The temperature pendulum is set to swing colder as an area of low pressure quickly sweeps across northern California on Tuesday, resurrecting the marine layer and igniting unstable air in the Sierras.
Based on the intensity of Tuesday’s conditions in the forecast, some elements of cold unsettled weather could persist through the end of the month.
Return of May Gray
Heavy fog and heavy fog have a good chance of reaching cities as far inside Sacramento on Tuesday if the area of low pressure develops as strongly as some of the weather models are predicting.
The U.S. Weather Service predicted the restoration of marine clouds along the coast and Gulf shoreline Tuesday morning, signaling that the cloud ceiling reached 3,500 feet above sea level. This means that high fog from the Pacific Ocean is likely to cast a cloud cover over much of the Bay Area.
A healthy sign of lows in recent weather model runs also raises the possibility that a counter-clockwise flow Tuesday afternoon will produce wind gusts in narrow swaths of San Francisco Bay, including the Golden Gate Bridge and the San Francisco River Delta -San Joaquin. As cold Pacific air moves inland, gusts will reach 30 mph, keeping daytime highs near average for late May: upper 60s on the coast, 70 onshore gulf and below 80 degrees in cities that overlook the Central Valley.
Low flow will also increase over the Sierra Nevada on Tuesday afternoon.
Sierra Thunderstorms
If the atmosphere was a Kirby game, the low would need to absorb enough atmospheric energy over the mountains to be able to spew out several thunderstorms.
Based on the readings of the European weather model, the air over the highest parts of the Sierra on Tuesday will have between 100 and 150 joules per kilogram of energy available for their development. These levels are suitable for afternoon thunderstorms that could produce 35 mph gusts, small hail and frequent lightning.
The Storm Prediction Center is forecasting a general severe weather threat Tuesday for parts of California, mainly along the Sierra Nevada.
Baron/LysFortunately, Tuesday’s thunderstorms will be short-lived.
Another bit of good news is that cold winds over the Bay Area are reaching the Sierras, causing daytime temperatures that are near average for May. This will help slow the ongoing snowmelt in the area.
Tuesday breakdown
San Francisco: Tuesday morning will be cool and slightly foggy from Ocean Beach to West Portal, while a mix of high fog and thin cirrus clouds moves warmer, drier air over downtown.
San Franciscans near the water will experience more southerly winds compared to recent days. Southwest winds will pick up after 1 p.m., gusting to 30 mph along Grand Highway, while steadier winds around 25 mph spread to Twin Peaks, Diamond Heights, Forest Hill, Billy Gott- Hill and Glen Park. Skies will be mostly partly cloudy throughout the day as highs reach the low 60s on the west side and the mid 60s east of Sutro Tower.
Some vapor fog will try to resurface around sunset, but it may not be fully effective until 9:00 p.m. Look for overnight lows in the mid-50s near the water and in the lower 50s in some hillside areas.
Pacific Coast and Peninsula: A cold and partly cloudy Tuesday is expected for residents along Highway 1 between Pacifica and Half Moon Bay. A mix of morning fog followed by a light southwest wind gusting to 30 mph will keep daytime highs in the 50s to 60s. By 1 p.m., some of that wind will spill into the San Bruno Gap, bringing similar temperatures to Daly City, Millbrae, and San Mateo.
A cold southwest flow will be blocked from reaching south of Millbrae and along the I-280 and 101 corridors, meaning daytime highs from Hillsborough to Menlo Park and Stanford will likely reach the mid-to-upper 60s Tuesday afternoon. Moist air will flow around an area of low pressure to the north around sunset, leading to a slow and steady return of heavy fog to the coast. That moisture will help keep temperatures Tuesday night across the peninsula from dipping below 50 degrees.
North Bay: An area of low pressure will deepen over Napa and Sonoma counties on Tuesday, bringing temperatures down rapidly in the Napa, Santa Rosa and Petaluma valleys. Partly cloudy skies with hints of fog are expected in some of the higher elevations around Guerneville and Healdsburg as daytime temperatures reach 60 to 70 degrees. Highs in the mid-70s are likely east of the Mayacamas Mountains and along interior Solana County.
Southwesterly winds will continue to pick up Tuesday afternoon along the coast of Sonoma and Marin counties, as well as San Francisco Bay and the Delta. Expect winds of 20 to 30 mph in Mill Valley, Tiburon, Sausalito, Benicia and Vallejo as the cooler air moves in. Waterside residents can expect temperatures in the low to mid 60s by the evening.
Additional moisture in the air outside the low pressure area will act as a blanket tonight, keeping overnight temperatures above 50 degrees.
East Bay: Temperatures will drop again Tuesday in Contra Costa and Alameda counties. An area of low pressure to the north will help turn cool, moist winds from the southwest toward the San Ramon and Livermore Valleys. On Tuesday afternoon, residents of Walnut Creek, Pleasanton and Dublin can expect partly cloudy skies and daytime highs in the lower 70s.
A gust of southwesterly winds Tuesday will also increase the chance of patchy fog in the Oakland and Berkeley hills, while most residents along the I-80 and I-880 corridors can expect a mix of clouds and sun. In Richmond, Oakland, Alameda and Hayward, look for daytime highs in the mid to upper 60s.
High clouds are likely to move across the region Tuesday night as moist air moves toward the Sacramento Valley. Winds are likely to be concentrated around the Delta region, where cities such as Concord and Pittsburgh can expect gusts of up to 30 mph. Winds will die down overnight, but moist air moving in from earlier in the day will help keep overnight lows closer to the lower 50s.
South Bay and Santa Cruz: Commuters along the Highway 101 corridor between Gilroy and Morgan Hill are in for a cloudy morning Tuesday morning as heavy fog from Monterey Bay moves north toward the Santa Clara Valley. This surge of moisture from the southwest will also cause fog to spread north of Pasatiempo Pass and along much of Highway 17 by 10am. Watch for reduced visibility in the Santa Cruz Mountains until the fog clears just before noon.
Most of the clouds will clear by midday Tuesday, but winds will pick up in most of Santa Cruz and Santa Clara counties. Watch for cool gusts of 20 to 25 mph from cool air around Monterey Bay, keeping daytime temperatures in the 50s to 60s in Capitol, Sokweli, Aptos and the Santa Cruz Boardwalk. Temperatures in the low 60s will also be widespread in the Santa Cruz Mountains as this cold air turns north toward Los Gatos.
A surge of cool air Tuesday afternoon will lose some of its strength as sunlight warms it by the time it reaches San Jose, but it will still be strong enough to keep daytime temperatures in the Santa Clara Valley only dropping below 70 degrees. With much of that cool air coming in from Monterey Bay, the air over Santa Cruz and Santa Clara counties will also be moist Tuesday afternoon and evening. The air will act as a blanket over the region, keeping overnight lows from dropping into the 40s.
Contact Gerry Diaz: [email protected]; Twitter: @geravitywave