Robert Louis Stevenson State Park Weather

Robert Louis Stevenson State Park sits at the trailhead for Mt. St. Helena, with the upper trail climbing to 4,341 feet -- the highest peak in the Bay Area's Coast Ranges. The park's base elevation of around 2,200 feet already puts you well above the fog line that routinely buries the Napa Valley in summer mornings. Expect noticeably cooler and windier conditions than the valley floor below.

Annual average highs run about 67°F with average lows around 46°F, but those numbers smooth over wide swings. Summer days are warm and dry, often reaching the mid-to-upper 70s at the trailhead, while winter days regularly hover in the low 50s with overnight lows dipping below freezing. Wind is a constant presence near the summit, where exposed ridgelines funnel gusts that can make a 65°F day feel much colder.

The park logs roughly 277 perfect weather days per year by comfort score, which is genuinely excellent for a Bay Area hike. Precipitation totals around 51.5 inches annually, most of it falling between November and March. How Bay Area microclimates shape conditions like these is worth understanding before you plan a trip -- elevation is one of the biggest drivers.

May, June, and August are the standout months at Robert Louis Stevenson State Park, and the comfort data backs that up clearly. May scores a 91, April scores a 90, August scores a 90, and June scores an 84 -- consistently the highest readings of any season.

May is especially good: temperatures average 79°F highs and 51°F lows, skies are reliably clear, and the hillsides still hold green from winter rains. The trail to the summit is dry enough for comfortable footing but not yet baked by July heat. August pushes the warmth higher (highs around 82°F) with even lower humidity, making it the most reliably sunny month of the year.

June can bring some morning cloudiness as the marine layer compresses before burning off, but by late morning the summit is typically clear and spectacular. For the full experience of hiking above the fog while watching it pool in the Napa Valley below, early summer mornings are your best window. Planning around the best hiking months in the Bay Area helps you make the most of the seasonal patterns at elevation destinations like this one.

March and April are solid shoulder-season alternatives, with April's 72°F average highs and a 90 comfort score making it nearly as good as summer.

Robert Louis Stevenson State Park is genuinely above the fog line for most of the year, which is one of its best features. The park averages only 1.1 fog hours per day and roughly 55 foggy days per year -- substantially less fog than Napa Valley towns or the Bay Area coast.

The elevation puts you above the marine layer that presses in from the Pacific during summer. On a typical July or August morning, the Napa Valley may be socked in with fog while the Mt. St. Helena trailhead basks in sunshine. This is the temperature inversion effect at work: cool, moist air settles into low valleys overnight while higher elevations stay clear.

That said, fog can reach the park during winter storm systems and in shoulder-season months when the marine layer pushes unusually deep inland. December and January occasionally see the park enveloped in cloud, which reduces visibility on the trail significantly. If you're driving up from Calistoga and see the valley in clear sunshine, the park will almost certainly be clear too -- both sit high enough to avoid the worst summer fog. The one exception is the true summit, which sits in its own weather pattern and can generate its own cloud cap on certain atmospheric setups. Why Bay Area elevation changes fog exposure so dramatically is worth reading if you plan to visit multiple times.

Winter at Robert Louis Stevenson State Park is genuinely cold by Bay Area standards. January is the coldest month, with average highs of only 51°F and average lows of 35°F -- below freezing territory on many nights. December and November are nearly as cold, with average lows in the 37-39°F range.

What makes winter hiking here more demanding than those numbers suggest is wind. The exposed ridgeline and summit area experience sustained winds that drive down the feels-like temperature significantly. A 45°F day with a 20mph wind feels closer to 35°F in exposed sections. Ice and frost are common on the upper trail from November through February, and snow dusting the summit a few times per year is not unusual.

Comfort scores reflect the difficulty: January scores just 62, the lowest of any month, and December and November both score in the mid-60s. The Bay Area's four seasons are real at this elevation even though the lower valleys stay mild. If you do hike in winter, treat it like a proper mountain hike -- layers, gloves, and waterproof boots are not optional. The nearby town of Middletown at lower elevation offers a useful weather baseline: if it's 55°F and pleasant in Middletown, plan for 45°F and windy at the trailhead.

Robert Louis Stevenson State Park receives about 51.5 inches of precipitation per year, which is substantially more than Bay Area cities at sea level and more than the Napa Valley towns nearby. Elevation drives this: the mountain forces moisture-laden air upward, cooling it and wringing out more rain and occasional snow.

The park sees around 81 rainy days per year, concentrated heavily in the wet season from November through March. December, January, and February are the wettest months, and the trails can become muddy and slippery during sustained rain events. The rainiest months in the Bay Area correlate closely with the park's worst conditions -- storm systems that drop 1-2 inches in the valley can drop 2-3 inches at this elevation.

Summer is reliably dry. June through September brings almost no rain, which is why those months score so highly for comfort. The trails dry out completely, the fire danger increases, and the views from the summit stretch 60-100 miles on clear days.

