What Is the Difference Between a Thunderstorm Watch, Warning?

The National Weather Service (NWS) generally uses a three-tiered approach, consisting of outlooks, watches, and warnings. This system is largely based on two variables: time until the event and certainty of the event.

Outlooks, like the one shown below for thunderstorms, 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 expected event. These typically cover very large geographic areas and also may be valid over a large period of time. In the case of the severe weather convective outlooks given by forecasters at the SPC, they are valid over a 24-hour period when first issued. 

A watch is issued by the NWS when conditions are favorable and 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. 

For example, the SPC may issue a severe thunderstorm watch like the one below when conditions are favorable for the development of deep, moist convection that may be severe and might contain strong straight-line winds, large hail, and/or tornadoes. Watches are issued for a smaller region than outlooks. Severe thunderstorm or tornado watches, for example, are usually less than one-third the size of the state of Kansas. Of course, if the outbreak is large, you may see two or more watch areas active. 

A warning, on the other hand, is issued by the local NWS forecast office when a particular weather hazard is either imminent or has been reported. Warnings are ordinarily issued when forecasters are certain of the impact of the weather hazard.

Persons in the warning area need to take action immediately to protect life and property. Depending on the specific hazard being forecast, warnings are normally issued 15 minutes prior to the event for severe thunderstorms or tornado warnings. Ordinarily, the geographic region covering a warning is much smaller than the watch area for severe thunderstorm or tornado warnings. Typically, these occur at the county level.

Lastly, the SPC also issues what are called mesoscale discussions. These are written as a forecaster-to-forecaster communication, so they frequently contain technical jargon describing the event. Nevertheless, they are usually issued a couple hours prior to the issuance of any watches or they may be issued to describe a current watch area.

They are usually about the same size as the watch areas. Mesoscale discussions can also be issued to describe a winter precipitation event, such as heavy icing due to heavy snow or blizzard conditions or moderate to heavy freezing rain like the one shown below.

Every outlook, watch, mesoscale discussion, and warning will have an associated text. Although highly technical, the text may have important information about the timing and intensity or movement of the weather described. You can find all SPC outlooks, watches and mesoscale discussions here.

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