The vineyards in Sonoma County are reawakening from their winter slumber this week, as grape vines that turned brown and dry after the fall harvest are beginning to sprout with green.

The annual ritual of spring in Wine Country, known as bud break, signals the start of the 2012 grape growing season.

"The vintage is about to begin," said Nick Frey, president of the Sonoma County Winegrape Commission.

Invigorated by a spate of warm, sunny days, tiny buds are slowly emerging on the vines. Within a few days, they will unfurl bundles of tightly-packed leaves, beginning a six-month growing season that will culminate with the frenzy of the fall harvest.

The buds are breaking a few days earlier than normal, growers said. Farmers in regions with moderate temperatures, like Carneros, are beginning to see the first impact of warm winter temperatures.

"We just started pruning our vines across our 250 acres here, and the sap was running like water," said George Rose, spokesman for J Vineyards and Winery outside Healdsburg.

"These vines have come alive," he said. "We're off to the races. And this is unbelievably good weather for grape to grow in."

But the nights are still cool, and the changing hues in the vineyard also mark the beginning of a tense period in Wine Country as growers prepare to battle frost.

When temperatures dip below freezing it can damage the tender green shoots and reduce the size of the grape crop, costing North Coast growers millions of dollars.

"The biggest problem with the early bud break is they're more susceptible to frost damage," said Andrew Avellar, president of Carneros Vineyard Management. "If we're already pushing here in early March, it makes for a very long frost season where we have to be on guard."

The early bud break follows a February that was so dry it broke records. Last week, the Sonoma County Water Agency declared this a "critically dry" year, a designation that could lead to significantly lower flows on the Russian River. Though water levels in reservoirs are in fair condition, the agency encouraged efficient water use.

Water is a critical weapon used by many growers to combat frost. Spraying vineyards with water on frosty nights coats the vines in a protective sheath of ice, safeguarding them from damage.

This year, growers in the Russian River watershed are facing new state rules to regulate frost protection practices. The rules, which are designed to protect endangered fish, were put on hold last month while growers challenge the regulations in Mendocino County Superior Court.

Even so, growers will carefully monitor water levels in the Russian River and coordinate efforts to ensure growers are not all pumping water at the same time, Frey said.

"I'm sure the National Marine Fisheries (Service) and other enforcement groups will be out making sure there's not any stranding of endangered fish," Frey said. "There's still plenty of regulations out there, and the growers need to be very cognizant of whether they're having any impact."

Dry, sunny weather will continue into the weekend although temperatures will cool slightly, with daytime highs dropping to the mid 60s by Saturday. There is a 20 percent chance of rain on Saturday and Sunday, according to the National Weather Service.

Warm days will accelerate bud break across the county's 57,000 acres of vineyards, where many of the vines are bulging with buds that have not yet broken through the surface.

"A little bud starts to swell," said Jim Murphy, owner of Murphy Vineyards, based in Alexander Valley. "Instead of looking tight and brown, they're like popcorn. The kernels are just slightly cracked, and it's more white. Definitely by Saturday we're going to have leaves showing."