The weather agency said snowfall amounts of more than 25 cm will be possible during the day, with an additional 10 cm possible in the evening for parts of Hamilton and Burlington.
Officials said the snow is a result of a squall off of Lake Ontario.
“Travel will be impacted, particularly along portions of the QEW, where very heavy snowfall will result in reduced visibilities at times,” the warning read.
“Motorists are advised to exercise caution.”
Visibility may suddenly drop to “near zero” at times, the weather agency said, in heavy and blowing snow.
The squall is expected to remain over the affected areas well into the evening before weakening overnight, the warning said.
Saturday is the second day in a row Hamilton and parts of Halton have been impacted by snow squalls.
“Some areas like the Burlington Waterfront saw over 30 cm of fluffy snow yesterday and today’s snow squall will last longer and potentially be even more intense,” Global News meteorologist Anthony Farnell said.
“Another 30 cm is likely for this area with pockets of 40 cm or more.”
Farnell said small shifts in wind direction can make a big difference.
“You can have whiteouts and knee-deep snow in one part of town and the other side gets only a couple centimetres,” he said.
Farnell said Friday’s and Saturday’s squalls are rare for the western side of Lake Ontario.
“You don’t often gets winds lining up from the east and a constant supply of cold air from an arctic high sitting to the north,” he said.
The rest of the GTA can expect to see light snow Saturday night with two to five cm of accumulation.