This photo series taken over the course of about 45 minutes illustrates pretty well the hope and disappointment of the summer monsoon season and chances for rainfall in this part of northwestern Arizona. Granted, the forecast was for at most isolated thunderstorms, which means exactly that – isolated and sparse coverage; but it’s always good if your particular cactus garden is under the rain column. Which mine wasn’t, and hasn’t been for several hot, dry weeks. Well at least we get some pretty clouds, a cooling breeze for a half hour, and a nice sunset. Better than nothing.
Monsoon weather generally arrives from the east, south, or southeast, rather than the west as it does most of the year. Photo snapped at 6:41 PM on Sun Aug 12, 2018. These storms came from the southeast of me and propagated to the northwest at the tail end of a hot and humid, but otherwise clear summer day. The plant to the right is Aloe x “Hercules”, a hybrid tree aloe, standing about 9 feet/3 meters tall.
Without going into tremendous detail, the reason we call it the “monsoon season” is because of this wind shift from west/northwest to east/southeast/south. The word monsoon derives from the Arabic word mausim, which simply means “season”. And while the summer monsoon is often accompanied by increased moisture resulting in thunderstorms in the second half of a southwestern desert summer, the word is very frequently misapplied to the thunderstorms themselves, rather than the seasonal directional wind shift that enables them.
20 minutes later at 7:02 PM, outflow winds from the parent thunderstorm arrive, gusting to about 20 or 25 MPH and cooling the evening down by 5 to 7 degrees F. Rain would be nice but the storms didn’t look strong enough or well-organized enough to hold up, so I didn’t expect anything precipitation-wise. Smart choice, because nothing happened.
Therefore it is technically quite incorrect to say, “We are having a monsoon today” when in fact you should simply say, “We are having a thunderstorm today”, because thunderstorms can be monsoonal in nature, or non-monsoonal. Thunderstorms in Arizona can occur in winter, spring, or autumn when wind patterns are coming from the west as they do for about 9 months of the year, making them non-monsoonal despite still being thunderstorms. Granted the majority of thunderstorms in the desert southwest occur in summertime, most specifically in July, August, and September, when the winds shift between 90 and 180 degrees and arrive from a completely different, southerly latitude bearing large amounts of tropical moisture, which then results in the storms. If the winds are coming from the west or northwest, off of the Pacific or the interior Great Basin deserts of North America, they are not monsoonal even if thunderstorms and rainfall are associated with them.
The storm clouds generated by uplift forced by outflow winds of the approaching thundercell to the southeast move rapidly over the landscape. 20 minutes before the sky in this northeasterly view was totally clear, and the wind calm. Now it’s blowing at about 20 MPH and the clouds are moving quickly to overtake the sky. Photo taken at 7:04 PM, only 23 minutes after the first photo at the top of this post.
And finally, one last point is that the true monsoon season in North America occurs primarily over mainland Mexico and the border states of the Desert Southwest. A monsoonal pattern is defined as a marked annual seasonal shift in wind directionality (as mentioned), moisture and precipitation profiles, and temperatures. So the “dry monsoon” of late spring and early summer gives way in a matter of days to the “wet monsoon” of mid-summer and early fall, and then a slower return in middle or late September to the prevailing westerly wind patterns that predominate most of the year. Unless your geographical region experiences an annual, reliable, and fairly sudden transition from persistent seasonal dryness to equally persistent seasonal wetness, you are not “having a monsoon.”
The most famous and strongest monsoonal pattern on earth occurs in South Asia (particularly India), with other prominent monsoonal shifts occurring in Southeast Asia, northern Australia, East Africa, the Arabian Peninsula, and southwestern North America. (And probably a few other places globally that I am not mentioning.) Monsoonal wet/dry climates are usually semitropical or tropical, not temperate or sub-Arctic/sub-Antarctic, and anywhere from 50% to nearly 100% of the annual rainfall might occur in the monsoonal period, with the remainder of the year between somewhat and nearly totally dry.
In the USA, the strongest monsoon pattern occurs in Arizona, New Mexico, and West Texas, with moderate influences in southern portions of California, Nevada, Utah, and Colorado. Occasional monsoonal moisture pushes from the south may reach as far north as southern Idaho and central Wyoming, but a cycle or two per year of tropical moisture being drawn into these more northerly latitudes under the right combination of atmospheric circumstances doth not a true monsoon make.
Last image of the series, 7:30 PM as the setting sun tints the clouds pink and the clouds fade and dissipate without producing any rain locally. Oh well, nice try. Maybe tomorrow….
Easterners in the USA don’t get to call their thunderstorms monsoonal whatsoever, because those storms are not associated with a dry-wet pattern shift, and frequently arrive with frontal airmass collisions moving from the west or north southwards into humid air from the Gulf of Mexico. Again, it’s the overall dramatic seasonal change and disparity between wet and dry accompanied with 90-180 degree wind shifts that defines a monsoon pattern. And that simply does not occur in the eastern half of the USA. Sorry, I know it sounds sort of romantic to use the word monsoon, conjuring up faraway exotic lands, but there’s an actual science definition to it as well. 😉
Meanwhile, enjoy your rains whenever they occur. We here in the deserts surely do. The annual summer monsoon with all its dramatic lightning, dust storms, and promise of cooling, life-giving rainfall is celebrated here, with very good reason.