St. Helena, in California's Napa Valley, typically enjoys warm, dry weather in late September, with daytime highs often in the mid-80s to low 90s°F, cool mornings in the 50s, and virtually no rainfall, as this falls squarely within the region's long dry season.

For travel, St. Helena is roughly 75 miles (about 1.5–2 hours by car, depending on traffic) from San Francisco International Airport, and closer alternatives include Oakland International Airport (similar drive time) or the smaller Charles M. Schulz–Sonoma County Airport (about 45 minutes away), though a rental car or car service is essentially necessary since there's no direct public transit or rail service into St. Helena itself.

Given the warm, dry days and cool mornings typical of late September, pack business casual layers—like light slacks or a skirt with a blouse or button-down, a blazer or cardigan for the cooler mornings/evenings, and comfortable shoes suited to walking on uneven outdoor terrain.