Los Banos, CA (December 2025) — Los Banos and much of the Central Valley are enduring one of the most stubborn tule fog outbreaks in recent memory, with dense, freezing fog settling over the region since late November and showing little sign of clearing. According to National Weather Service (NWS) briefings, the current fog pattern began around November 20–23 and has persisted far longer than a typical one-week radiation-fog event.
Repeated Dense Fog Advisories issued by NWS Hanford throughout early December confirm the fog’s unusual duration and intensity. Forecast discussions indicate that a strong ridge of high pressure over California is preventing storms, wind, and daytime warming from breaking up the low stratus layer, allowing fog to rebuild every night. As of mid-December, meteorologists expect the pattern to continue until the high-pressure system shifts, which models suggest may not occur until mid- to late December.
The extended fog layer has pushed Los Banos temperatures well below seasonal norms. NWS climate data show that typical December highs in Los Banos reach 56–57°F. Over the past two weeks, highs have repeatedly stalled in the upper 40s — roughly 8–10°F colder than average. Nighttime temperatures have hovered in the low 40s, with some Central Valley locations dipping to the freezing mark.
While Los Banos has not reached record low temperatures (the city’s coldest December temperature on record is 14°F on Dec. 22, 1990), the lack of sunlight and daytime heating is noteworthy. NWS Hanford meteorologists stated in recent briefings that if this pattern persists, December 2025 may rank among the coldest in recorded Valley climate history.
Travel conditions have been hazardous throughout Merced County, especially during morning and evening hours. CHP and county officials have repeatedly warned of sudden zero-visibility pockets on I-5, Highway 152, and rural areas near Los Banos and Merced. The sun has been obstructed due to the fog unusually thick fog layer.
According to NOAA and NWS analyses, several factors have converged to create ideal conditions for the unusually persistent tule fog blanketing Los Banos and the broader Central Valley. The region’s exceptionally wet autumn left soils heavily saturated, a fact confirmed by satellite measurements and NOAA hydrology reports. This moisture allows the ground to cool rapidly at night, quickly bringing surface air to its dew point and generating dense radiation fog. At the same time, calm and cold overnight conditions have limited atmospheric mixing, allowing the fog layer to deepen and refill each morning. Compounding this, a strong, stable ridge of high pressure has settled over California. NWS has described the ridge as “resilient” and “blocking,” meaning it is preventing storms, wind, and daytime warming from dispersing the fog. As a result, cold air and low stratus clouds remain trapped at the surface, creating the prolonged, freezing fog event now impacting the Valley.
Some online posts have speculated about “radioactive fog,” but NOAA clarifies that the term radiation fog refers only to radiational cooling, which is the natural process where the ground loses heat at night through infrared radiation. Weather.gov explains that the phenomenon is purely atmospheric and has no connection to nuclear or ionizing radiation.
While tule fog is a well-known winter feature in the Valley, long-term scientific studies show that fog days have declined by roughly 50% since the 1980s, partly due to changes in air pollution. Multi-week fog episodes have become less common as a result. However, researchers note that when conditions do align, such as after unusually wet periods, dense valley fog can still form and persist.
Los Banos, at roughly 65 feet of elevation on the west side of the San Joaquin Valley floor, experiences the same basin-wide atmospheric conditions as Merced and surrounding areas. Residents who grew up with frequent tule fog decades ago may remember long stretches of frozen, gloomy days, but many locals say the current 2025 event is the worst they have seen in years.
According to NWS Hanford, meaningful relief will likely require either a shift in the high-pressure pattern or an incoming storm capable of mixing the air mass. Forecast models show limited change through the coming week, meaning Los Banos should expect continued freezing fog, reduced visibility, and colder-than-normal daytime temperatures.
Until then, officials emphasize caution on the roads and patience as the Central Valley endures one of its most persistent winter fog events in recent history. This is a true “tule fog season” that has returned with unexpected force.
