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Rare September rains bring respite to drought-stricken inland California

The showers and thunderstorms, however, are unlikely to make a serious dent in the rainfall deficit

Rain drenches streets in Chinatown in San Francisco on Sept. 18. The rainstorm is a dramatic shift of events for many residents after a record heat wave and grueling wildfire season. (Brontë Wittpenn/San Francisco Chronicle/AP)

Much-needed rainfall and thunderstorms are hitting central and northern parts of California, bringing relief to places that typically see little precipitation in September. An upper-level low-pressure system, an occurrence more likely in winter, is churning off the coast of Northern California. It follows unprecedented heat across much of California at the start of September, when a prolonged heat wave shattered thousands of records across the West.

Summer months are usually dry across California, and the first half of September was no different, with most of California — including the cities of Sacramento, which saw an all-time high temperature of 116 degrees on Sept. 6, and nearby Davis — left dry and baking. The notable exception to the dry conditions was the heavy tropical rainfall from the remnants of Hurricane Kay in far-southern parts of the state. In the past few days, the unusual September rainfall has fallen from Santa Barbara up through the California/Oregon border.

Driest, wettest, hottest: Sacramento’s troubling trifecta of extremes

At Davis’s University Airport, 1.7 inches of rainfall were recorded in the past 24 hours. While this type of rainfall may not be unusual in many spots in the United States, it is highly out-of-the-ordinary in the Davis area, which on average sees less than a tenth of an inch of rainfall in September.

“There’s been some minor flooding, road flooding and such in the heavier thunderstorms, and there’s been reports of minor ash flows and debris flows and burn scar areas,” Chris Hintz, a meteorologist with the National Weather Service in Sacramento, told The Washington Post.

On the subreddit for the University of California at Davis, students shared photos of the downpour and the subsequent flooding, with students swimming in a flooded-out underpass. Photos also showed minor flooding at the school’s student union building.

A photo shared by the city’s Twitter account showed minor flooding at a local underpass, with the city asking drivers to take alternate routes in and out of town.

In Sacramento, just over an inch of rain has fallen in the last three days, well over the September average of just 0.09 inches of rain. From June through September, the city averages just 0.36 inches of rain a year, meaning that the city saw nearly three times its annual summer rainfall in just three days.

The big rainfall winner was just north of Davis in Woodland, where 4.11 inches of rain were measured in the past 48 hours, according to the National Weather Service. Notable rain fell also fell in Central California. In San Luis Obispo, a daily rainfall record of 0.32 inches of rain was set on Sunday.

The mountains of Santa Barbara County picked up the most rain in the greater Los Angeles area, with 4.07 inches of rain recorded at Rancho San Julian. The city of Santa Maria also smashed its daily rainfall record on Monday, tallying 1.77 inches in 24 hours. The former record was just 0.16 inches, set back in 1959.

The weather pattern that has brought scattered showers and thunderstorms to the area is expected to hang on for another day or two, with the quasi-stationary large low-pressure system off the coast forecast to begin moving inland on Wednesday, Hintz said.

Flash flood watches have been hoisted for the burn scar of the Mosquito Fire — California’s largest fire of the year — which as of Tuesday morning is just 39 percent contained, having burned 76,000 acres northeast of Sacramento.

Volcano-like plumes spread above intense Northern California fire

When heavy rain falls over burn scars, ash and debris flows can sometimes be triggered, especially on steeper terrain, which can make for dangerous firefighting. The wet weather does bring some advantages for firefighters too, with chillier temperatures and high humidity both helpful in fighting the blaze.

“Fire activity has slowed down, but the firefighters have not,” the U.S. Forest Service wrote in its Tuesday morning update. “While the rain presents a different set of challenges to the fire-suppression effort, crews continue to work, taking advantage of the lull in fire activity to secure the fire perimeter and increase containment before warm, dry weather returns.”

Unfortunately, the rainfall in the region will not be enough to make a significant dent in the severe-to-extreme drought conditions that persist throughout the state.

“This little bit is not going to make much of a difference in the overall drought picture but the fact that we’re starting to see significant storms like this is favorable,” Hintz said.

After the low-pressure system passes through the region, the medium-to-long-range forecast for Northern California shows a return to hot and dry conditions, with the latest runs of the American (GFS) model showing the potential for several days with highs in the 90s, or even topping 100 degrees, in California’s Central Valley by late next week.