A mud-covered Highway 101 sign after rain and catastrophic debris flows hit California in January.
Photo: AP

Turns out the East Coast doesn’t have a lock on butt weather. A Pineapple Express storm will mainline tropical moisture into Southern California starting on Tuesday. Up to ten inches of rain could fall this week, but this is no regular March Miracle.

The deluge could trigger mudslides on the parched and charred slopes left behind from this winter’s fires and drought. Mandatory evacuations already underway, underscoring how dangerous the storm is.

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Until now, California has been suffering from a severe lack of precipitation this winter. December-February marked the second-driest winter on record for the Golden State. It was also its third-warmest, helping bake in drought conditions further.

The southern half of the state has been hit particularly hard. Los Angeles, Santa Barbara, and San Diego have less than a 3 percent chance of reaching their median precipitation total for the water year, which turns over on Oct. 1. That’s because despite there being more than six months until October, California receives the vast majority of its precipitation in winter.

But this storm is likely to change those odds dramatically for Southern California in particular. Daniel Swain, a climate scientist and California weather expert, has dubbed this storm the strongest of the year to hit the state. The storm is an atmospheric river, which is exactly what it sounds like. A plume of moisture extending from the tropical Pacific will crash into the California coast starting on Tuesday.

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A satellite view of precipitable water shows this week’s atmospheric river starting to curl from Hawaii toward California.
Graphic: CIMSS

In his blog post, Swain warned that this atmospheric river could be record-setting, portending that models are underestimating rainfall totals:

When measured in terms of the “vertically integrated water vapor transport” (IVT, i.e. the amount of water in motion in the entire column of air above your head), the inbound atmospheric river may be among the strongest (or perhaps *the* strongest) on record for the months of March or April in Southern California. With water vapor fluxes of this magnitude, it is possible that the models are actually underestimating the precipitation potential with this event (and they’re spitting out some huge totals for much of SoCal already).

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It will be aided and abetted by another low pressure system that will act as a precipitation amplifier. Rather than just wringing out heavy mountain snows—which are on tap, too—this storm will also produce copious rain in the foothills and valleys starting on Tuesday and picking up into Wednesday and Thursday.

At its peak, the storm could produce rainfall rates approaching three-quarters of an inch per hour. This is a nightmare scenario for drought-parched Southern California, though. Rainfall rates that high can trigger floods and debris flows on normally stable slopes.

But after months of hardening up due to drought and the largest wildfire in California history (coupled with millions of dead trees from the last drought), slopes are primed to either give way or funnel water rapidly downhill rather than soaking into the soil.

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Deadly mudslides swept through Montecito, CA, located just east of Santa Barbara, in early January. The town is in the middle of a 250-mile stretch of land currently under flash flood warnings that runs from north of San Luis Obispo to south of Newport Beach. The National Weather Service has warned debris and mud flows are “likely in and around recent burn areas in the watch area” and that the areas under flood watches could expand. 

Areas around Santa Barbara that are near burn scars from the Thomas, Whittier, and Sherpa fires are under a mandatory evacuation orders. The county government is also recommending that people living in the scar left by the Alamo Fire evacuate as well.

The chaos in the foothills and valleys will ease slightly at higher elevations of the Sierra Nevada. Instead of mudslide-triggering rain, heavy snow will fall above 8,000 feet in elevation. That will help build on snowpack that’s currently just 48 percent of normal for this time of year. The National Weather Service is forecasting up to five feet of snow, making this week’s nor’easter look like a quick flurry.

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