(NBBJ) - Napa County’s wine producers enjoyed strong growth in case sales through their tasting rooms last year and even stronger growth in consumer spending on wine purchases at those venues, and membership in the county’s winery-based clubs is rebounding at a strong clip as visitors are convinced to become purchasing members, according to a recently updated annual survey of several hundred mostly North Coast vintners.

But there is clear room for improvement suggested by the even higher rate of attrition from Napa County winery clubs and below-average typical length of membership — just short of two years, according to the 2013 Tasting Room Survey of 552 vintners by Silicon Valley Bank and Wine Business Monthly.

Napa County tasting rooms sold 11.9 percent more wine last year than in 2011 and harvested 13.2 percent more revenue, respondents said. Average growth was 9.9 percent by volume and 9.4 percent by value. Vintners in Sonoma and Mendocino counties followed that volume-over-dollars pattern, with 9.2 percent and 9.7 percent volume growth and 8.7 percent and 6.6 percent dollar growth, respectively.