A cold blast is forecast to hit the North Coast tonight, bringing below-freezing overnight temperatures through the weekend.

The sharp dip is expected to fill shelters and could threaten outdoor plants.

"Overall, this may be some of the coldest weather we've had this winter," senior AccuWeather meteorologist Bob Smerbeck said Wednesday. "Anyone with susceptible plants better have a game plan beginning Thursday night."

Daytime highs are expected to be in the upper 40s with cloudy skies and rain possible Saturday night, Smerbeck said.

The chilly conditions will continue through the beginning of next week, he said.

It's not the first cold snap this winter. Temperatures in Sonoma County dropped to 26 degrees Dec. 19. But Smerbeck said it will be the longest period of freezing weather so far.

In Santa Rosa, temperatures will drop to 29 degrees overnight tonight, 32 on Friday and Saturday and 30 on Sunday.

Ukiah and the Napa Valley will have slightly lower temperatures.

It's all part of a cold front coming down from western Canada and sweeping across the country.

Smerbeck said the normal high this time of the year in Santa Rosa is 58 and the normal low is 39.

"We're going to have a string of days where there could be frost," he said.

The CHP warned black ice could form on roads.

"If the roadway appears wet, however there is no spray coming from the vehicles' tires around you, be aware," Sgt. Diana McDermott said.

Meanwhile, homeless shelter operators said they were expecting more people trying to get in out of the cold.

John Records, who runs the Mary Isaak Center on Hopper Street in Petaluma, said he would put out extra sleeping mats and make more space for people if possible.

"It's terrible to be on the street in weather like this," Records said. "People anticipate it and they find a safe place to be."

Jennielynn Holmes, program manager for Catholic Charities in Santa Rosa, said the waiting list to get in her three shelters doubled Wednesday morning. Her group is passing out hot drinks and warm clothes to people who may not be able to get in.

"We're handing out more items than ever before," she said. "All the things that can keep people warm and help them live through the night."

You can reach Staff Writer Paul Payne at 568-5312 or paul.payne@pressdemocrat.com.