Monsoon moisture flows every summer from Mexico to the American Southwest, driven by a persistent high pressure center that sits over the Four Corners region (and sometimes paired with a low that sits over Yuma).

As the sun warms the humid air, it rises, and the moisture condenses, and thunderstorms sprout, bringing much-needed afternoon rain to the mountains and deserts.

As patterns go, the monsoon is remarkably stable. It traditionally begins around July 4 and wraps up Labor Day weekend. Blips or wobbles in the spinning high can send moisture into California, triggering storms there (especially the Sierra), or cause the whole pattern to dry up for a few days until the high reforms, and the moisture recharges.

Arizonans, in particular, eagerly await the arrival of the monsoon.

Those afternoon thunderstorms take the edge off the heat in Phoenix, and make for stunning landscape photographs. In Flagstaff, meanwhile, the forest grows dangerously dry by the end of June, especially if winter snowfall was poor (as it was this year, for example). Every July, Monsoon rains transform Flagstaff from dry, hot, and dusty to green, lush, and cool.

How stable is the monsoon?

That's a question I find myself pondering. The monsoon has changed, a little, over my lifetime. It isn't quite as regular in its arrival and departure dates, and those rain-free summer interludes, once extremely rare, have become quite a bit more common.

But the monsoon remains largely intact, and I find that immensely comforting. Not only do those summer thunderstorms remind me of happy summer days gone past, they also suggest, at least for now, that the monsoon remains robust despite our climate's obvious changes. At least, for now.

— July 29, 2018

Andy Lewicky is the author and creator of SierraDescents

Dan Conger July 29, 2018 at 9:32 pm

It has been particularly harsh in the Sierra this summer with more mudslides and flash floods than I can remember in my lifetime (I’m in my forties). This monsoon season has been more intense than usual.