‘Inside slider’ system to deliver Thursday’s dry winds to the Bay Area

Low-pressure systems get called inside sliders when they travel long distances over land but move quickly from Canada to California. In most instances, these fast-moving systems end up carrying all the characteristics of a storm without rain or snow. That’s because inside sliders lose almost all their moisture as they travel over the deserts that cover most of Nevada and Utah and are practically bone-dry by the time they reach California.

Thursday’s low-pressure system will quickly evolve into an inside slider as it loses all its water vapor over Nevada’s Great Basin, arriving in California with next to no moisture. 

Inside sliders often are able to kick up powerful winds out of the northeast as they approach the Bay Area — especially in the North Bay. These winds, which can eventually reach the status of Diablos, can quickly dry out the air around the region. This leads to clouds fizzling away and water in plants quickly evaporating out into the parched atmosphere.

While these systems often lack rain and snow, they still manage to pack a punch.

It’s looking like Thursday will host its fair share of these northeast winds, with some of the gustiest conditions expected between 4 a.m. and noon. Some of the highlands just outside of Napa and Santa Rosa could easily gust to 35 mph, with gusts up to 45 mph possible in the Mayacamas Mountains and highlands at or above 2,000 feet.

Thursday is looking to host an array of breezy northeast — or offshore — winds. Weather models are trending toward gusts over 35 mph across parts of the North Bay’s highlands. These warm winds are likely to dry the region’s wet soils by a fair amount, though not enough to raise any immediate fire concerns.

Fortunately, these conditions will be short-lived. The inside slider will quickly move out to Southern California by the afternoon, and its northeast winds are expected to fizzle away for the second half of the day. A sea breeze is then looking to form along Highway 1 after 2 p.m., streaming moisture back to the coast, bays and eventually inland valleys.

The fast-moving nature of the inside slider system and the saturation of soils across the region will both play a role in mitigating the fire concerns that would have otherwise been much higher.

San Francisco: High pressure returns as sunshine radiates across the city. The warmth from the sun will coincide with the return of dry, northeast winds that will generally be light. Look for daytime highs peaking around 1 p.m. as they reach the lower 60s from Muni’s Ocean Beach station to the Carl and Cole Street station. Warmer weather is expected east of Sutro Tower, where downtown, SoMa and the Mission are likely to reach the mid-60s. Winds will then begin to shift, blowing out of the west after 2 p.m. and making way for cooler, moister Pacific air.

Temperatures will quickly fall to the upper 50s and lower 60s by the second half of the afternoon as gusts reach 30 mph near Lake Merced and Golden Gate Park, while gusts up to 25 mph spread out to the rest of the city.

Thankfully, all the added moisture off this afternoon’s winds will linger overnight, helping to keep nighttime temperatures from falling below the mid-40s.

Pacific coast and Peninsula: Light, dry winds off the hillsides of San Mateo County will keep clouds from forming this morning along Highway 1 between Pacifica and Half Moon Bay. Sunshine will prevail across most of the coast and bayside as high pressure builds over the Peninsula through the rest of the day. This will raise temperatures to the upper 50s on the coast and lower 60s along the San Bruno Gap, where commuters along the BART stations at Daly City, Millbrae and San Francisco International Airport can also expect a cool breeze in the afternoon.

Temperatures will be a little warmer as you head south of Millbrae along Interstate 280 and Highway 101, where residents in Redwood City, Foster City and Atherton can expect daytime temperatures closer to the mid-60s by 3 p.m.

A shift in the winds is expected after 3 p.m. across wide portions of the Peninsula, leading to a steady 3-5 degree fall in temperatures for the second half of the afternoon. Look for gusts up to 25 mph. These winds will then drop off after sunset, keeping nighttime temperatures from falling below the mid-40s.

North Bay: Northeast winds are slated for the highlands of Sonoma and Napa counties this morning, with gusts over 35 mph possible along the Mayacamas Mountains in areas that are over 2,000 feet in elevation. These winds will be much lighter in the Santa Rosa, Napa and Petaluma valleys, but they’ll reel in enough warm and dry air to the valley floors so that daytime temperatures reach the lower 70s. The exception will be some of the forested areas along the Russian River, like Healdsburg and Guerneville, where upper 60s are more likely by afternoon.

The winds over Sonoma and Napa counties will drop off by 1 p.m. and make way for westerly winds off the Pacific coast and San Francisco Bay. Look for 35 mph gusts at Point Reyes and Bodega Bay, while 25 mph gusts will be possible all along the bayside. Ferry riders taking off from Sausalito can expect gusts closer to 30 mph over the open water, along with temperatures closer to the upper 50s. Temperatures across most of the bayside and Solano County’s delta region will generally reach the lower 60s this afternoon before cooler air slips in this evening.

Temperatures will then gradually drop overnight, falling to the mid-40s by the coast, bayside and delta. Thanks to drier air away from the shoreline, upper 30s and lower 40s are expected across the inland valleys.

East Bay: Pesky northeast winds are slated for the Oakland and Berkeley hills through noon, gusting to 25 mph at times. The breeze will expand out toward the Caldecott Tunnel and usher in a warm afternoon across Alameda and Contra Costa counties. People heading in and out of the Dublin and Pleasanton BART station can expect temperatures in the upper 60s by the afternoon. Widespread upper 60s are on tap for most of the San Ramon and Livermore valleys, though a few areas around Walnut Creek could briefly hit the 70-degree mark if enough warm air filters in.

A shift in the winds begins after 2 p.m., with more westerly winds rolling into the Interstate 80 and 880 corridors. This will shave off daytime temperatures by 3-5 degrees in Richmond, Albany, Oakland, Alameda and Hayward, meaning highs are likely to only reach the lower 60s.

Winds are forecast to drop off overnight, leaving behind a trail of moisture that will keep nighttime temperatures above the lower 40s.

South Bay and Santa Cruz: A warming trend is in store for the Santa Clara Valley as warm, dry air filters through over the course of the day. Look for breezy conditions as 10-15 mph winds circulate the valley and raise temperatures to the upper 60s this afternoon. Some of this warmth will be offset by a light cool breeze off San Francisco Bay, meaning those taking BART at the Berryessa and North San Jose station can expect temperatures closer to the lower 60s after 2 p.m.

Warm, dry air will continue to spread south along the Highway 17 and 101 corridors this afternoon, raising temperatures to the upper 60s in Morgan Hill, Gilroy and Los Gatos. Some of this warm air will make it west of Highway 17 and into Felton, Ben Lomond and Scotts Valley where temperatures will likely reach the mid-60s. The same can’t be said for coastal residents in Soquel, Aptos and Capitola where a light breeze off Monterey Bay will keep daytime temperatures closer to the 60-degree mark.

Reach Gerry Díaz: [email protected]; Twitter: @geravitywave