May gray has arrived in the Bay Area, and it’s here to stay.

The foggy weather pattern, synonymous with summer in San Francisco, settled in along the coast over the past few days, spreading farther inland every morning. But coastal parts of the Bay Area could be due for a brief switch-up Tuesday. 

On Tuesday morning, low clouds and fog are expected to encroach again on San Francisco’s west side; however, the forecast is a bit unpredictable due to the possibility of a coastal eddy developing. An eddy is a region of counterclockwise spinning clouds which result from varying wind speeds and directions. Eddies occasionally form between Point Reyes and San Francisco during spring and summer mornings.

Coastal eddies can result in clear skies along the Peninsula and in downtown San Francisco, but enhanced cloud cover in the Monterey Bay and other portions of the coast. If the eddy does develop Tuesday, places like Daly City and San Francisco could see sunny weather for part of the day. 

Temperatures are expected to be in the mid- to upper 50s at the coast Tuesday. If the eddy persists until the afternoon, beaches under sunny skies may reach the lower to mid-60s. Inland areas will reach the upper 70s to mid-80s. Another eddy may develop Wednesday, but there is low confidence in this forecast.

Tuesday breakdown

San Francisco: Fog is expected to hug the coast at least part of the day, meaning temperatures are expected to reach only the upper 50s west of Twin Peaks. If a coastal eddy develops, downtown may be warmer than Monday, with sunshine prevailing a bit earlier and the low clouds returning slightly later in the day. Highs should reach the mid-60s. However, there is a chance clouds and fog persist all day, keeping temperatures below 60 degrees. Lows will be in the lower 50s with a high likelihood of clouds and fog returning overnight.

North Bay: The ebb and flow of low marine clouds is expected in Marin and Sonoma counties, but what time the sunshine could prevail in the morning remains in flux depending on the development of a coastal eddy. Regardless, highs should be a few degrees warmer than Monday, in the lower to mid-70s in Santa Rosa, San Rafael, Petaluma, Napa and Vallejo. Eastern Solano County is expected to be mostly sunny all day, with highs in the lower 80s in Fairfield and mid-80s in Vacaville. Lows in the North Bay will be in the lower to mid-50s. 

East Bay: Fog is unlikely to spread as far inland as it did Monday, meaning a sunnier start to the day east of the Oakland and Berkeley hills. It will also be slightly warmer, with highs near 80 in Livermore, Dublin, Danville, Walnut Creek and Concord. Temperatures will also increase a couple of degrees near the bay shoreline, with highs in the mid- to upper 60s in Oakland, Berkeley and Alameda and the lower to mid-60s in Hayward and Fremont. Lows will be in the lower to mid-50s with fog likely returning.

Pacific Coast and Peninsula: The development of a coastal eddy will determine just how much sunshine each portion of San Mateo County receives. It should be a bit warmer than Monday along the bay shoreline, with highs in the mid-60s in San Bruno, upper 60s in San Mateo and lower 70s in Redwood City. 

Temperatures in Daly City, Pacifica and Half Moon Bay will struggle to crack the upper 50s, but if the clouds burn off for even a few hours, highs may reach the lower 60s. The sea breeze will pick up in the afternoon gusting 20 to 30 mph. Lows across the Peninsula will be in the lower 50s, with fog likely spreading all the way toward San Francisco Bay by Wednesday morning.

South Bay and Santa Cruz: A coastal eddy may spin up in the morning in Monterey Bay, which would keep fog going most of the day in Santa Cruz. Highs along the coast will likely drop a couple of degrees as a result, to the lower 60s. Scotts Valley will be in the upper 60s, and Boulder Creek and Ben Lomond should reach the mid-70s. Lows will be in the lower to mid-50s.

While the coast may be cloudier than Monday, the South Bay should trend sunnier. Skies should start off sunny Santa Clara Valley, allowing temperatures to climb faster and higher, reaching the lower 70s in San Jose, Gilroy and Morgan Hill and the upper 70s in Sunnyvale, Palo Alto and Milpitas. Lows in the South Bay will be in the lower 50s.

Reach Anthony Edwards: anthony.edwards@sfchronicle.com