The driest February in the Bay Area in 156 years is coming to a close — with no end in sight to the unusually parched, hot weather — and nearly a quarter of California is now under drought conditions, federal officials announced Thursday.

If this week ends without a drop of rain, as is forecast, it will be the first time since 1864 — when the Civil War was raging and covered wagons roamed the American West — that San Francisco had no measurable rain in February, normally one of the wettest months of the year.

And the news in the Sierra Nevada is just as grim. On Thursday, the snowpack, the source of one-third of California’s water supply, was 46% of its historical average, down from 92% on New Year’s Day, with no precipitation falling this month over key Sierra Nevada watersheds for the first time since records began in 1921.

When will it really rain next? Not until mid-March at the earliest, forecasters say.

With only about a month left in California’s winter weather season, the likelihood that the state will end its rainy season in April with below-normal rain and snow conditions is nearly certain, experts say, raising concerns about fire danger this summer.

“We’ll most likely end this water year below average,” said Sean de Guzman, chief of snow surveys and water supply forecasting for the state Department of Water Resources. “We just don’t know how far.”

Altogether, 23.3% of the state is in “moderate drought” — up from 9.5% last week, according to the U.S. Drought Monitor, a weekly report issued by the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture and the University of Nebraska-Lincoln.

That’s the most of any week in 13 months. Another 46.4% of California is now classified as “abnormally dry.”

Sunny, dry weather, with temperatures in the 60s and 70s, is forecast for at least another week across Northern California. Apart from a slight chance of sprinkles on Sunday, the next time significant showers are possible in the Bay Area is between Monday, March 9, and Wednesday, March 11, when the long-range forecast shows a 40% chance in San Jose, Oakland and San Francisco, according to the National Weather Service.

The culprit for the dry weather? A large ridge of high pressure air, similar to the one that created much of the state’s 2012-2017 drought, that has set up off the West Coast and deflected storms to Washington state, southern Canada and Mexico that otherwise were on target to hit California.

“California and parts of the Southwest dried out while the Northwest observed surplus precipitation,” wrote David Miskus, a NOAA meteorologist who issued this week’s Drought Monitor report.

“During the past 60-days, less than 25% of normal precipitation had fallen on much of California and western Nevada,” he added, “creating deficits exceeding a foot in parts of the Sierra Nevada, and 4 to 8 inches along the coast.”

With no rain in sight for the remainder of the month it appears that Feb 2020 will go down in the record books as the driest on record in many locations. #cawx pic.twitter.com/PkXDHi0p32 — NWS Bay Area (@NWSBayArea) February 24, 2020

Overall, 23.6 million acres of California now is in “moderate drought,” an area stretching across most of the Central Valley and including Sacramento, Mendocino County and parts of Contra Costa, Solano and Napa counties in the Bay Area.

One piece of good news: Because of wet winters last year and in 2018, the water levels in major reservoirs around the state are at near or above average.

“We have some dry conditions across the state, but our reservoirs are doing well,” said Chris Orrock, a spokesman for the state Department of Water Resources. “If we have to go through two dry years, then we’ll start looking at what additional conservation measures we need.”

On Thursday, Shasta Lake, the state’s largest reservoir, was 78% full — 107% of its historic average for late February. Lake Oroville was 64% full, or 92% of its historic average. And San Luis Reservoir near Los Banos was 70% full, or 83% of average for this time of year.

But the Sierra Nevada snowpack — the source of nearly one-third of California’s water supply for farms and cities — is taking a major hit.

For the first time since measurements began in 1921, de Guzman said, no precipitation fell at all in February over the Northern Sierra Eight-Station Index. The closely-watched weather stations are located in watersheds from Mount Shasta to south of Lake Tahoe that empty into the major reservoirs of Northern California.

And the lack of rain this February has been historic across Northern California.

This winter began with several significant storms shortly after Thanksgiving. But since then, there has only been one day in the Bay Area, Jan. 16, when San Jose, Oakland and San Francisco all received more than 1 inch of rain.

As a result, the seasonal rainfall totals are looking meager. Since Oct. 1, San Jose had received just 3.8 inches of rain through Wednesday night, or 36% of its historical average for that date. Oakland was at 39%, with 5.8 inches. And San Francisco was at 50% of normal, with 8.8 inches. Even in the Central Valley and Southern California, where some storms broke through earlier this year, dry conditions have been the norm. Fresno is at 47% of normal rainfall for this time of year and Los Angeles at 68%.

Because of reservoir and groundwater storage, it typically takes at least two years of dry winters for a major drought to emerge in California. A wet winter next year could return conditions to normal. But the lack of rain this year is generating concerns about increased fire risk this summer and increasing the chances of major blazes, due to lower moisture levels in grasses, brush and trees.

On Thursday, fire crews in the Mendocino National Forest in Glenn County battled a wildfire in remote brush and timber that began when a controlled burn spread out of control in an area that normally would be wet this time of year. The fire, which is 68 acres, was 40% contained, and 75 firefighters were working to stop its spread. No structures were threatened.