At 4:52 a.m. Saturday a forecaster at the National Weather Service’s Bay Area office in Monterey rewrote a few paragraphs almost nobody outside a fire camp reads, and moved the North Bay into them.
The paragraphs live in the fire weather section of the Area Forecast Discussion, the shop-talk product where Bay Area forecasters explain their thinking to each other and to the agencies that fight fire. Thursday’s version projected Energy Release Component values — an index of how much energy is available to a fire in the fuel on the ground — climbing above the 97th percentile across the interior Central Coast. That was the only area Thursday’s text named. Saturday’s version added a second: the “Mid Coast to Mendocino (North Bay) zone.” It also cut the forecast for overnight humidity recovery above 2,000 feet from 35% to 45% down to 25% to 40%, “with some favored locations struggling to reach even 25%.” The first place it named was the Mayacamas.
“Near critical fire weather conditions are expected through the weekend and potentially into next week,” the section reads.
No fire weather alert says any of it.
No warnings anywhere in California
As of Saturday morning there wasn’t a single Red Flag Warning or Fire Weather Watch in effect anywhere in the state. The weather service’s active-alert feed listed 29 alerts across California: 15 extreme heat warnings, seven heat advisories, three beach hazards statements, two high surf advisories and two air quality alerts. Nineteen Red Flag Warnings were running nationwide at that same moment, and every one of them was in Oregon, Washington, Idaho, Montana or Wyoming.
That gap is not an oversight. It’s how the product is built.
Wind buys the warning, not dryness
California’s weather service offices work from criteria published by the interagency coordination centers that run the state’s fire-danger reporting. Those criteria come down to a grid crossing wind speed against humidity. For Northern California west of the Cascade and Sierra crest, an overnight humidity that tops out between 31% and 45% only rates a warning if the sustained 20-foot wind runs 12 mph or better. The grid adds that “the wind event should be expected to last at least 8 hours.”
The zone forecast puts the North Bay Interior Mountains above 2,000 feet at 35% to 45% tonight, inside that band. The ridge-top wind forecast for the same hours is west at 5 to 10 mph, gusting to 20.
Drop the humidity a little further and the wind threshold falls with it. Below 31%, a warning comes into play at sustained winds of just 6 to 11 mph — which the ridges are forecast to see. The Area Forecast Discussion’s figure for locations above 2,000 feet, 25% to 40%, reaches into that range. The North Bay’s own zone forecast does not.
Fuel dryness never appears in the grid. It’s assumed before the grid starts: the grid takes as given that 10-hour fuel moisture is at or below 6%, “annual grasses are cured, and that no wetting rain (greater than 0.10 inch) has fallen in the last 24 hours.” Dry fuel is the cost of admission. Wind is what buys the warning.
The two measures are built to miss each other. The Northern California Geographic Area Coordination Center keeps the definitions fire agencies work from. It describes the Energy Release Component as a function of the fuel model and of live and dead fuel moisture. Of the influence wind speed has on the number, it says flatly: “wind speed has none.” It calls the index “the best fire danger component for indicating the effects of intermediate to long-term drying.” The number that tracks how dry the country has gotten is blind to wind by design. The warning that would tell anyone about it is triggered by wind.
What the weekend actually looks like
The zone forecast for the North Bay Interior Mountains calls for highs of 92 to 100 today and 93 to 101 Sunday, with afternoon humidity of 20% to 30%. Tonight’s low there is forecast 4 degrees warmer than Friday night’s, and the overnight humidity 10% drier. The Sonoma Coastal Range gets 90 to 98 with the same 20% to 30% afternoon humidity and ridge gusts to 30 mph.
Forecasters also flagged nocturnal drying, a pre-dawn drop in humidity that they wrote “will increase the burn period or time in which fire behavior may remain active.” In plain English: a fire that would normally lie down overnight doesn’t.
There’s already a fire in that zone
The Woodside Fire has been burning above Timber Cove on the Sonoma coast since Thursday afternoon. Cal Fire’s incident feed put it at 151.5 acres and 40% containment as of 8:50 a.m. Saturday, with the Sonoma-Lake-Napa Unit in command. The Press Democrat, the San Francisco Chronicle and Bay City News have all been covering it.
On Friday evening, at the request of an officer on the incident, the weather service wrote a spot forecast for that fire alone. Most of it concerns onshore wind. The last paragraph does not. A “low probability but higher impact scenario,” the forecaster wrote, is northeast wind Saturday night into Sunday, sliding downslope off the higher ground east of Highway 1 and leaving “little to no marine layer influence and warmer and drier conditions all of the way down to the ocean.”
The same office’s longer-range discussion expects a stronger version of that Sunday night into Monday: a burst of offshore flow “not strong enough for a Wind Adv, but definitely elevate Fire Weather concerns.”
The forecasters aren’t hiding the reasoning
Farther north, the weather service’s Eureka office got to the same place from the other direction. Saturday’s west-northwest wind, its forecasters wrote, carries enough moisture to hold afternoon humidity in the 20s instead of the teens. “This will keep the fire threat from becoming critical,” they wrote. Sunday night into Monday reads differently to them. That stretch, they wrote, “is starting to look like a more critical time period.”
The Monterey office closed its fire weather section with the only instruction available when there’s no warning to hang it on: people in these areas “are advised to avoid activities that may spark a fire,” and fire crews should “exercise extra caution when executing any prescribed burns, especially where fuels, topography, and weather align.”
An Area Forecast Discussion is not an alert. It doesn’t turn a map red or interrupt anything. It’s forecasters talking to one another, and what they’re telling one another this weekend is that the Energy Release Component across the interior North Bay is projected to climb past the 97th percentile — and that the wind, the one ingredient that would put that in a warning, isn’t quite there.
Both offices named the same window to watch. Sunday night into Monday.