Boston Mayor Michelle Wu issued a cold emergency for the city beginning Friday night, as Massachusetts braces for dangerously cold wind chills. Wind chills as low as minus 10 are expected Friday night into Saturday across the state as an arctic cold front moves through the region.

With a massive snowstorm barreling toward Massachusetts and New England, forecasters are urging anyone planning to travel over the weekend to avoid doing so if at all possible. The National Weather Service issued a Winter Storm Warning blanketing the entire state,

All of Massachusetts is under a winter storm warning ahead of a massive weather system that could dump 1-2 feet of snow across the region this weekend. Hour-by-hour futurecast maps: See when huge winter storm arrives in Mass.

What happens when heavy ice, deep snow, and an arctic plunge line up over the same stretch of the country? Rather than a single “snow map,” forecasters monitoring Winter Storm Fern have been more concerned with a line that is constantly moving: where the precipitation falls as snow versus sleet or freezing rain.