three storms poised to pummel the San Francisco Bay Space
The storm door is wide open in the San Francisco Bay Area. Three systems sweeping the region over the next week bring a wintry mix of cold temperatures, heavy rain, isolated thunderstorms, small hail, high altitude snow and the first atmospheric flow of the season.
The first storm, the weakest of the three, arrived on Thursday night, causing light rain in the area overnight, with more showers on the way on Friday morning and afternoon. A cold front associated with the storm will blow up the region this morning, creating instability in the atmosphere and offering a 10% probability of individual thunderstorms, heavy rain, small hailstones and brief strong gusts of wind.
“The main front is moving through and is currently over the South Bay,” said Drew Peterson, a forecaster with the National Weather Service at 7 a.m. daily, day-to-day. “
The cold air entering behind the storm will lower temperatures, with highs on Friday afternoons in the lower to mid 50s in the valleys and along the coast and in the 40s in the hills and ridges. The snow will drop to 4,000 feet, and Mount Hamilton at 4,265 feet and Mount St. Helena at 4,327 feet could see light dusting of snow.
The main threat on Friday will likely be slippery roads as oil from cars has been building up on the roads recently, Peterson said.
Friday nights and Saturdays are mostly dry and allow people to exercise outdoors. Overnight temperatures drop into the 40s and 30s, while Saturdays are mostly sunny and slightly warmer with temperatures between the mid and late 50s.
People shelter from heavy rainfall along Lake Merritt in Oakland, California on January 4, 2021. A cold front that crossed the San Francisco Bay Area included high winds and brief intense rains.
Douglas Zimmerman / SFGATE
The second system will sweep the area on Sunday and Monday bringing in widespread light to moderate rain showers, another shot of cold air, and snow at altitudes above 2,000 feet. Other peaks could have light clouds of dust, including Mount Diablo in East Bay and Mount St. Helena in North Bay.
Peterson said snow could affect some roads like Skyline Boulevard that runs through the mountains in the Santa Cruz and San Mateo counties.
Monday night is expected to stay dry before the strongest system of the three – an atmospheric river – arrives Tuesday through Wednesday. These systems carry long plumes of water vapor in the atmosphere – 250 to 375 miles wide on average – and are often referred to as rivers in the sky. On the west coast, they mostly come from the South Pacific and collect incredible amounts of moisture on their way from the tropics over the ocean.
The heaviest rain will be Tuesday night through Wednesday, and Peterson said areas like San Jose that are protected by mountains could see up to 1.5 inches, more exposed areas like San Francisco and Oakland up to 3 inches, and mountain locations 6-20 Inch .
Meteorologists are closely monitoring this event as it could lead to mud and rock slides and flooding in lower and poorly drained areas. Higher rainfall rates could trigger debris flows in burn scars from forest fires, particularly in the Santa Cruz Mountains, where the CZU Lightning Complex charred 86,509 acres in August.
The cool, choppy weather will last until the end of next week and the following weekend.
“We’re seeing a shift to cooler weather by remembering the month and possibly beyond as La Niña dwindles,” Peterson said.
La Niña, the cousin of the better-known El Niño, is the cooling of equatorial waters in the eastern and central Pacific, which can affect atmospheric conditions worldwide. In La Niña conditions, the Bay Area may experience drier weather as the storms are driven into the Pacific Northwest, and that is exactly what developed at the start of this rainy season.
“We’re starting to see the La Nina weaken and that has an immediate effect on our weather,” said Peterson. “At least I think that’s part of what we’re seeing here. It’s part of the story, but not the whole story.”