October is a transition month worth watching -- its 52°F average high and comfort score of 68 reflect the shift from dry summer to wet winter. Early October can still be excellent; late October often brings the first significant storms of the season. Check the forecast carefully for any October or November hike.

Yes, Mt. St. Helena is reliably windy, and the upper sections of the trail are among the windier spots in the Bay Area's Coast Ranges. The summit sits at 4,341 feet with no significant terrain blocking the wind from any direction. On a day with 15mph winds in Calistoga, expect 25-35mph gusts on the exposed ridgeline near the top.

Wind speed generally increases with elevation along the trail, so conditions at the trailhead parking area can be deceptively calm before you reach the fully exposed upper trail. The north-facing aspects tend to catch more wind in winter, while summer afternoons bring a persistent onshore flow that funnels up the mountain from the west.

For hikers, the practical impact is significant. Wind chill on the summit can take a pleasant 68°F afternoon and make it feel 10-15 degrees colder. Bringing a wind layer -- not just a fleece but something that actually blocks wind -- is important even in summer. May and June hikers sometimes get surprised by strong afternoon gusts after a warm morning start.

Winter and spring storms can bring gusts exceeding 50mph near the summit, making hiking genuinely dangerous. Check a mountain-specific weather forecast, not just the valley forecast from Calistoga, before any cold-season hike. Dressing in layers for variable Bay Area conditions is solid advice for this trail specifically.

Layering is the right approach for Robert Louis Stevenson State Park in every season because the elevation creates a significant temperature gap between the trailhead and the summit, and conditions can change quickly.

In summer (June through September), start with a moisture-wicking base layer and pack a wind-resistant shell in your daypack. Morning temperatures at the trailhead might be a comfortable 65°F, but summit winds can make it feel like 50°F within the same hike. Sun protection matters too -- at 4,341 feet you're above much of the atmospheric haze and UV exposure is higher than it feels.

Spring and fall require a more substantial layer system. A mid-layer fleece or insulated jacket plus a windproof outer layer covers the range from warm lower trail to cold, exposed summit. April might be 72°F at the trailhead but feel like the mid-50s with wind near the top.

Winter hiking here means treating it like a proper mountain outing. Insulated jacket, gloves, hat, and waterproof boots are standard. Ice on the upper trail is common from December through February -- microspikes are worth carrying from November onward. Jeans are a poor choice in any season because they hold moisture and dry slowly.

Footwear matters more at this park than at most Bay Area hikes. The trail to the summit covers roughly 10 miles round-trip with significant elevation gain. Proper hiking boots with ankle support and grip outperform trail runners, especially on the rocky upper sections.

The weather at Robert Louis Stevenson State Park and Calistoga can be dramatically different despite being only about 8 miles apart by road. The elevation difference -- roughly 2,000 feet from the Napa Valley floor to the park trailhead, and another 2,000+ feet to the summit -- drives most of the contrast.

Calistoga is one of the hottest spots in the Bay Area during summer, regularly hitting 95-100°F in July and August when hot air pools in the enclosed Napa Valley. The trailhead at the park sits at a comfortable 75-80°F on those same days. This temperature inversion is reliable enough that on a scorching Calistoga afternoon, the park offers genuine relief.

In winter, the relationship partially reverses. Calistoga's valley floor, while chilly, often catches sun while the mountain sits in cloud or wind. Calistoga averages warmer minimum temperatures in winter than the exposed park trailhead.

Fog is another point of divergence. Calistoga sees summer morning fog regularly as cool marine air flows through the Petaluma Gap and into the valley. The park, sitting above that fog layer, often starts summer mornings in clear sunshine while Calistoga waits for fog to burn off. This is the temperature inversion dynamic that makes Bay Area elevation changes so significant. If you're deciding between a valley hike and an elevation hike on a given day, checking conditions at both spots is always worthwhile.

December and January are the worst months at Robert Louis Stevenson State Park, with January scoring only 62 out of 100 for comfort -- the lowest of any month. The combination of cold temperatures (average highs just 51°F), frequent rain, possible ice and snow on the upper trail, and persistent wind makes winter hiking here demanding rather than pleasant.

November is nearly as challenging, scoring 65, and October's score of 68 reflects a sharp seasonal drop from summer's 87-90 range. The problem is not just rain: it is the interaction of cold air, wet trails, and wind. A January day at the summit can easily feel like the mid-20s with wind chill factored in.

The fire season consideration also affects timing in the other direction. Late summer and fall occasionally bring dry, hot north winds (Diablo winds) that raise fire danger in the area significantly. The park may close during high fire danger periods in September and October. Check CalFire and park status before any late-season trip.

The irony is that the worst weather months still produce some striking days. A winter storm clearing in January can leave the summit dusted with snow and the valley buried in fog below -- genuinely spectacular if you're prepared for it. But those days require careful timing and full winter gear. For casual hikers, sticking to the May through August window is the straightforward choice, and planning hikes around Bay Area seasons helps avoid unpleasant surprises.