National Weather Service

Storm-Based Warnings


Effective 10/1/07

NOAA LogoThe Storm-Based Concept National Weather Service Logo County vs. Storm-Based
  • Polygons aren't just the future, they are the present!
    • Private sector companies are using Lat/Lon information at the end of warning products to:
      • Graphically plot warning polygons for customers.  TV meteorologists and Weather Radio manufacturers are already taking advantage of this information.
      • Allow Emergency Managers to determine what tornado warning sirens to sound.
      • Determine what phones to ring.
  • The Storm-Based concept allows us to:
    • Focus on the true area threatened by a given storm.
      • Reduce the area not threatened
    • Easily handle storms moving along county borders.
    • Support weather radio evolution to alert areas smaller than a county.
    • Support the development of graphical/web-based products that show where the greatest threat exists
  • The size of the polygon will relate to the location and extent (in time and space) of the threat.
  • By evolving from “county-centric” to “specific threat” areas we will improve the accuracy of warnings and minimize the impact on residents outside the polygon area.
  • The main concern with the Storm-Based concept:
    • How do we relay polygon coverage into words for audio broadcast without causing confusion?  Include specific locations, communities, landmarks, etc.

Example of Warning by Polygon

  • In this example, the storm is moving east. The highest tornado threat exists over a small portion of Sumner County, as shown by violet box (threat area) compared to the thick orange box (county border).
  • We can use the storm-based approach to graphically illustrate the highest threat area, which is in the southern rural areas of Sumner County, by using polygons.
    • Is there a need to warn people in the northern part of Sumner County of an imminent tornado threat (e.g. sound sirens, urge folks to move to the basement) in this example?
    • As the illustration shows, the polygon warning approach offers the potential of reducing warning aerial coverage, where no imminent threat exists, by 50% or more compared to whole-county warnings. is the U.S. government's official web portal to all federal, state and local government web resources and services.