If you have been unclear about the National Weather Service’s new Warning by Polygon system, then this should be informative.
Now that the severe weather season is underway, we have gotten a chance to utilize our new Warning By Polygon system that was implemented nationwide on October 1st 2007. This marks a major change in how the National Weather Service warns the public of impending threats by transitioning from traditional county-based warnings to now issuing warnings based on individual storms or lines of storms. Instead of a warning being in effect for an entire county where many locations may not in fact be threatened, the warning will now strictly be in effect for the areas that will be impacted.
The graphic below is a snapshot from our webpage during the severe weather event on April 11, 2008. In this snapshot, the areas in orange are covered by Severe Thunderstorm Warnings while the area in red is under a Tornado Warning. These polygons can be viewed on our homepage and may also be seen overlaid with radar data on the radar page.
In the example below, portions of Sanilac and Huron County are under a Severe Thunderstorm Warning while eastern Sanilac County is under a Tornado Warning. When receiving warning information visually, the polygon method provides enhanced information regarding which areas are included in the warning and threatened by the storms. The new method reduces the area that is “false alarmed” by being able to exclude areas that do not face an immediate threat of severe weather. In this example, the tornado warning for Sanilac County covers only 27% of the county. Persons not included in the tornado warning would not need to seek shelter from the storm at this time. This new method of warning supports the latest technology and allows more precision with warnings.
The next several years will continue to be both an adjustment period and a transition period. In many cases, dissemination methods continue to be county based and will take time to transition to polygon based. For example, many television broadcasts are already utilizing the new polygon warnings while both NOAA Weather Radio All Hazards and most tornado sirens will still alert the public by county. You may always view the latest warning information on our website at www.weather.gov/dtx and then click on Local Radar.
SOURCE: NWS Spotter Newsletter
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