Among the hardest-hit areas last week was the community of Springville, Tulare County, in the foothills of the Sierra Nevada. A staggering 8.14 inches of rain was recorded there in 48 hours, while neighboring Peppermint received 11.96 inches. Now the beleaguered town is, once again, at risk of significant flooding as the next atmospheric river arrives.
Closer to the coast, a levee failure in the town of Pajaro, near Watsonville in Santa Cruz County, caused the Pajaro River to inundate virtually the entire community. More than 1,000 residents had to be evacuated.
The National Weather Service’s Weather Prediction Center has declared a level 3 out of 4 risk area for excessive rainfall on both Monday and Tuesday, affecting the state’s coast and interior. Such a designation connotes at least a “moderate” risk of flash flooding.
Accompanying the deluges will be a wintry side to the system, as well. Up to 10 feet of snow is expected in the highest peaks of the Sierra Nevada. The southern Sierra already is buried beneath its biggest snowpack on record, and some places farther north, like the Central Sierra Snow Lab off Interstate 80 near Donner Pass, have recorded in excess of 54 feet of snow so far this winter, third most on record.
With another monster of a mountain winter storm on the way, there is no clear end in sight.
Recent storminess produced extreme precipitation, rotating thunderstorms
Friday’s atmospheric river made severe impacts as heavy rainfall combined with rapid snowmelt to pour copious water into rivers and streams. In addition to the foot of rain that fell in Tulare County, a number of other extreme 48-hour totals were logged:
- 10.23 inches: Mountain Home State Forest in Tulare County
- 7.49 inches: Wawona in Mariposa County
- 7.48 inches: Miramonte in Fresno County
During the height of the episode, seven flash flood warnings were in effect simultaneously up and down the Sierra Nevada.
In the mountains, several communities were forced to evacuate as avalanche danger spiked into the “extreme” category. Among the evacuees was Tony Phillips, the creator of Spaceweather.com. On Friday, Phillips posted on his website that he’d be fleeing his residence ahead of the impending danger. He posted an update on Monday that read “Once-in-a-generation snowstorms and an avalanche in California have stranded Dr. Tony Phillips in the mountains of the Eastern Sierra with only 10 sled dogs and a satellite dish to update Spaceweather.com. Everything is okay!”
The ensemble of wild weather even included a pair of tornado warnings Saturday, prompted by supercells or rotating thunderstorms that flared up over the San Joaquin Valley. Saturday’s supercell in Tuolumne and Calaveras counties dropped an EF1 tornado that uprooted oak and pine trees and toppled power poles. Winds were estimated at 90 mph along the twister’s 0.8-mile path.
Another supercell produced funnel clouds northwest of Fresno on Sunday.
The coming storm: Monday night into Wednesday
Low pressure near Vancouver Island in Canada is swirling a filament of moisture-rich air toward the U.S. West Coast. It has origins in the west central Pacific about 6,000 miles away near the Federal States of Micronesia. A measure of atmospheric moisture, known as precipitable water, ranges between 1.3 and 1.7 inches. That means every column of atmosphere contains roughly that amount of water. Like a sponge being squeezed out and continually replenished, that self-replacing air mass will unload exceptional precipitation over the West.
The Weather Prediction Center’s moderate risk outlook area includes coastal Northern California and the northern Sierra Nevada on Monday, and then expands south along the central and southern coast of California and the central Sierra into Tuesday.
“Significant amounts are likely over the next 24-hrs, with [high-resolution models] showing high probabilities for accumulations of 3 inches,” wrote the Weather Prediction Center.
Snowfall levels will hover between 5,500- and 6,500-feet elevation in most areas, though they may dip slightly lower in Oregon and southern Washington.
The heaviest precipitation will come down through Tuesday evening, but sporadic downpours and heavy mountain snow showers are likely through at least midday Wednesday, and possibly longer.
“Unlike the previous event, this atmospheric river will be associated with a rapidly [intensifying] surface low rather close to the NorCal coast,” wrote Daniel Swain, a climate scientist at the University of California Los Angeles, in a blog post. “This will raise the potential for widespread and possibly damaging winds well beyond what was observed in the previous storm.”
- Rainfall totals of 3 to 7 inches in the coastal range and 4 to 8 inches in the Sierra Nevada below the snow line are likely. In the Central Valley, expect a general 1 to 2.5 inches, with a few totals in the 3 to 4 inch range not impossible.
- Flooding is a significant concern, especially considering many areas are running 5 to 10 inches above average for March. Soils are saturated and can’t absorb much excess moisture. Flood watches remain in effect until early Wednesday morning for most of central and northern California. The danger will be greatest in the 4,000- to 7,000-foot range. That’s where the higher terrain will boost rainfall rates and where a rapidly melting snowpack may combine to virtually double how much water is flowing into area streams and rivers.
- Snowfall totals of 1 to 3 feet are likely above 6,000 feet, with 5 feet or more above 8,000 feet elevation. Totals nearing 10 feet can’t be ruled out above 9,000 feet elevation, where blizzard conditions and winds gusting over 75 mph are also expected.
- High wind watches and warnings also blanket northern and central California. Onshore winds of 45 to 55 mph are expected at the coast, with 45 mph winds in the San Joaquin Valley and gusts over 60 mph in the high terrain.
- Avalanches, particularly along the east slopes of the Sierra Nevada, will remain a concern. The Eastern Sierra Avalanche Center in Mammoth Lakes has issued an avalanche watch, writing “Travel in avalanche terrain is not recommended. Avalanches may run long distances, into mature forests, valley floors or flat terrain.”