On October 1, 2007 the National Weather Service introduced Storm Based Warnings for tornadoes, severe thunderstorms, flash floods, and marine hazards, which are more specific and accurate for short-duration weather events. With this new warning system, the National Weather Service can specify areas within a county and refer to commonly known landmarks like roads or rivers, which could be affected by severe weather.

  Why Storm Based Warnings?

  • Storm Based (Polygon) Warnings focus on the true threat area, which are not restricted to geopolitical boundaries such as state and county lines.
  • Polygon warnings are easily displayed by the media to the public showing areas of maximum threat to severe weather and better shows who is at risk.
  • They significantly reduce the warning area to the approaching severe weather event.
  • Reduced cost due to polygon warnings is around $100 million each year, if the typical polygon warning is 1/4 the size of a typical warned county. 
  •  Emergency Managers can make better decisions on what resources are required and in which locations.
  •  Schools and businesses can more accurately determine if/what safety procedures are required to ensure the safety of students and workers alike.
  •  Agencies such as the FAA and airlines will be able to make better risk assesments.
  •  This warning system in partnership with the private sector is able to support a wider warning distribution area through cell phone alerts, pagers, and web-enabled PDAs.
 The Process...
Instead of issuing warnings by county, your local NOAA NWS Weather Forecast Office is able to narrow their focus on a portion of a county/counties that have the greatest threat for being impacted by severe weather. The warned area is defined by latitude and longitude coordinates and depicted by polygons. Utilizing Doppler radar algorithms, the calculated movement of severe storms can be indicated through this technique as well.

For audio broadcasts, portions of counties are described by compass points (e.g. northeast, south central etc.). The use of familiar landmarks such as highways or rivers as reference points help describe the warned area.

Warning polygons can shrink in area, but never expand. If a severe storm is expected to track outside of the current warning area, a new Storm Based Warning will be issued for the region now at risk. When severe weather is no longer threatening, your local NOAA NWS Weather Forecast Office allows the warning to expire.

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