A Winter Storm Watch is a National Weather Service (NWS) alert that conditions are favorable for a winter storm that could reach the local office's warning criteria. It goes out ahead of the event, while timing, track or amounts are still uncertain, and it can cover heavy snow, heavy sleet, ice, blowing snow or a combination of them.
How confident the watch is
Under NWS Instruction 10-513, the national directive for winter products, offices issue a watch when there is about a 50 percent or greater confidence that a hazardous winter event will meet local warning or impact criteria.
The same directive says a watch should not be issued within 12 hours of the event's start, so it is a longer-range heads-up rather than a last-minute alert. Offices describe the window differently: the Lincoln, Illinois office says its watches usually go out at least 24 hours ahead, and the Twin Cities office describes them as generally within 72 hours.
One watch for several winter hazards
Since 10/01/17, the Winter Storm Watch has also covered hazards that once had their own watches. The NWS folded the Blizzard Watch and the Lake Effect Snow Watch into it.
That makes the text of the watch more important than its name. When only blizzard conditions are possible, the directive has the WHAT section read "Blizzard conditions possible," while other watches include generalized amounts, such as more than 6 inches of snow possible.
What happens after a watch
A watch is a step in a sequence, not a final call. Warnings and advisories are tied to a higher bar, about 80 percent or greater confidence, so as the storm comes into focus the office may move to a Winter Storm Warning, Blizzard Warning, Ice Storm Warning or Winter Weather Advisory, or drop the watch if the threat fades.
What a watch does not tell you
Because the watch points to local warning criteria, the same headline can stand for very different snow totals in different regions. The NWS notes that the snow needed for a warning in the Northern Plains is typically much higher than in the Southeast.
Read the forecast amounts and timing in the watch text, and pair it with impact tools such as the Winter Storm Severity Index, to judge what the storm may actually do where you are.