MONTREAL -- Fashion forecast for Tuesday: Dress for mild weather. But also for freezing weather ... you know what, just wear layers to be safe.

Why? Generally, because it's Montreal in December.

More specifically, because forecast temperatures in the city are expected to hit a relatively balmy high of 8C at 7 a.m. Tuesday before dropping consistently throughout the day to hit a low of -9C by the time the clock strikes midnight.

We've done the math for you: that's a 17C swing in 17 hours.

Weather, Dec. 9, 2019.

And that swing will be even more of a shock to the system than it may sound, as the wind chill will make it feel like -16C by Tuesday night.

The unseasonably mild temperature dropping so quickly could make walking and driving Tuesday afternoon and evening - especially during rush hour -  particularly treacherous, so caution is advised on Montreal roads and sidewalks.