The NWS issues both watches and warnings when weather conditions/events
threaten life and/or property. This includes watches and/or warnings for winter storms, hurricanes and tropical storms, tornadoes and severe thunderstorms, flash floods, high wind, fire weather, freezing rain/drizzle, wind chill and blizzards just to name a few. Additionally, the NWS issues advisories for weather events that are less serious than a watch or warning, but may cause significant inconvenience. An example is a winter weather advisory. If caution is not exercised, it could lead to situations that may ultimately threaten life and/or property. When issuing watches or warnings the NWS generally uses a three-tiered approach as shown in the diagram below.
This approach is largely based on two variables, time until the event and certainty of the event. Outlooks are issued well in advance of the event when conditions are ordinarily uncertain. For example, the Storm Prediction Center (SPC) may issue a severe thunderstorm outlook three or more days in advance of the severe thunderstorm event. The outlook area will typically cover a large geographic region as shown by the purple line in the diagram below.
A watch, on the other hand, is issued by the NWS indicating that conditions are more favorable (forecasters are more certain) for a particular weather hazard to occur. A watch is a recommendation for planning, preparation and increased awareness. Persons in the watch area should remain alert for changing weather, listen for more information and think about what to do if the danger materializes. For example, the SPC may issue a severe thunderstorm watch when conditions are favorable for the development of deep, moist convection that may be severe (strong straight line winds, large hail and tornadoes). Depending on the specific hazard being forecast, watches are normally issued from several hours to a day or so before the event occurs. Hurricane watches, for example, are normally issued 48 hours in advance of the anticipated onset of tropical-storm-force winds, but severe thunderstorm watches may be issued only a few hours before thunderstorms are impacting the watch area. Also, in many cases, geographic coverage is much smaller than the outlook area as shown by the orange parallelogram shown above. A warning is issued by the NWS indicating that a particular weather hazard is either imminent or has been reported. Warnings are ordinarily issued when forecasters are very certain of the impact of the weather hazard. Persons in the warning area need to take action immediately to protect life and property. Again, depending on the specific hazard being forecast, warnings are normally issued 15 minutes prior to the event for severe thunderstorms or tornado warnings or within 36 hours for hurricane warnings. Ordinarily, the geographic region covering a warning is smaller than the watch area can be quite small as shown by the red dot in the image above for severe thunderstorm or tornado warnings. As you can see, the criteria used for outlooks, watches and warnings are highly dependent on the kind of event being forecast. Larger scale weather events such as winter storms or hurricanes ordinarily impact larger geographic regions that are issued with more advance notice. Severe thunderstorm and tornado watches and warnings, however, impact a smaller area and have a shorter advance notice.
Most pilots are weatherwise, but some are otherwise™ Scott Dennstaedt Weather Systems Engineer CFI & former NWS meteorologist