Halloween Forecast: Chilly Trick-or-Treating, Light Rain Late Saturday and More High-Tide Flooding

A deep upper-level trough from the Gulf of Alaska is pushing cold air into California this weekend, the National Weather Service's Bay Area office said in its Thursday afternoon forecast discussion. The result will be a chilly Halloween with a chance of light rain.

Saturday, Oct. 31

Forecasters expect highs well below average on Saturday, in the upper 50s to low 60s. Showers are likely to develop early Saturday on the interior Central Coast and spread north into the Bay Area by late Saturday. "Light rain/drizzle may start during prime Trick or Treating hours," the discussion said, "but the highest chances for precipitation will fall after midnight/early Sunday morning." The weather service puts the chance of thunderstorms on Saturday and Sunday below 15%, with the best odds over the ocean and along the coast.

How much rain?

Not much. Most of the moisture from this system is headed for Southern California. The Bay Area is forecast to get only a few hundredths of an inch, possibly up to a tenth. The North Bay coastal mountains and the interior Central Coast could see a tenth to a quarter inch. Weather service meteorologist Rachel Kennedy told SFGATE earlier this week that it's "an active start to the season" and that "El Niño is happening already regardless."

High tides

Coastal flooding remains a concern along the San Pablo, San Francisco and Monterey bays through Saturday. The weather service said a coastal flood advisory is in effect until 6 p.m. Saturday for parts of the region, and until 5 p.m. Friday for others. At the San Francisco tide gauge, Saturday's high tide is expected at 3:44 p.m., about 1.1 feet above normal, or 6.9 feet MLLW. Forecasters said the advisory could be extended into Sunday if the tidal anomaly falls more slowly than expected.

Next week

Temperatures should warm early next week, with the interior reaching the upper 60s to low 70s and the coast staying in the upper 50s to 60s. The next trough is now expected around Wednesday or Thursday. Clocks also fall back an hour early Sunday: here's our guide.