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.

The Storm Prediction Center is forecasting a general severe weather threat Tuesday for parts of California, mainly along the Sierra Nevada.


Fortunately, 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.


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