UPDATE 10/5/19, 1 p.m. -- Here is our latest update with specific information from today, the day the red flag warning is set to begin:
UPDATE 7:25 p.m. -- The National Weather Service's Eureka office just tweeted out, confirming that there is now red flag warning in Mendocino or Humboldt counties, though conditions will be dry with some wind at higher elevation:
MENDOCINO Co., 10/4/19 -- If you look out your window (or if you're lucky enough to be outside on this glorious autumn day) you're likely some cool breezy winds rustling those still green leaves. Well, those cool winds will not be resulting in substantially elevated fire risk in Mendocino County, according to the Eureka office of the National Weather Service (NWS) -- however, a bit south and east, in Sonoma and Napa counties, a red flag warning has been issued.
The red flag warning means that people in those areas should expect critical fire weather, low humidity with high temperatures and winds, that make it more likely for a fire to catch and spread -- from Saturday evening into Sunday morning.
Now we all know that the weather doesn't respect county lines, so what exactly is going on here? Why is there a red flag warning in the hills east of Cloverdale but not in the hills southeast of Hopland? Well, we spoke a meteorologist at the Bay Area office of the NWS, in Monterey to get some answers.
Here is a bigger map from the NWS office in Sacramento, showing the red flag warning extending up through the Sacramento Valley:
The short answer is that the systems used by the NWS have to communicate with other emergency responders and political units, meaning that it still makes more sense to make warnings at the county level, so that decision makers can respond in a coordinated way.
It's also the case that the current forecast predicts the most dangerous conditions in southeast Lake Co., along the Napa-Sonoma line, and other points east; north and western Sonoma Co. in contrast, will not experience very dangerous conditions, and by the time you move a little farther north into Mendo, the conditions really just won't be dangerous enough to merit a red flag warning.
One added wrinkle is that the decision to declare a red flag warning is done at the individual NWS office, and while Sonoma Co. is lumped into the Bay Area zone, Mendocino Co. is overseen by the Eureka office of the NWS. So while NWS Bay Area believes that conditions are forecasted to be severe enough in eastern Sonoma to merit declaring a red flag warning across most of that county (even though conditions will be relatively mild around Cloverdale), conditions on average across Mendo will remain mild, and so NWS Eureka did not believe it merited a warning across this county.
Now, for you weather nerds still reading, let's talk a little about what a red flag warning means and why this one has shaped up the way it has.
Basically a red flag warning means that weather conditions will result in critical fire weather, which in turn means: high heat, high winds, and low relative humidity (RH). In this case there is a forecast for that to occur Saturday night into Sunday morning. This kind of weather often occurs in the early fall, when we experience gusty "off-shore" winds -- that is, winds that blow from land towards the ocean, a reversal of the normal daytime pattern in coastal California. These winds are also often warmer and drier.
Perhaps the most famous example of such winds, are the Santa Ana winds in Southern California. Conditions similar to the those, hot, dry fast winds, prevailed on the night that the Redwood Fire started, two years ago next week. That event, called by some the "2017 Northern California Firestorm," and referred to by Cal Fire as the "2017 Fire Siege," was driven by such winds.
But the varied geography of our region means that things are a bit more complicated. Winds are often faster at ridge lines, and as NWS Meteorologist Drew Peterson explained, the Napa-Sonoma county line roughly follows a ridge line, meaning that that region will experience particularly fast winds, as will portions of Yolo and Lake counties.
Another important effect is what's called "thermal belting," which, in this area, is the tendency for hills and low mountains to stay warmer overnight than the neighboring valleys. The reason for this is that cool air tends to sink lower, settling in the valleys, leaving a layer of warm air above it, a situation also called an inversion layer. What this means is that these same hills and ridges with higher winds also end up being warmer and maintaining a lower relative humidity, this also keeps vegetation in these areas drier over night, and into the next day.
And that is the reason why these red flag warnings can also occur overnight, as this one is scheduled to, while the temperatures in the valleys drop.
It also explains a bit why though there's a red flag warning to the south, Cal Fire is confident dropping the burn ban up here early next week: