Sonoma County growers are nearing the halfway point of the 2018 grape harvest as light rainfall over the weekend has not so far disrupted picking.

The rain that fell in Santa Rosa, measuring only 0.11 of an inch, and a storm that’s forecast to enter the area on Monday night should not cause significant problems with the harvest, local vintners said.

Rain can become a problem during the annual county grape harvest when it lingers and grapes aren’t allowed enough time to dry, which could lead to fruit rot, said Rhonda Smith, viticulture farm advisor for the UC Cooperative extension in Sonoma County.

“It’s significant rainfall that doesn’t have a break on it,” Smith said of poor conditions that could lead to bunch rot.

A weather expert predicted a storm system will reach the region at 6 p.m. Monday and will bring an estimated half inch of rain to Sonoma County before leaving the area by Wednesday, said Drew Peterson, a meteorologist with the National Weather Service in Monterrey.

“It’s an early season storm,” Peterson said. “They aren’t as cold and as wet as later season storms.”

The forecast for later in the week, with highs in the low 80s and mostly sunny skies from Friday to Sunday, will be much more welcoming to local winemakers, Peterson said.

Some grape varieties — such as chardonnay and pinot noir — are more susceptible to rot because they have thinner skins and typically are packed more closely in their clusters. Those varieties are now at the tail end of the harvest as vineyard managers start to focus on Bordeaux varieties that have thicker skins such as cabernet sauvignon, merlot and petit verdot.

“It’s not ideal timing.” said Karissa Kruse, president of the Sonoma County Winegrowers trade group, of the rain. She said there is still some amount of chardonnay grapes out in areas such as the Sonoma Coast and the Russian River Valley that have not fully ripened.

Some growers — such as Dutton Ranch in Sebastopol — ramped up picks in between the weekend rain and the expected precipitation early this week to get more grapes into the wineries, she said. Others are removing more leaves from the vines so that they can dry quicker after rainfall.

Duff Bevill, founder of Bevill Vineyard Management in Healdsburg, has been warning his winemakers they should get ready to pick as soon as possible, if they have any white grapes still on the vine with sugar levels indicating they are near ripeness.

“If you got ripe fruit out there, you need to get in,” Bevill said.

You can reach Staff Writer Bill Swindell at 707-521-5223 or bill.swindell@pressdemocrat.com. On Twitter @BillSwindell.