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.
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.
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