A Winter Storm Warning is a National Weather Service alert that a significant winter weather event is expected, with snow, sleet, ice or blowing snow, alone or in combination, reaching the local office's warning criteria. It means conditions threaten life or property and travel may become difficult to impossible.
What triggers it
NWS Instruction 10-513, the national winter products directive, gives two main paths. A single element such as snow, sleet or ice can meet the office's event-based warning criteria, or a mix of snow, ice, sleet and blowing snow can qualify when at least one precipitation element reaches warning level.
Offices issue warnings at about 80 percent or greater confidence. The directive also allows an impact-driven warning when amounts will fall short of published criteria but the timing, such as early in the season or during rush hour, makes high impacts likely.
Why the numbers differ by office
Each office sets its own amounts. As examples, the Lincoln, Illinois office lists at least 6 inches in 12 hours or at least 8 inches in 24 hours, while the Twin Cities office uses 6 inches or more for an event up to 48 hours long.
The NWS notes that the snow needed for a warning in the Southeast is typically much lower than in the Northern Plains.
How it fits with other winter warnings
This is the general-purpose winter warning. When wind-driven snow and visibility are the threat, offices use the Blizzard Warning, which has its own wind and visibility criteria, and when ice accumulation is the main concern, the Ice Storm Warning applies.
Since 10/01/17, many Great Lakes offices have handled lake-effect snow under the Winter Storm Warning instead of a separate Lake Effect Snow Warning. The directive still lets regions keep that separate product at their discretion.
What the headline does not tell you
The headline alone does not say where the heaviest snow will fall within the area or how fast it will come down. Totals can vary sharply over short distances when narrow snow bands set up, so the forecast amounts, timing and observed snowfall rates matter more than the headline alone.