A day before Halloween, a storm system approaches the SF Bay Area

A cold front will keep temperatures cool and bring a chance of light rain to the San Francisco Bay area Wednesday night into Thursday, but conditions are expected to dry out as trick-or-treaters take to the streets on Halloween night. The one exception: North Bay could see lingering showers.

After a brief break from the rain, a stronger system is poised to deliver another round of rain from Thursday into Friday. The storm door is open and the Bay Area is finally getting some notable rainfall, the National Weather Service says.

“This is the most significant rainfall we’ve seen so far this season,” said Dial Hoang, a meteorologist with the weather service’s Bay Area office.

The two pushes of rain are coming to Northern California as a low pressure system sets up off the coast of the Pacific Northwest. While the low pressure remains north of California, a cold front associated with the system will move into the state Wednesday into Thursday. Another cold front will bring more significant impacts to the region Friday into Saturday.

High clouds began to fill the skies over the Bay Area Wednesday morning, signaling that rain is on the way. The first showers are expected to arrive in the North Bay late Wednesday night, with the rain extending south of the Golden Gate and reaching San Francisco and Oakland around midnight. The South Bay will see rain early Thursday morning, Hoang said.

The good news is that conditions will likely dry out by Thursday afternoon in most of the area, just as trick-or-treaters begin donning their costumes. The outlook is not quite as good for North Bay, where light scattered showers may continue overnight into Thursday. Fortunately, showers are likely to be light and you may not even need to open your umbrella. “We might only see a few hundredths of an inch Thursday night in North Bay,” he said.

Rainfall totals with the first cold front are generally expected to be light, with most areas picking up a few hundredths to 0.1 inches. The North Bay valleys, Oakland, San Francisco and western San Mateo County could see up to 0.25 inches, while the North Bay valleys have a chance to record up to 0.5 inches, Hoang said.

The other cold front is the wetter system. The outlook shows it arriving Friday into Saturday morning; on Wednesday there was some uncertainty about the exact timing. From Friday through Saturday, areas south of the Golden Gate are expected to measure 0.5 to 0.75 inches, while the North Bay could pick up 0.75 to 1 inch, Hoang said.

Temperatures have been below normal in recent days, and the cool weather will continue until Saturday, the weather service says. On Wednesday, afternoon highs are expected to be 5 to 10 degrees below average, with mid-to-upper 60s across inland areas, temperatures around 60 to 62 in San Francisco and Oakland, and upper 50s along the Pacific coast. Thursday and Friday will see similar highs in the afternoon, and temperatures are likely to drop on Saturday before warmer weather arrives on Sunday.