Rain Shadow Desert: Why the Rain Often Skips The Desert

A 2009 storm wets the Antelope Valley | Photo: Rennet Stowe/Flickr/Creative Commons License

Today provides another reason for Angelenos struggling through their hour-long commutes in the rain to envy those of us with the good fortune to live in the desert: it looks like the large rainstorm now bearing down on Southern California may skip us, or at least most of us.

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Northern California's getting drenched as we speak, with the small Sonoma County town of Venado recording seven and a half inches of rain yesterday. (An inch or so was more typical in Northern California's bigger cities.) Officials are issuing warnings about flash flooding in Bay Area canyons already soaked by rain earlier this week.

Southern California has gotten off more lightly so far, with mild rains wetting pavement -- though a few places places like Topanga Canyon have gotten more in the last day. But forecasters are saying that the rain will get a bit more insistent over the weekend, with a heavy storm due to hit the area Sunday.

And yet it looks like the desert may stay relatively dry. The Antelope Valley, that part of the desert closest to the Pacific Ocean, has a better chance of getting rained on this weekend than the rest of the desert: according to wunderground.com, a 50-50 chance of rain today and a 30% chance over the weekend. (Obviously, that forecast will likely have changed if you follow that link later in the weekend.) Cities on the edge of the desert like Victorville may get some slop-over from mountain storms.

And aside from that, most of the desert seems to have about a one in five chance of seeing any rain at all on Sunday, while the rest of the region is due to be inundated. And some parts of the desert, including the Imperial Valley and most of the Mojave east of Interstate 15, are expected to stay dry.

This isn't all that much of a surprise, of course. It's a desert for a reason. Storms coming in off the Pacific Ocean get caught up in the mountain ranges, which squeegee most of the precipitable water out of them. The result is what the geographers call a "rain shadow desert." And California has some pretty effective rain-shadow-casting mountain ranges between the coast and the desert: the San Gabriels, the Sierra Nevada, the San Jacinto/Santa Rosas, the San Bernardinos, and a whole host of smaller yet still prodigiously tall sets of mountains have sealed off the California desert from almost all of the rain enjoyed by its coastal neighbors.

You can see the result as you cross the mountains: as less precipitation reaches the other side, the vegetation changes to a droughtier regime. You see fewer conifers and chaparral plants, and more succulents like yuccas and cacti, and more open soil.

All this is not to say we're not getting any storm here in the desert. It's hitting pretty dramatically here in the Morongo Basin as I write this. Clouds are lowering, looking truly menacing to the west in the vicinity of San Gorgonio Peak. It's cold, relatively speaking, and it's windy enough that I wondered if I'd wake this morning to broken tree limbs. The deciduous trees in the neighborhood are now officially stripped of their leaves. We're definitely having a storm here.

We're just not getting any rain to speak of.

Winter rain does come to the desert almost every year, when a storm has so much moisture in it that some gets past those mountain range squeegees. Unlike summer monsoon rains, which are sudden and occasionally violent and very localized, winter desert rains will settle in for days at a time. They can be quite gentle, drizzles that last for a week, or they can be persistent, soaking storms that blanket upper elevations with snow. However they arrive, winter storms generally contribute about 75 percent of the precipitation the Mojave desert gets in a year.

And aside from the spots mentioned above, it looks so far like this isn't going to be one of those winter storms.

I wouldn't be upset to be proven wrong, though. We could use some rain.

Chris Clarke is an environmental writer of two decades standing. Director of Desert Biodiversity, he writes from Joshua Tree regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here.

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