone who regularly travels to SE Oregon has experienced it, and the role of climate cycles had many impacts on that region's history.

    As an example, when Abert Lake, which is on the Lake-Harney county border, dried up in the 1930s, the receding water uncovered a Pioneer wagon that contained the skelton of a girl with the telltale marks of a murder by Indians. Which tells you that Abert had dried up before, or how would the wagon have gotten there?

    There are cycles within cycles within cycles there. Rivers and lakes rise and fall regularly; same with groundwater tables. In the early 1900s, a wet period lasted for a couple decades. In the mid-1920s, the wet cycle ended abruptly. In 1924 alone, the Christmas Valley about 50 miles west of Abert lost 80% of its homesteaders. The federal government ordered their abandoned cabins burned.

    In the early 20-teens, we were there in a wet year. The water was so high that some roads were on the brink of being washed out. A couple years later, it was bone dry, and there was a gigantic lightning-caused grass fire in Malheur County, which borders Idaho. When we went through in 2019, the lakes were full and the creeks were torrents. There had been a flood a month before we arrived, and Lakeview was saved the old-fashioned way, by townspeople stacking sandbags 24 hours a day.

    The point is this: Be VERY careful when trying to analyze the high desert climate. In particular, departures from "average" are useless. Not only are the records brief and incomplete, but the temp and precip numbers are HUGELY variable even during the short period that climate alarmists know about but ignore because the facts don't conform to the narrative.

  7. Some scientists believe that cosmic rays are part of the cause of lightning as discussed in this article https://www.scientificamerican.com/article/experts-do-cosmic-rays-cause-lightning/ . With the trend of increasing cosmic rays, this indicates that additional lightning will result from the Southwest monsoon. To the extent that additional cosmic rays correspond to additional ozone depletion and more intense UV reaching the surface of the earth and as a result more trees being harmed and killed, the vegetation will be even drier by midsummer and the Southwest monsoon will correspond to even more wildfires.

    If more intense UV reaches the surface of the earth, this would tend to support an upper level ridge over the Southwest which would tend to support the Southwest monsoon since the dry air over the Southwest can potentially be heated more by a given increase in solar radiation than air with higher humidity to the east. To the extent that evaporation increases as a result of the potential for more frequent extreme high temperatures as a result of more intense solar radiation reaching the surface of the earth, even drier vegetation will further increase wildfires. However the tendency for an increase in wildfires is offset to at least some degree by that a stronger Southwest monsoon and additional evaporation and resulting greater availability of water vapor as a result of more intense solar radiation supports additional precipitation during thunderstorms in at least some parts of the area affected by the Southwest monsoon in any given year.

  8. The SW monsoon is fascinating. I live in Palm Desert part time, and am there right now - we just had a weekend of downpours and flash floods. Unfortunately, the humidity hasn't gone away with the return of sunshine this week...yuck!


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