The National Weather Service office in Great Falls issued a Special Weather Statement at 5:15 a.m. Wednesday after patchy dense fog settled into the Helena Valley overnight, dropping visibility below a half mile in some areas. Commuters heading out early Wednesday faced conditions that could change quickly and without much warning.
The NWS advised drivers traveling through the valley to stay alert for rapidly shifting visibility — the kind of fog that can be clear one moment and wall-to-wall white the next, particularly in low-lying areas and along the interstate corridor. The Helena Valley's geography tends to trap cold air and moisture in ways that make localized fog more intense than what's happening just a few miles away on higher ground.
The fog was expected to begin dissipating after 9 a.m. as temperatures rose and the sun got to work. Morning fog in the valley is a recurring feature of the shoulder seasons, when overnight temperatures drop quickly but soil and air retain enough moisture to produce these conditions.
No accidents or road closures were reported in connection with the fog as of the time of the statement. Residents are encouraged to monitor NWS Great Falls for updated conditions during the fall fog season.