Above the Fog: What September Actually Looks Like on Howell Mountain

Drive up White Cottage Road most mornings this month and you'll cross an invisible line without a sign to mark it. Below, St. Helena is still wrapped in the kind of gray that keeps porch lights on past seven. Above, the light is already sharp and the air smells like crushed grape skins instead of wet asphalt. Nobody up here needs a weather app to know which side of that line they're on. You just look down.

That line has a number. The Howell Mountain AVA begins at 1,400 feet, the elevation where the marine fog rolling in from San Pablo Bay typically stops climbing. It's the only Napa Valley sub-appellation defined purely by altitude rather than rivers, roads, or property lines, and the boundary runs right through the intersection of Deer Park Road and White Cottage Road. Cross it heading east and you've left the fog behind, at least on the Napa side.

That last qualifier matters more than most write-ups about this mountain let on.

Two Mountains, Not One

The version of Howell Mountain that shows up in most wine writing goes like this: elevation escapes the fog, grapes get more sun, wines get more structure. All true, and none of it explains why two vineyards four miles apart can be having completely different mornings.

Clif Family Winery owns one block, Cold Springs, that sits just above the Napa-side fog line, and another, Croquet, on the Pope Valley side of the same ridge. Cold Springs runs cooler with morning fog and breezy evenings. Croquet runs about ten degrees warmer with a shorter growing season and almost no fog at all. The reason comes down to which way the cold, wet air moves once it clears Mount St. Helena to the north. It gets pushed around unevenly, pooling into some folds of the mountain and skipping others entirely, which is why La Jota's winemaking team has spent years mapping which blocks catch the marine layer and which never see it.

So the honest version isn't "Howell Mountain is above the fog." It's that Howell Mountain has two separate relationships with the fog depending on which slope you're standing on, and that split shapes everything from bud break to which side of the mountain gets the first frost warning in November.

The numbers back this up in a way that's easy to overlook. Howell Mountain's average mean temperature runs 56.8°F, a couple degrees cooler than St. Helena's 58.6°F and Pope Valley's 59°F just over the ridge. But the mountain also gets more rain overall, 40.74 inches a year against 35.4 inches down in St. Helena, because being exposed above the fog cuts both ways: less insulation from heat, but also less shelter from storms that hit the ridge directly.

What the Mountain Is Doing Right Now

That exposure showed up again this week. According to a harvest report published this week by the Napa Valley Register, Bremer Family Winery's Dan Pescetti described the past several days on the mountain as a mix of moisture, cool temperatures, and fog, with a welcome high of 79 degrees that finally let tannin development catch up to the sugar levels already built into the fruit. The season's first measurable rain landed on the mountain during that stretch, and Merlot has now moved onto the watch list for picking in the coming days.

It's a useful reminder that "above the fog" doesn't mean immune to weather. It means a different weather, one that can turn cool and damp just as easily as it turns bright and dry, sometimes within the same week.

The soil is part of why none of that moisture turns into mud. Howell Mountain's ground is a mix of volcanic tufa and red clay, both of which drain fast even under heavier rainfall than the valley floor gets. It's the same porous ground that stresses vine roots into producing the small, concentrated berries the mountain is known for, and it's why a hiking trail up here can take a real storm and still be walkable within a day or two.

The Trails Are Finally Worth the Climb

Speaking of trails, this is the month Las Posadas State Forest stops being a summer-heat gamble and starts being the reason people up here own hiking boots at all. The Linda Falls Trail, a shaded 1.6-mile out-and-back that stays cool under tree cover most of the way, runs into a small waterfall that's more of a payoff after the first fall rains than it is in the dry stretch of August. Push further into the forest and Inspiration Point opens up the kind of valley view that doesn't need a caption, while the Moore Creek and Canyon Road Loop, at nearly 11 miles and over 2,000 feet of climbing, is where the mountain's serious trail runners spend their weekend mornings once the heat breaks.

None of this is undiscovered. Every May, Pacific Union College hosts the Angwin to Angwish trail race through these same forest paths, drawing runners from as far as Tennessee and Arizona, with the Redwood Trail Alliance handling trail maintenance and Sportago, the outdoor shop down in St. Helena, running bib pickup. It's a small event, but it's a real one, and it's a sign that the trail network up here gets used by people who live near it, not just visitors passing through.

The Campus Fills Back Up

If harvest and trail conditions are the mountain's two biggest seasonal shifts, the third is happening on a fixed schedule regardless of weather. Pacific Union College's fall quarter opens with new student orientation running September 16 through 20, with instruction beginning September 21. For a town built around a single crossroads, that's not a small event. The parking lot fills, the Howell Mountain Market sees its heaviest foot traffic of the year, and Angwin's rhythm shifts from the quiet of late summer back into something closer to a college town's ordinary bustle.

It's worth remembering that PUC isn't a recent addition to the mountain. It's been here since 1909, when the Pacific Union Conference bought the old Angwin Resort property specifically for its springs, its lumber, and the healthful air at elevation, the same air that later got Howell Mountain its own wine appellation. The college and the vineyards around it have been running on the same climate logic for over a century.

A Date Worth Knowing

For anyone curious what a season like this one turns into once it's bottled, the Howell Mountain Vintners & Growers Association holds its Harvest Celebration on November 7 at the Culinary Institute of America at Copia in Napa, with pours from more than thirty member wineries. It's the closest thing the mountain has to a public report card on the vintage currently coming off the vines, the same vintage Pescetti and his neighbors are watching right now with an eye on the sky.

Whatever the fog does between now and then, it won't do it evenly. That's been true of this mountain since before anyone drew a line at 1,400 feet, and it's still the most useful thing to understand about living here in September, above the fog on some mornings, right in the middle of it on others, depending entirely on which side of the ridge you happen to be standing on.

If you're curious what any of this means for a specific property, on either side of that ridge, the Jeff Earl Warren team has spent three generations learning the difference firsthand. Work With Us.