Several regions in California, including the Central Valley and northern areas, have experienced persistent fog for more than two weeks. NASA reported that satellite imagery showed fog stretching approximately 400 miles across the Central Valley during late November and early December.
The National Weather Service noted that while much of the Pacific Northwest is preparing for rain, snow, and possible flooding, central California remains an exception. “The exception continues to be in the Central California Valley where low clouds and fog are expected to remain stuck, keeping temperatures much cooler than surrounding cloud free areas,” the agency stated on Thursday, Dec. 11.
NASA identified this weather event as tule fog, named after a marsh plant native to the region. Tule fog typically forms in the valley during colder months when winds are light and soil moisture is high. The phenomenon becomes especially dense when moist marine air combines with calm winds and clear skies. “The Central Valley is fertile ground for the formation of tule fog, a persistent radiation fog, in late autumn and winter,” NASA explained.
This type of weather pattern usually occurs from November through February. However, NASA noted that “researchers have found…tule fog has been forming less often in California in recent decades.”
Jan Null, a former National Weather Service forecaster now teaching meteorology at San Jose State University, attributed this year’s heavy fog to specific early fall conditions: “The perfect recipe for radiation fog … is for there to be low-level moisture, clear skies and light winds,” Null said. “Right after it quit raining (in late November), we had high pressure build over us and that’s when we got all three of those ingredients.” He also cited abundant soil moisture from an unusually wet autumn as a contributing factor.
NASA added that precipitation totals across most of central and southern California from September through November 2025 ranked among the top 10% on record. These storms not only contributed to dense fog but also improved drought conditions statewide; as of Friday, Dec. 12, more than 90% of California was considered drought-free.
Following rainfall ending Nov. 21, temperatures dropped sharply—highs fell into the 40s and 50s compared to earlier highs in the 60s and 70s according to National Weather Service data. Inversions trapped cold air near ground level while higher elevations remained warmer; this temperature structure helped maintain persistent low-lying clouds.
Null said breaking this inversion would require a weather system capable of mixing temperatures between elevations: “A weather system with significant temperature mixing to cool down higher elevations and warm up lower elevations is necessary to ‘break the inversion.'”
Dakari Anderson from the National Weather Service mentioned a potential incoming system around Wednesday, Dec. 17 but said it was uncertain whether it would bring enough precipitation to clear out lingering fog layers. Anderson added that Sacramento area temperatures would likely stay in the upper 40s or low 50s until then due to cold air trapped by clouds.
Higher elevations above the valley’s fog may see warmer-than-usual temperatures—possibly seasonal records—with forecasts predicting mid-50s up to upper-60s.
“(The fog) could go through Christmas right now,” Null said. “We’ll have to keep on watching it.”
Forecasts as of Friday indicated continued weekend fog for Modesto, Merced, and Fresno with lows expected in the 40s.
Driving during these conditions can be hazardous; officials advise motorists use caution if travel cannot be avoided.
