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HOPLAND -- St. Helena winemaker George Vierra says wine snobs will find it "unfathomable" that climate changes may be transforming Mendocino County into a higher-quality grape-growing region than neighboring Sonoma County.

Or worse yet, the touted Napa Valley.

"Yet such a shift is supported by available data and expert analysis," said Vierra.

For Vierra it's a surprising development after 35 years of overseeing wine production for Napa Valley icons such as Charles Krug and Robert Mondavi wineries, and his own Vichon and Merlion labels.

"I was among the many who believed Mendocino vineyards were probably 'too hot' to produce quality grapes for high-end premium wines," Vierra said.

Yet, an expert analysis of 50 years of summer highs shows that the Ukiah Valley is now a cooler grape-growing region than Napa Valley.

The findings are attracting the scrutiny of winemakers such as Vierra and climate experts who specialize in weather-related effects on grape-growing regions.

Vierra's own quest to understand changes in Mendocino vineyards took root last year after he assisted in making a wine from organically grown pinot noir grapes in Hopland.

"I was amazed," said Vierra.

Vierra said he unexpectedly found acidic levels of the Hopland grapes comparable to those grown in the famously cool Carneros region at the southern tip of Napa and Sonoma counties. That's important because grape acidity falls significantly slower in cool regions, a process that can be sensed in the taste of a finished wine.

To better understand why, Vierra sought the opinion of nationally known climatologist Gregory Jones of Southern Oregon University. Jones the year before had compiled, analyzed and published findings from a study of 50 years of Wine Country temperature data.

Jones' findings substantiated a warming trend in Sonoma and Napa counties, which he said is already altering grape-growing conditions. Jones said a pattern of warmer overnight temperatures appears to be stewing some cool-loving varietals.

Jones, and now Vierra, are convinced the climate shifts occurring in inland Mendocino County to the north deviate sharply from Sonoma and Napa.

Jones' study, for example, found that the number of hot summer days over 95 degrees in the Ukiah Valley has steadily declined over five decades while temperatures in Napa have risen.

In the 1950s, Ukiah posted an average of 51 days where the high temperature reached 95 degrees or more, according to Jones' study. The average for the last 10 years is just 29 days.

The shift is so dramatic that for three summers in a row Ukiah has on average posted cooler temperatures than Napa.

"If we didn't have the supporting data, no one would believe it," said Vierra.

Charlie Barra isn't surprised. A Redwood Valley grape grower for 60 years, Barra knows from experience that conditions have changed.

"I know it's cooler at night because my own vineyards are cooler," said Barra, who has more than 100 acres of grapes.

Last summer, a seminar at Gloria Ferrer Winery in the Carneros district focused on global warming and the wine industry. It touched not only on how winemakers can reduce their carbon footprints but also on how climate change can affect premier grape-growing regions. Some winemakers said they had witnessed local changes, such as more heat spikes and fewer spring frosts, and those changes were influencing the way they manage vineyards.

In a recent e-mail exchange between Vierra and Jones, the Oregon weather expert theorized that a warmer Central Valley is now pulling more cool air across inland Mendocino County from the Pacific Ocean. Frequent daytime breezes are helping keep maximum temperatures down.

Jones said because Mendocino vineyards are being exposed to cooler air for longer period of times, the ripening process has slowed, which helps raise grape quality.

In contrast, Jones said Napa Valley and inland Sonoma County vineyards seem to be experiencing warmer overnight temperatures because of their proximity to the warmer Central Valley.

For Vierra, the findings help explain the surprising quality of pinot noir grapes that were being harvested on vintner Dan Fetzer's Jeriko Estate north of Hopland. Fetzer's decision a decade ago to plant the pinot vineyard was considered risky, given Hopland's reputation for warm weather.

But Vierra learned the Jeriko vineyards are brushed by constant winds from the northwest. "It's even cooler there than Ukiah," said Vierra.

Vierra became so convinced about the new role cooling temperatures are playing in Hopland grape production that he organized a blind tasting earlier this summer. Jeriko's pinot noir was put up against 10 highly rated pinots produced from vineyards in the more recognized Carneros district and the Russian River Valley.

Jeriko pinot came out third in the rankings.

The same pinot grapes are used to produce a sparkling Jeriko Brut Rose, which this summer won a gold medal during the prestigious Orange County Fair judging.

"It's clear to me inland Mendocino County has become cool enough to produce some very high-quality wine grapes," said Vierra.

Climatologist Jones believes the cooling-off in Mendocino and warming of summer nights in Napa and Sonoma are part of complex and evolving regional climate shifts that are likely to intensify over time.

The best grape-growing regions of the past may not be best in the future, said Jones.

At industry and academic conferences across the West, Jones tells listeners that some of today's inland grape-growing regions eventually will become too hot, forcing producers to shift operations northward.

For the coastal regions of Wine Country, however, the outlook is not so bleak.

All of Jones' data modeling suggests that coastal regions will be less affected by warming because of the moderating effects of the Pacific Ocean.You can reach Staff Writer Mike Geniella at 462-6470 or mgeniella@gmail.com.