Two lows are heading towards Northern California today, with the first system arriving late Thursday evening through Friday morning. These systems will bring moderate winds and rain showers to the Bay Area this weekend, which will go a long way in keeping soils healthy. Some of the precipitation will then lead to beneficial additions to California’s snowpack as the low-pressure systems bring snow shower waves to Tahoe and many of the ski resorts in the Sierra Nevada.
On Friday afternoon the first low rolls into the Sierra Nevada. Bay Area residents heading into the Sierra foothills Friday afternoon will contend with gusts of 20 to 30 miles per hour on sections of I-80 between Auburn and Donner Pass. Scattered gusts of up to 50 mph are possible after sunset at Donner Pass and around Mammoth Lakes after sunset as snow showers gather.
Friday’s peak winds over the Tahoe Territory and Sierra Nevada, with gusts of 20 to 30 mph along I-80 and up to 60 mph on peaks over 6,000 feet — including Kirkwood.
Chronicle of San FranciscoThese winds will be followed by brief snow gusts above 3,000 feet, dropping between a tenth and 2 inches of snow at those elevations from Friday afternoon through Saturday morning, while Donner Summit and peaks above 6,000 feet will see up to a half foot of snow through Saturday noon. This is good news for those traveling to Tahoe on Saturday morning, as weather patterns like the Canadian and American have reduced the amount of snow forecast from this first depression.
A snowy Saturday night and Sunday for the Sierra Nevada
The cold front from the second low will sweep over the Sierra Nevada after midnight Sunday, blanketing Tahoe, Truckee, Donner Pass and most of the I-80 pass with heavy snow gusts, wind gusts up to 45 mph and whiteout conditions up Sunday afternoon.
The cold front from the second low will sweep over the Sierra Nevada after midnight Sunday, blanketing Tahoe, Truckee, Donner Pass and most of the I-80 pass with heavy snow gusts, wind gusts up to 45 mph and whiteout conditions up Sunday afternoon.
When the second low pressure system sweeps over northern California after 4 p.m. Saturday, visitors and residents in the Tahoe area will want to shut the hatches. This system’s cold front will pour laps of heavy snow and strong winds with gusts up to 45 mph in parts of Tahoe and over 60 mph above 3,000 feet. The worst of these conditions will peak overnight and into Sunday morning. When all is said and done, weather models are projecting anywhere from 2 inches to a half foot of snow on the ground around Tahoe and most of I-80 from Auburn to Reno and about 1 to 2 feet of snow above 4,000 feet.
Residents who take a southerly route this weekend and make it to Yosemite and Mammoth Lakes will encounter similar snow totals, with the windward foothills in Yosemite Valley seeing up to 9 inches of snow, while the surrounding mountains easily over a foot and a come half snow by Sunday afternoon. Keep up to date with road closures and alternative routes from Caltrans.
Cyclists ride through the Twin Peaks in San Francisco on Wednesday. Two lows are expected to hit the Bay Area and the rest of Northern California this weekend, beginning with widespread rain and snow on Friday.
Juliana Yamada/The ChronicleThursday collapse
• San Francisco: A low-pressure system will slowly approach San Francisco this morning, bringing cool air and cloud streams over Sutro Tower, Bernal Heights, Mount Davidson and several other hills near West Portal. Sunshine will be hard to come by on the west side today as the air gets wetter in Sunset and Richmond counties and clouds darken. Some of these clouds will eventually flow east of Sutro Tower into Castro, downtown, and SoMa before discharging into San Francisco Bay. Look for daily highs just making it to the 60 degree mark between Muni’s lines from West Portal to the Embarcadero, while commuters heading to Glen Park and Sunnydale this afternoon expect highs in the mid to high 50s.
Wind picks up around Ocean Beach, Golden Gate Park and the Presidio from about 4 p.m. Watch out for gusts of up to 20mph over most of the west side after sunset. These wind gusts will reach up to 30 mph and spread east of Highway 1 towards Twin Peaks by 6 p.m. Weather models project the majority of these wind gusts will stay west of Church and Market, but gusts of 15 to 20 mph cannot be ruled out for downtown, the Tenderloin, Japantown and SoMa tonight.
If you plan to be out after 10pm tonight, be sure to have a raincoat or umbrella handy. Drizzle begins to develop on slopes and coast. The depression will then pour its first round of rain showers into San Francisco after midnight.
• Pacific Coast and Peninsula: Once the peninsula’s foothills and peaks thaw from the morning lows in the 30’s, clouds will begin to pour into the coast. Winds on the coast will hit 20mph this afternoon thanks to a cold front spreading eastward and filling the San Bruno Gap with dark clouds. BART riders at San Francisco International Airport and Daly City will feel the afternoon wind picking up on their way home as they hit up to 30 mph along the gap.
