Meteorological Winter Is Here: Latest Forecasts For The Bay Area

SAN FRANCISCO — The winter solstice is still two weeks out, but the season is already here in a meteorological sense, and the Bay Area may kick things off on an appropriately rainy note.

Differing from astronomical seasons, meteorologists and climatologists measure the periods in three-month cycles, based on their annual temperature patterns. In the Northern Hemisphere, meteorological winter covers December, January, and February.

Citing high odds for a strong El Niño persisting into the spring, and even chances of an "historically strong" pattern developing, climatologists have continued to favor a wetter-than-average winter ahead for the Golden State, including in the Bay Area.

In most El Niño winters, the Golden State tends to be rainier than usual from January to March. In moderate to strong El Niño winters, California tends to see a strong southern jet stream and atmospheric rivers during that period.

The latest available seasonal outlooks remain in line with initial projections, even expanding expectations for a wetter trend to include all of California between December and February. An update is due by the end of next week.

California's water year is off to a slow start

While the prospect of another wet winter lies ahead, state officials note the water year is off to a very slow start. State officials measure "water years" from October through September annually so that the numbers capture the full extent of the fall and winter months when California gets most of its rain and snow.

According to the Department of Water Resources, the initial water supply forecast on Dec. 1 had to contend with a dry October and November, with little in the way of storms so far.

"California's water year is off to a relatively dry start," said Karla Nemeth, the DWR Director. "While we are hopeful that this El Niño pattern will generate wet weather, this early in the season we have to plan with drier conditions in mind."

The early conditions spurred the State Water Project to forecast allocating just 10 percent of the supply requested by 29 public water agencies for next year. Fortunately, there is plenty of time for things to change, and last winter's endless parade of atmospheric rivers has put California in a relatively good position.

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Last year, for the first time since 2006, the State Water Project delivered all water allocations to the public water agencies and farmlands it serves.

"The initial SWP allocation forecast is looking ahead to 2024 and only applies to water deliveries in the year ahead," the DWR wrote this month. "It has no bearing on current water supplies or water captured in 2023. Most reservoirs in the state remain above average for this time of year."

Rain returns to the forecast

There is some good news in the near term: The National Weather Service expects a passing storm system to bring rain and lingering showers to the Bay Area late Tuesday into Thursday before a cold front brings a winter chill to the air.

Forecasters said the latest models have trended wetter than earlier forecasts, with portions of the North Bay picking up as much as an inch of rain. Hazardous beach conditions will remain a concern through Thursday.

"Rain is expected starting early Wednesday across the North Bay, spearing south and east through the afternoon," NWS Bay Area wrote Tuesday. "There's still some uncertainty with how much rain we'll get, but the overall message is that this next round of rain will be beneficial rain with no real flooding concerns."

After the rain passes, temperatures are set to plunge. Frost and freezing overnight temperatures will be possible in the North Bay Valleys. Forecasters expect overnight lows will run about 10 degrees colder across the board by Friday.

"A colder airmass will funnel into the region at the end of the week," forecasters warned Tuesday. "Colder overnight lows become the next concern with Friday and Saturday morning looking to be the coldest mornings with temperatures dipping into the 30s across the interior, which may result in frost/freeze conditions."

New climate outlooks have also tilted in favor of a wetter trend reemerging around the middle of the month.

Patch reporter Rachel Barnes contributed to this report.