Warm winter rains pounded the Mother Lode foothills and mountains at near-record and record-breaking rates Thursday, unleashing mud slides, rock slides, and runoff that swelled streams, creeks and rivers, and flooded some key bridges, including the Highway 49 Cement Plant Bridge at Caleveritas Creek south of San Andreas.
Now this wet warm kiss of a storm is about to change costumes to become another potential low-elevation snow, freezing cold beast by Saturday and Sunday.
Flash flood warnings were issued Thursday for parts of western Tuolumne County and eastern Calaveras County, and 24-hour rainfall totals included 2.32 inches at Telegraph Hill between Sonora and Columbia, 3.72 inches at the Calaveras Ranger Station near Avery, 4.74 inches at Rail Road Flat, 2.3 inches at Pinecrest, and 6.1 inches at Tiger Creek Powerhouse on the Mokelumne River near West Point.
Before Thursday, the previous Sonora record rainfall for Feb. 14 was set in 1937 with 1.87 inches.
Out of sand
Flooding was such a widespread problem Thursday in the Columbia area they ran out of sand for sandbags at Columbia Airport, said Jason Terry, an analyst with Tuolumne County administration. More sand was available at the Tuolumne Road Yard, 18870 Birch Street; Tuolumne City Fire Department; the Jamestown Road Yard, 18188 7th Ave.; and the Big Oak Flat/Groveland Road Yard, 11240 Wards Ferry Road.
Maximum wind gusts in the Mother Lode between 12:01 a.m. and 12 p.m. Thursday included 68 miles per hour near Twain Harte, 37 mph between San Andreas and Angels Camp, 54 mph east of Coulterville, 46 mph east of Groveland, and 70 mph near Big Oak Flat Road in Yosemite National Park.
Many roads were closed due to flooding, mud and rocks, erosion and undermining, downed trees and downed power lines.
Closed roads, blocked roads and flooded roads before sundown Thursday included Big Oak Flat Road in Yosemite, Marshes Flat Road at First Creek, Shell Road outside Jamestown, Rawhide Road near Sonora High School and Longeway Road out near Crystal Falls, Twain Harte Drive at Highland, Parrotts Ferry Road south of the New Melones-Stanislaus River bridge, and Highway 26 at Woodhouse Mine Road southwest of West Point.
Shortly after 3 p.m., Tuolumne County Roads Division workers closed Kewin Mill Road between Comstock Ranch and Belleview Elementary School due to flooding. A detour was expected to be in place in Cedar Ridge with a single lane, with flaggers, traversing Broken Pine Road, Hitching Post Road, Pack Trail Road to Old Oak Ranch Road.
A river in the sky
Thursday’s storm was a warm, wet atmospheric river in the sky that stalled over Calaveras and Tuolumne counties long enough to unload historic volumes of rain at intensities approaching 2 inches or more per hour in some locations. There were concerns that high rain levels would melt significant snowpack depths at elevations up to 6,000 feet.
Shortly after 2 p.m. Thursday, two men stopped their pickup on South Green Street to watch whitewater rising in Sonora Creek next to Coffill Park. One of the men remarked, “Looks big enough to surf.”
Unlike last week’s storms, Thursday’s storm brought no low-elevation snow but it still wreaked havoc with exposed Pacific Gas and Electric lines and other utility lines throughout the Mother Lode.
As of 4 p.m. Thursday, there were more 440 Pacific Gas and Electric customers without power in Groveland and east of Groveland, and more than 100 customers without power along Jacksonville Road outside Jamestown.
Outages were also reported near Phoenix Lake-Cedar Ridge and in south Sonora towards Jamestown. More than 500 customers were without power in the Glencoe-Rail Road Flat area, and there were more than 100 customers without power near Jesus Maria and scattered throughout the Mountain Ranch area. There were also more than 100 outages reported for PG&E customers in San Andreas.
Erosion and crumbling slopes
A mudslide and debris flows forced the closure of Highway 26 at Woodhouse Mine Road southwest of West Point, Caltrans District 10 communications staff said Thursday afternoon. Caltrans crews were working with heavy earth-moving machines to clear the road. There was no estimated time of reopening.
At 12:07 p.m., California Highway Patrol personnel in Calaveras County responded to reports of erosion and crumbling slopes at Highway 4 and Utica Powerhouse Road near Angels Creek in Angels Camp. Parts of a road were reported to be falling apart and washing away.
Flash flood warnings were issued before 11 a.m. Thursday for western Tuolumne County and eastern Calaveras County and remained in effect through 1:45 p.m., with thunderstorms producing heavy rains and flash flooding expected at locations including Sonora, Chinese Camp, Tuolumne City, Phoenix Lake-Cedar Ridge, Mi-Wuk Village and Twain Harte.
An earlier flash flood warning came shortly after 10 a.m. when r adar indicated thunderstorms producing up to two and a half inches of rain with mudslides were reported and sections of Highways 26 and 49 were closed. Flash flooding was to be expected in San Andreas, Rail Road Flat, Glencoe, Mountain Ranch, Pioneer, Cooks Station, Lily Gap, Hodson, Copperopolis, Copper Cove Village and Angels Camp.
Chris Hintz, a National Weather Service meteorologist in Sacramento who put out the flash flood warnings, said at 12 p.m. thunderstorms had already dumped 1 inch of rain in the past hour, and 1 to 2 inches total were possible in the warning area before 2 p.m.
Authorities urged motorists to avoid crossing flooded roads, noting that most flood deaths occur in vehicles.
Snow levels were rising as the warm, wet atmospheric river storm dumped temperate rains on vulnerable volumes of snowpack at elevations up to 6,000 feet. There were concerns that warm rains on snow could melt snowpack and increase runoff volumes at lower elevations.
About 11 a.m. in downtown Sonora, one deluge of wind-blown rain and hail came down so steadily it generated roaring noises that sounded like an active waterfall on South Green Street.
Earlier Thursday, workers with Tuolumne County’s Roads Division closed Red Hills Road and Sims Road in the Chinese Camp area because of unsafe driving conditions.
Precipitation measurements as of 12:01 a.m. Thursday showed the Mother Lode's primary watersheds, including the Stanislaus and Tuolumne river basins, with 29.5 inches since the current water year started Oct. 1. That's 124 percent of average for the date Feb. 14. Nearly half that precipitation —14 inches — has come in February.
In Merced County, road crews re-opened northbound and southbound State Route 59 from Sandy Mush Road to Mission Avenue at 9:45 a.m. Thursday. The low-lying, flood-prone road was closed 10 days ago on Feb. 4 in both directions due to flooding at the Mariposa Creek bridge.
Forecasts for the Sonora area and up and down the Highway 49 corridor show rain showers continuing Friday, then possibly changing to snow showers Friday night. Chances of rain or snow in Sonora are 90 percent Saturday, 70 percent Saturday night and there’s a 60 percent of snow showers in Sonora on Sunday.
Snow levels were expected to rise as high as 8,000 feet by Thursday evening, then drop again back down over Friday and Saturday to as low 1,000 feet by Sunday morning.
Overnight lows in the Sonora area are supposed to be around 40 degrees early Friday, 34, early Saturday, 31 early Sunday, and 26 early Monday. Dry weather is expected early next week.
Contact Guy McCarthy at email@example.com or 588-4585. Follow him on Twitter at @GuyMcCarthy.