A snow squall is a brief, intense burst of heavy snow with strong, gusty winds that can drop visibility to near zero within moments. Squalls often accompany strong cold fronts and usually last less than an hour in any one place.
How a squall differs from a snowstorm
A snowstorm can last hours or days. A squall typically passes in 30 to 60 minutes, and accumulations are usually an inch or less, so the danger comes from how fast conditions change rather than how much snow piles up.
What produces them
National Weather Service (NWS) research lists the favorable setup as instability with steep lapse rates near the ground, frontogenetic forcing, low-level moisture and a temperature profile cold enough for snow.
Researchers describe squalls as convective systems in environments with low convective available potential energy (CAPE) and strong wind shear. NWS State College compares a squall to a summer thunderstorm, with heavy precipitation in a short time and strong winds.
Why roads turn dangerous
Visibility can fall to almost nothing in an instant, often while traffic is moving at highway speed. Behind the squall, strong cold advection and cooling from melting snow can freeze wet pavement into ice, a flash freeze that keeps roads hazardous after the heaviest snow has passed.
NWS research links squalls to multi-vehicle pileups on highways and interstates and cites Storm Data showing they were Pennsylvania's second leading cause of weather-related deaths, behind floods, over the ten years ending in 2008.
How the Weather Service warns on it
The NWS issues a Snow Squall Warning, a short-fused warning focused on the specific areas facing an intense burst of snow and wind, usually lasting 30 to 60 minutes. A Significant impact tag marks squalls expected to pose a substantial threat to travel and triggers a Wireless Emergency Alert.
Safety guidance is simple: avoid or delay travel until the squall passes, and if already driving, slow down, turn on headlights and leave extra room.