A Snow Squall Warning is a short-fused National Weather Service alert for a brief, intense burst of snow with gusty winds that sharply cuts visibility and can quickly turn roads icy. Like a tornado or severe thunderstorm warning, it is drawn as a polygon around the squall's path, and it is usually valid for 30 to 60 minutes.
What triggers a warning
Under NWS Instruction 10-513, offices issue the warning based on radar or satellite indication, or reliable reports from sources such as state transportation departments, webcams and road sensors.
The squall has to meet at least one of two conditions: visibility of 1/4 mile or less in snow with road temperatures below freezing, or plunging temperatures that could cause a flash freeze along with a significant drop in visibility from falling or blowing snow.
The SIGNIFICANT tag
Each warning carries one of two impact levels. The default, with no tag, is for squalls where mitigating actions and context reduce the threat to travel, while a SIGNIFICANT tag marks a substantial threat to safe travel.
Only warnings tagged SIGNIFICANT activate Wireless Emergency Alerts, which reach phones in the squall's path.
How it fits with other winter alerts
Polygon-based Snow Squall Warnings became operational at NWS offices on or about 11/01/18. They are not issued while a Winter Storm Warning or Blizzard Warning is in effect, but they can be issued during a Winter Weather Advisory if conditions are expected to worsen briefly.
The key difference is duration. A snow squall usually lasts on the order of 30 to 60 minutes, while a winter storm can last for hours or days, and a blizzard requires conditions lasting at least three hours.
What it does not tell you
Snow squalls are often tied to strong cold fronts and move in and out quickly. The polygon covers the expected path only for the warning's 30 to 60 minute window, so conditions just outside it, or after it expires, can still change as new squalls develop.