The National Weather Service (NWS) is the United States government agency, part of the National Oceanic and Atmospheric Administration (NOAA) within the Department of Commerce, that issues the nation's official weather, water and climate forecasts and warnings. Its work runs from national centers that forecast large-scale hazards to local forecast offices that warn individual counties.
How it is organized
The NWS mission is to provide weather, water and climate data, forecasts, warnings and decision support for the protection of life and property and the national economy. Its structure runs from headquarters in Silver Spring, Maryland, to six regional headquarters and then to local Weather Forecast Offices (WFOs).
Alongside the WFOs sit 13 River Forecast Centers that watch the nation's waterways and the National Centers for Environmental Prediction (NCEP), made up of nine centers, including the Storm Prediction Center, Weather Prediction Center, Climate Prediction Center and National Hurricane Center.
Who issues what
Local WFOs issue advisories, warnings, statements and short-term forecasts for their county warning area around the clock. They are the source of tornado, severe thunderstorm, flood and winter storm warnings.
A notable exception is severe thunderstorm and tornado watches, which the Storm Prediction Center issues for the whole country. Other national centers supply outlooks and guidance that local offices build on.
Watches, warnings and advisories
The NWS glossary defines a warning as an event that is occurring, imminent, or has a very high probability of occurring. NWS Baltimore/Washington describes a watch as meaning the hazard is possible, and a winter weather advisory as covering conditions expected to cause significant inconvenience but not serious enough for a warning.
Criteria for some products are set locally. For example, NWS Indianapolis uses a different heavy snow warning threshold from other NWS offices serving Indiana.
What it does not tell you
A product's name describes the type and certainty of a hazard, not the exact conditions at your location. Two places under the same warning can see very different weather, so read the text of the product and watch radar and reports as the event unfolds.