It will hit 90 degrees in the North Bay today, and the sky tonight is forecast to stay mostly clear. Sometime around 1:18 a.m. Wednesday, the water at the mouth of the Petaluma River will stop rising. About 40 minutes after that, it will stop rising at Napa. Parking lots, parks and low roads along the bay will be wet, and there will not have been a drop of rain.
The National Weather Service issued a coastal flood advisory late Monday night for the shoreline of San Francisco and San Pablo bays. It runs until 4 a.m. Thursday. The forecast calls for up to 1.8 feet of water above ground level in low-lying spots near the shore and tidal creeks, with, in the agency’s words, “flooding of lots, parks, and roads with only isolated road closures expected.”
This is a minor advisory. Nobody is being told to leave. But buried in the boilerplate is a sentence the weather service doesn’t usually bother to write down.
The part they said out loud
Here is the advisory, verbatim:
“These predictions include both astronomical tides and roughly 3-6 inches of additional water mainly from a combination of thermal expansion and accumulated sea level rise since the tidal datum was established (1983-2001).”
Read that again. The federal forecast for tonight’s high tide has several inches of sea level rise baked into the number. Not projected. Not modeled out to 2050. Added to tonight’s arithmetic, because the water is already there.
That’s what a sunny-day flood is. No storm, no swell, no atmospheric river — just the ordinary moon-driven tide arriving on top of an ocean that sits higher than it used to. Take away those 3 to 6 inches and tonight is an unremarkable summer high tide that nobody writes an advisory about.
When it actually gets here
The number in the advisory — 7.6 feet, 1.8 feet above normal, at 11:55 p.m. — is measured at the San Francisco tide gauge. It’s not when the water peaks here.
Tides take time to push up the bay. Using the weather service’s own tide predictions, the high water that crests in San Francisco at 11:55 tonight reaches the entrance of the Petaluma River around 1:18 a.m., the mouth of Sonoma Creek around 1:33, Mare Island Strait around 1:42, and the city of Napa on the Napa River around 2 a.m.
So if you keep a boat at Cuttings Wharf, or you drive one of the low roads out past the marshes on your way to a 5 a.m. shift, the hour that matters to you is not the hour on the advisory. It’s an hour or two later.
The second peak comes just after midnight Thursday — 7.3 feet at the San Francisco gauge, 1.5 feet above normal, at 12:48 a.m., which again means closer to 2 or 3 a.m. up here.
Nobody is measuring it here
Which raises a question: how does anyone know what the water is doing on the Sonoma or Napa shoreline?
They don’t, exactly. There’s no real-time federal tide gauge on the San Pablo Bay shoreline of either county. The National Oceanic and Atmospheric Administration publishes tide predictions for the Petaluma River, Sonoma Creek, the Napa River and Mare Island — but those are calculated offsets, not instruments reporting water levels right now. The nearest gauge actually taking a reading tonight is across the bay at Richmond.
So the North Bay learns what its own shoreline is doing by doing math on a measurement taken in San Francisco.
What’s out there to flood
The Sonoma and Napa edge of San Pablo Bay is not the Embarcadero. It’s marsh, levee, farm road and highway, and a lot of it sits below the tide already.
The Napa-Sonoma Marshes Wildlife Area alone runs about 15,200 acres of baylands and tidal slough between the Napa River and Sonoma Creek, most of it former salt ponds. At Sears Point, the Sonoma Land Trust reopened 960 acres to the tide in 2015 on land that had sunk 6 feet below the marsh around it; a sloped levee is what stands between that water and Highway 37, the rail line and the farms behind it. Out at Skaggs Island, the ground has subsided 3 to 7 feet, and the things sitting behind those levees include a PG&E substation and the SMART tracks.
None of that is failing tonight. The advisory forecasts wet parking lots, not a levee breach. But it’s the ground the tide is rising against.
Highway 37
Which brings you to the road everyone in Sonoma, Napa and Marin already has an opinion about.
Caltrans has been blunt in writing about what happens to State Route 37, the 21 miles of highway between Novato and Vallejo that tens of thousands of North Bay commuters use every day.
“Nearly the entire length between Novato and Vallejo is predicted to become permanently submerged as sea levels rise if modifications are not made,” the agency says on its own project page. Portions of it, Caltrans adds, “will be completely inundated by 2050.”
The highway has closed for flooding before — the State Coastal Conservancy points to closures in 2017 and 2019. Caltrans’ own engineering report for the corridor is grimmer than the public page: under conditions that exist today, with no additional sea level rise at all, a routine one-year storm already puts water across about 960 feet of the roadway roughly every year or two.
There’s a $430 million project for the Sears Point-to-Mare Island stretch, scheduled to break ground in August 2027 and finish in late 2029. It will add a lane, widen the Sonoma Creek bridge and raise the pavement where flooding is worst.
It will not fix the problem, and Caltrans says so plainly: “the Project does not address sea level rise.” That job belongs to a future rebuild — a raised causeway across the marsh — and the project scheduled to break ground in 2027 is designed for a 20-year life.
The quiet part
Here’s the thing that took the longest to run down.
Neither county has assessed what sea level rise does to its own bay shoreline.
Sonoma County has an adopted sea level rise vulnerability assessment, and it’s a serious document — but it covers Bodega Bay and the outer coast. The bay side, the side that floods tonight, isn’t in it. Napa County doesn’t have one at all. Under a 2023 state law, every local government on the bay has to produce a shoreline adaptation plan by Jan. 1, 2034. Across the bay and around the Carquinez Strait, Solano County, Vallejo, Benicia, Fairfield and Suisun City have started. Sonoma and Napa have not.
The counties studied the ocean and skipped the bay.
Tonight
As of Tuesday morning, the tide had not made it onto the pages where a resident would look for it. Napa County’s alert center read, in full, “There are no active alerts.” Sonoma County’s emergency site still led with the all-clear from the Ledson Fire. Caltrans’ road-conditions feed reported no restrictions on Highway 37, 121, 29 or 12.
That’s probably correct. It’s a minor advisory, and the most likely outcome tonight is a flooded parking lot at a boat launch and some standing water on a levee road at 2 a.m., gone by breakfast, seen by almost nobody.
But the weather service counted it. At the San Francisco gauge, there were zero days of high-tide flooding in all of 2025. There have been four already in 2026. NOAA’s middle-of-the-road projection has that same gauge at about 10 such days a year by the 2050s.
The gauge at the Golden Gate has been reading the ocean since September 1897, when William McKinley was in the White House. Over that stretch the water has come up about 8 inches a century, and it is still coming. Tonight it lends 3 to 6 of those inches to an ordinary high tide, which can carry them onto a levee road in Sonoma County at 1 a.m.
Then it goes back out, and comes again Thursday.