of state under drought as rain returns
California is expecting its first rain of the season this weekend, a major shift in weather that’s likely to bring scattered showers and chilly breezes to the Bay Area, and freezing temperatures and snow to the Sierra.
While the unavoidable turn toward winter, starting Friday, is sure to offer at least some relief from this year’s unrelenting fire season, forecasters warn that wildfire risk is likely to persist for another month — possibly much longer. The state’s hills and valleys remain tinderdry. And the prospect of dangerous, seasonal winds generally lingers into December.
In the long term, climate models project lessthanaverage precipitation through most of winter, because of a La Niña weather pattern, raising concern about continued dryness and even the beginning of a prolonged drought across much of California.
“We’re certainly in a droughtrisk posture statewide at the moment,” said Noah DiffenMuch
baugh, a climate scientist and professor of earth system science at Stanford University. “Having the odds tip toward a warm, dry winter suggest the potential for deepening drought conditions.”
Warm, rainless weather ahead would follow scant precipitation last winter. Much of Northern California saw only half as much rain as usual during the winter months, when the bulk of the state’s precipitation occurs. All but the southern part of the state now stands a possibility of backtoback dry years and, with that, an elevated fire risk as well as tighter water supplies.
Many of California’s reservoirs are a bit emptier than typical this time of year, though it usually takes two years of belownormal precipitation before water managers start to worry. Lake Shasta, the state’s largest reservoir, is at 77% of its normal level for the week, and Lake Oroville, the second largest, is at 69%. Some small water agencies with less robust reserves have already begun to put water restrictions in place.
Currently, twothirds of California is considered to be in at least some degree of drought, according to the U. S. Drought Monitor. While the state inevitably turns dry in the fall after its long, mostly rainfree summers, aridity hasn’t been nearly as widespread in almost two years, shortly after the last major drought.
San Francisco saw only one onehundredth of an inch of precipitation in October, the month when the first rains of the season usually arrive. Heat this summer, with record average temperatures in California in both August and September, has only exacerbated the dry conditions.
The future will hinge largely on the fate of the current La Niña, climate experts say.
The weather pattern is defined by cooler water and easterly winds in the equatorial Pacific, which often result in less rain in the subtropical latitudes of the Americas — and as far north as Southern and Central California. Conversely, La Niña often means more rain in northern latitudes, including the Pacific Northwest.
“It’s the strongest singular factor for seasonal precipitation in our forecast,” said Matthew Rosencrans, a meteorologist with the Climate Prediction Center, a division of the National Oceanic and Atmospheric Administration, which produces an annual longterm outlook for winter.
The agency’s outlook this year shows enhanced odds of dry weather in California as far north as the Bay Area and Lake Tahoe. Northern Oregon and Washington, on the other hand, are pegged for wetter conditions.
As history will tell, however, La Niña’s expectations don’t always hold. And because the Bay Area and most of Northern California sit between the spots where La Niña’s mark is strongest, the pattern is a less reliable at forecasting conditions here.
Jan Null, a meteorologist at Golden Gate Weather Services, says that during the last four moderate La Niñas, which resemble the current one, Southern California has received 80% or less precipitation than normal. For Northern California, three of four have correlated with abovenormal precipitation and one with below normal.
“Here it’s kind of a tossup,” Null said.
The last moderate La Niña, in the winter of 201112, however, was significant. Much of the state saw less than threequarters of average seasonal rainfall, a shortage that turned out to be the start of California’s fiveyear drought.
The wintry front forecast for this weekend, coming from the Gulf of Alaska, is expected to move into the Bay Area on Friday morning, according to the National Weather Service.
Scattered rain showers are likely through the weekend between Sonoma County and the Central Coast. Overnight temperatures are expected to dip well into the 40s with daytime highs in the low 60s.
In the Sierra, low temperatures will drop below freezing, with some valleys hitting single digits, according to the weather service. Light snow is expected to start falling Friday above 6,000 feet, with more snowfall, and possible accumulation Sunday morning.
The forecast models differ on total weekend precipitation, though none expect too much. Many spots in the Bay Area may be spared any rain, with the largest totals in the region being only a few tenths of an inch.
“It does not look like that significant of a rain event,” Null said. “But any amount of rain is going to be positive as far as beginning to end the fire season. … You have to begin somewhere.”
Many of California’s most devastating wildfires, including the recent blazes in Wine Country and Paradise, have come on the cusp of fall and winter.
While the low temperatures are forecast to stick around through at least next week, helping reduce the risk of fire, rain and snow will pass by Sunday, according to the weather service.
Currently, 17 large fires are burning in the state, though the spread of the blazes has tapered off. They include the August Complex, the monster fire that has scorched more than 1 million acres in the coastal mountains north of Lake County. It’s the largest fire in modern California history.
A recordshattering 4.2 million acres have burned across the state this year.