With the unusual weather making its way through southern Ontario, Environment Canada has issued a fog advisory for Guelph and Waterloo Region.
The agency says near-zero visibility is expected on Wednesday afternoon and into the evening.
The fog advisory also covers all of Wellington County, along with most of southwestern Ontario.
Visibilities should improve with rain that is expected Wednesday night, but patchy dense fog may redevelop, the weather office said.
Drivers are being cautioned to slow down, watch for tail light ahead and be prepared to stop.