Drivers on Highway 92 between Half Moon Bay and San Mateo will also experience gusts of 25 to 30 miles per hour as drizzle develops in the foothills tonight. Total cloud cover during the day will help keep daily highs in the upper 50s along Highway 1 while mid-50s are found in Daly City, South San Francisco and Millbrae.
Total cloud cover will hold overnight lows in the lower 50s along the coast, while towns along the 101 corridor south of Millbrae are likely to drop to the upper 40s tonight thanks to cloudbursts. Expect drizzle and light rain spreading to the San Bruno Gap and most of Highway 1 by midnight as the cold front approaches the peninsula.
• North Bay: Today will be the last bitterly cold morning of the week as cloud cover slowly settles over the valleys of Petaluma, Santa Rosa and Napa – along with some of the peaks in the Marin Headlands. Look for morning lows in the mid-30s in the inland valleys. Commuters traveling on Highways 101, 29 and 17 after 9 a.m. will notice an increase in winds after 1 p.m., with gusts reaching 30 mph by the afternoon. Skies will also darken as clouds move onto the Sonoma County and Marin County coasts and spread east.
Portions of Napa and Sonoma counties, including Healdsburg and Guerneville, will hit the 60 degree mark between 1 p.m. and 3 p.m., just before most clouds gather. The cool air off San Pablo Bay and the Delta keeps the rest of the Banana Belt running 101 from Novato to San Rafael and towns along Highways 37, 80, and 780 in Solano County in the upper 50s.
Look for regional overnight lows that fall only in the 40s and 50s thanks to cloud cover overhead. Residents along the Marin Headlands and Sonoma coast will then see drizzle and light rain showers around 11 p.m., which will continue to spread into Napa and Sonoma counties after midnight.
• East Bay: One of the warmest areas in the East Bay this morning will once again be Alameda, where the warm waters of San Francisco Bay will keep lows below 40 degrees. Most East Bay residents along the I-80/I-880 corridors will wake up to another round of morning temperatures in the upper 30s, while wind-sheltered sections of the Oakland and Berkeley hills are likely to drop back to freezing before sunrise – including valleys near the Caldecott Tunnels.
After 9:00 a.m., temperatures will rise steadily to the mid-40s in the coldest spots east of the I-680 corridor. Cities such as Concord, Walnut Creek, and the San Ramon and Livermore valleys warm to the upper 40s and low 50s by 10 am and reach their daily highs in the upper 50s just after 1 pm. Daily highs in Bay Area cities like Oakland, Fremont, Hayward and Richmond will edge closer to the 60 degree mark. But clouds will spread quickly over Alameda and Contra Costa counties this afternoon as northwesterly winds — gusting to 30mph — spread dark clouds to the rest of the region.
Clouds will then act as a blanket over most of Contra Costa and Alameda counties tonight, increasing overnight temperatures by another 5 to 7 degrees. This means the lows will only fall into the mid to high 40s, with some isolated patches of the lower 40s near Moraga and parts of the San Ramon and Livermore valleys.
• South Bay and Santa Cruz: This will be the last bitterly cold morning this week for the Santa Clara Valley and Santa Cruz Mountains. Clouds from a low pressure area will blanket most of the region tonight and protect bayside residents in San Jose, Milpitas and Sunnyvale, as well as coastal residents in Santa Cruz, Aptos and Capitola from further cold snaps this week. But these clouds will heave a hefty amount of moisture and wind onto the coast.
After 1 p.m., expect windy conditions in mountains and valleys with gusts in excess of 20 mph hovering over East San Jose and the foothills of the Santa Clara Valley. These winds contribute to clouds pouring ashore in the afternoon and evening. Residents along the Highway 101 and 17 corridors will see daily highs into the low 60’s. Some of the warmest places will be San Jose, Los Gatos and Morgan Hill, where mid-60s are possible. This warmth will be contrasted by the upper 50s and lower 60s available for the Santa Cruz Mountains and coast, where more widespread winds gusting up to 30 miles per hour will occur this afternoon and evening.
After sunset, the South Bay and Santa Cruz Mountains will catch some of the heat from earlier in the day thanks to cloudy skies. This will keep most of the Santa Clara Valley and mountains in the 40’s overnight. After 10pm watch out for erratic drizzle in the foothills and mountains
Gerry Díaz (he/she) is a weather forecaster for the newsroom of the San Francisco Chronicle. Email: [email protected] Twitter @geravitywave