Los Banos, CA (December 12, 2025) — The National Weather Service (NWS) Hanford office has issued a Dense Fog Advisory along with an Experimental Fog Severity Index that places the region at Level 3: Moderate Transportation Risk until Saturday, December 13 at 11:00 am. This means motorists should be prepared for significantly reduced visibility, in some cases as low as ½ mile or less, along with slower traffic conditions due to the fog.
The National Weather Service (NWS) Hanford office uses an Experimental Fog Severity Index to communicate how fog is expected to impact transportation across Central California. Rather than simply indicating whether fog is present, the index categorizes conditions by how significantly visibility reductions may affect drivers and traffic flow. The system is designed to help motorists, emergency responders, and transportation agencies better understand the potential risk level on roadways.
The Fog Severity Index includes three primary levels. Level 1 indicates no transportation impacts, with fog not expected to meaningfully affect visibility or travel. Level 2 represents a low transportation risk, where visibility may be reduced to around one mile or less and traffic may slow below posted speed limits. Level 3, which is currently in effect for portions of the San Joaquin Valley, indicates a moderate transportation risk, meaning visibility may drop to ½ mile or less in some locations and drivers should anticipate slower traffic and more hazardous driving conditions.
For Los Banos, Merced, and surrounding areas, the Level 3 designation signals that fog-related travel impacts are likely, particularly during overnight and early morning hours. According to NWS guidance, this level is associated with increased caution for motorists, as reduced visibility can make it difficult to see other vehicles, road hazards, or sudden changes in traffic patterns. Major travel corridors near Los Banos and Merced, including Interstate 5 and State Routes 152, 41, 99, and 165, are among the roadways where these impacts may be most noticeable.
The National Weather Service emphasizes that the Fog Severity Index is impact-based, meaning the focus is on how conditions affect travel rather than just meteorological measurements alone. While fog may be patchy in nature, areas that experience sudden drops in visibility can still pose a significant risk to drivers. Motorists traveling in or through the Los Banos and Merced region are urged to remain alert, slow down, and be prepared for rapidly changing visibility conditions while the Level 3 designation remains in effect.
