The National Weather Service office in Great Falls issued a Dense Fog Advisory for the Canyon Ferry area, Gallatin Valley, and Missouri Headwaters beginning the night of October 13 and running through noon on Wednesday, October 14. Visibility could drop to as low as one quarter of a mile in areas where the fog is thickest.
The NWS flagged an additional hazard layered on top of the reduced visibility: below-freezing temperatures overnight could produce a glaze of ice on road surfaces, with bridges and overpasses carrying the highest risk. That combination — fog that cuts sight distance down to a few car lengths and ice that removes traction — makes for conditions that punish inattention, particularly on the canyon road out to Canyon Ferry Lake and on elevated sections of I-15 and U.S. 12.
Drivers heading out early Wednesday morning should slow down, increase following distance, and treat bridges as suspect until temperatures climb. The advisory expires at noon, but ice on shaded overpasses can persist well past the official end of an advisory on days that stay cool. The NWS Great Falls office is the point of contact for updated conditions.