A Weather warning generally refers to an alert issued by a meteorological agency to warn citizens of approaching dangerous weather. A weather watch, on the other hand, typically refers to an alert issued to indicate that conditions are favorable for the development of dangerous weather patterns, although the dangerous weather conditions themselves are not currently present.
In the United States, governmental weather warnings and watches are issued by the National Weather Service, which is itself a branch of the National Oceanic and Atmospheric Administration. The NWS defines a watch as "the risk of a hazardous weather or hydrologic event [increasing] significantly, but its occurrence, location, and/or timing is still uncertain" and a warning as "hazardous weather or hydrologic event [that] is occurring, is imminent, or has a very high probability of occurring" [1]. Additionally, the NWS breaks down weather warnings and watches based upon the specific type of hazardous weather. These warnings and watches include, but are not limited to, flooding, severe local storms, tropical cyclones, and winter storms. The Severe Weather Terminology article has much greater detail on NWS warnings.
The Met Office, the NWS's counterpart in the United Kingdom, does not issue separate weather warnings and watches, but has an analogous system of flash warnings and advance warnings that serve the same general role of weather warnings and weather watches, respectively. Other official meteorological departments may use similar systems but use different terms.
MetService is the National Meteorological Service of New Zealand, and is designated by the Minister of Transport to provide New Zealand's authorised meteorological warning service (). MetService issues Severe Weather Outlooks, Watches and Warnings under a Code of Practice that enables others to distribute this information in the national interest. Weather Warning Criteria are published on its website. Like the United States NWS, MetService breaks down weather warnings and watches based on the specific type of hazardous weather - heavy rain, heavy snow, severe gales, and other weather likely to cause significant disruption to the general public or specific industry groups. MetService also provides Severe Thunderstorm Outlooks, Watches and Warnings to deal with heavy rain and strong wind gusts caused by thunderstorms, as well as large hail and damaging tornadoes.
The Met Office and other weather services have three colour coded warning levels. Yellow: Be aware. Possible travel delays, or the disruption of your day-to-day activities. Amber: Be prepared. Could cause road and rail closures, interruption to power and the potential risk to life and property. Red: Take action. Widespread damage, travel and power disruption and risk to life is likely. Avoid dangerous areas.
The Swedish Meteorological and Hydrological Institute has developed its own warning level terminology. Class 1 means forecast of weather that can mean some risks and disturbances to transport and other parts of society. Class 2 is for weather that can mean danger, damage and larger disturbances. Class 3 is for weather that can mean big danger, serious damage and major disturbances. This can refer to many types of weather related events such as wind, flooding, snow, forest fires etc. Sweden does not have as severe weather as some other countries, so class 3 events in Sweden would normally not cause very large international headlines.