These pop-up storms, often tied to cold fronts, can turn winter calm into winter chaos in just minutes. Snow squalls are brief but intense, with heavy snow, gusty winds, and near-zero visibility. More common in late winter and early spring, they can happen when temperatures and air masses mix suddenly.

Tens of millions of Americans are bracing for a massive winter storm Sunday, which is forecast to bring the heaviest snowfall and coldest temperatures to parts of the country in over a decade. The polar vortex has already prompted states of emergency in the Plains,

As a deep winter chill begins to take over the northern half of the country, a new major winter storm will move across the U.S. this weekend into early next week.