A Fire Weather Watch is a National Weather Service (NWS) alert that a Red Flag event, meaning weather and fuel conditions that meet the local office's Red Flag criteria, has a high potential to develop in the coming days. It alerts land managers and the public before a Red Flag Warning would be needed.
How far ahead it goes out
NWS Instruction 10-401, the national fire weather directive, has offices issue the watch 18 to 96 hours before the expected onset of criteria. Its stated purpose is to alert users at least a day ahead for resource allocation and firefighter safety.
Office pages describe the window in their own terms. The Chicago office says up to 72 hours ahead, and the Grand Junction office says generally the next 24 to 72 hours.
Not a low-confidence hedge
The directive says a watch should not be issued, or kept going, just to signal low confidence or borderline conditions. It is meant for a real, high-potential threat.
A watch ends in one of three ways: it is canceled, upgraded to a Red Flag Warning, or allowed to expire. Before issuing either product, forecasters are expected to coordinate with local fire and land managers and with Predictive Services at the National Interagency Fire Center or the regional coordination centers.
What sets the bar
The watch points to the same local criteria as the warning. In Grand Junction, for example, one path is relative humidity of 15 percent or less with sustained winds or frequent gusts of 25 mph or more, together for at least 3 hours in a 12-hour period.
That office also counts widely scattered dry thunderstorms, about 15 percent coverage or more, as a Red Flag condition. That office says Red Flag events normally require critical fuels as well, as judged by the land management agencies.
What a watch does not tell you
A watch describes potential weather and fuel conditions, not a fire. Because criteria differ by office, the same headline can mean different wind and humidity numbers from one region to the next, so read the text for the expected values and timing.