Water from heavy rains breaches a levee in Novato, Calif. on Feb. 14. (Terry Chea/AP)

The snow that fell atop California’s higher elevations this winter remains deep. And the waterways emerging from the mountains are already swollen to their banks and beyond by the frigid runoff from the snowpack.

Forecasts are calling for a stretch of wet weather across the Western United States, especially in Northern California, so meteorologists and emergency officials are keeping watchful eyes on river gauges and radar reports. All it takes is one thunderstorm parked over a snow-covered area to wreak havoc downstream.

Unstable weather, due to a cold cutoff low wandering into the state from Nevada, started causing problems Thursday, with effects expected through the weekend. A flash flood watch for Death Valley National Park and the Owens Valley was issued Friday by the National Weather Service office in Hanford, Calif., citing the possibility of “heavy precipitation in the Sierra (Nevada mountains) along with mid-slope snowmelt.”

At the higher elevations, though, it’s the white stuff that’s forecast to fall, with 8 inches of snow expected along the ridgetops. Despite the calendar showing it’s almost Mother’s Day, conditions prompted the Weather Service to send out a winter weather advisory for the central Sierras.

The latest snow survey by the California Department of Water Resources shows conditions are primed for amazing amounts of melt, with runoff forecasts in watersheds across the state coming in well above average.

Making those numbers even more astounding, the survey followed a dry April across the state. According to the Center for Western Weather and Water Extremes, most of California received less than 20 millimeters (0.8 inches) of rain, with parts of the Southland taking in less than 1 percent of normal.

Models show this low-pressure system exiting to the Pacific Ocean via San Luis Obispo County, halting its retrograde movement with a swing southward past Point Conception, and eventually heading back east, south of the border into Baja California. As a result, forecasts called for chances of spotty showers and thunderstorms almost everywhere in Southern California, but nothing of severity.

Still, as of Friday, downpours were flooding roadways northwest of Bakersfield and in Santa Barbara County, while gutters were overflowing in the Palm Springs area. In the Tejon Pass community of Gorman, an automated gauge reported nearly a quarter-inch of rain in 10 minutes.

Meanwhile, the Los Angeles area received a mere dampening, with sprinkles during commencement ceremonies at the University of Southern California.

Wind also was an issue, with gusts up to 40 mph roiling Lake Tahoe, causing waves up to 4 feet high.

Once this low departs, there will be a few days of calm before the next rainmaker: “Medium-range models are continuing to indicate a robust frontal system (for May) moving across the area bringing the potential for widespread showers late Wednesday and Thursday,” according to the National Weather Service office for the Bay Area.