Californians are beginning to wonder: Is the state heading back into a drought?

While experts say it’s still too early in the winter rainy season to say for sure, the evidence is accumulating, and the rain is definitely not.

La Niña conditions in the Pacific Ocean — the cooling of ocean waters near the equator that have historically increased the chances of dry winters in California — are strengthening and should last through the spring, federal scientists said Thursday.

December has so far brought almost no rain across the state, and with no significant storms forecast for the next two weeks in the Bay Area, the month is on track to become the driest December in San Jose history since records began in 1893 and the third driest in San Francisco history back to 1849.

The Sierra Nevada snow pack, which got off to a good winter start with several storms in November, has melted down to 34 percent of its historic average.

The unusually warm, dry weather has fueled historic fires across Southern California, and prompted warnings from the National Weather Service about increased fire risk this weekend in Northern California. And the U.S. Drought Monitor reclassified 44 percent of California as “abnormally dry” on Thursday — up from 22 percent three months ago.

That’s the bad news, but experts and optimists point out that roughly half of the annual rainfall in Northern California falls in December, January and February, and big storms in the coming months could wipe out this year’s winter precipitation deficits.

Also, even if this winter does turn out to be drier-than-normal, reservoirs across the state are brimming. They were filled during the soaking storms last February and March that ended California’s historic five-year drought that parched the state from 2011 to 2016. With all that water stored now, and groundwater tables boosted back in some areas, the chances of water shortages next summer are low.

But still, with the previous drought still so recent in the Golden State’s memory, and this winter off to an uncomfortably dry start, water managers are beginning to get nervous. What if last winter was a blip in a longer drought? Australia had a drought that lasted more than 10 years recently, interrupted by one or two wet winters.

“It is still too early to be truly alarmed about the lack of rainfall, but we are certainly watching it closely, and I think it’s appropriate to express some concern,” said Doug Carlson, a spokesman for the state Department of Water Resources in Sacramento.

For the past few weeks, a huge ridge of unusually high atmospheric pressure has sat off the West Coast. Such ridges and troughs of high and low-pressure air come and go daily around the world and help shape much of its weather. During the drought, one such ridge blocked storms from bringing rain to California to such an extent that it became known as the “ridiculously resilient ridge.”

A similar ridge is back now, and it’s unclear when it will dissipate.

On Thursday, the National Weather Service issued a fire weather watch for the Bay Area down to Monterey County through Sunday morning. Wind gusts up to 50 mph, warm temperatures and low humidity will increase fire risk, particularly in the Santa Cruz Mountains, Diablo Range, East Bay hills and Big Sur areas, forecasters said.

“People need to be aware we do have the potential for fires to grow rapidly over the weekend,” said Charles Bell, a meteorologist with the National Weather Service in Monterey. “It’s easy to sometimes get lulled into this thought up here, it’s December, we can’t have fires.

“It’s just very dry out there. We have to be on guard.”

Since Oct. 1, Northern California has had a mediocre rain season. Some storms in November increased the moisture levels in plants and dampened the ground. But with nothing new for weeks, San Jose is now at 43 percent of its historic rainfall average for this date, with 1.58 inches, while San Francisco is at 52 percent with 3.16 inches and Oakland is at 63 percent with 3.15 inches. This time last year, every one of those cities had already had twice as much rain.

Southern California is desperately dry.

Many areas there have not had significant rain in nine months. Los Angeles, for example, has had only .11 of an inch since Oct. 1, or 4 percent of the historic average. San Diego has had even less, .02 of an inch, or 1 percent of normal, and Palm Springs, Irvine and Palmdale have had no rain at all.

“It’s still early,” said Jan Null, a meteorologist with Golden Gate Weather Services in Saratoga. “We could have two or three wet weeks that make up a lot of the deficit. But the likelihood of a below normal year should be a real concern. People should probably begin treating it like a drought year.”

After the worst drought since California became a state in 1850, Gov. Jerry Brown last April rescinded his emergency drought declaration. Huge storms brought flooding in downtown San Jose and other areas, buried Sierra ski resorts in snow and wrecked the spillway at Oroville Dam, the tallest dam in the United States.

On Thursday, scientists from the National Oceanic and Atmospheric Administration issued an update on La Niña conditions in the Pacific Ocean. They said that water continues to cool along the equator, and there is an 80 percent chance they will persist through the spring.

“La Niña is still on the weaker side, but it is strengthening,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center in Maryland. “We see no reason that this won’t persist through the winter.”

Although it’s no guarantee, La Niña conditions have historically increased the chance of dry winters in California, just as El Niño conditons — warmer than normal water along the equator off Peru — have increased the chances of rain.

According to an analysis by Golden Gate Weather Services, there have been 21 La Niña winters in California since 1950. Rainfall totals have been below average in 14 of those winters in San Francisco and 17 in Los Angeles.

If Northern California remains dry through New Year’s Day, as the forecast suggests, this December would be the third driest on record in San Francisco history with .02 of an inch of rain, dating back to 1849, according to the National Weather Service.

San Francisco recorded no December rainfall in 1876 and 1989. San Jose, which has received just a trace amount (less than one-hundredth of an inch) this month, would eclipse the current low of .04 of an inch in 1989. And Oakland International Airport, currently at .02 of an inch, would break the mark of .11 of an inch set in 2011.

“We hope that 2018 gives all of us a new start, as well as Mother Nature,” said Carlson. “We need more rainfall and snow.”