SCIENCE & NATURE

What criteria does the National Weather Service use to issue a PDS tornado warning?

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The National Weather Service issues a PDS (Particularly Dangerous Situation) tornado warning when a tornado has a strong potential to produce significant damage and fatalities, typically based on radar signatures showing strong rotation, damage reports, or both. PDS warnings indicate an unusually dangerous and potentially long-lived tornado situation.

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PDS MeaningParticularly Dangerous Situation
Primary CriteriaStrong rotation on radar (mesocyclone/debris signature) or significant damage already occurring
Warning ColorRed polygon on weather alerts
Damage PotentialIndicates strong to violent tornado with major damage expected
FrequencyIssued for only the most dangerous tornado situations

What is a PDS Tornado Warning

A PDS tornado warning is the most urgent type of tornado warning issued by the National Weather Service. It alerts the public to an exceptionally dangerous tornado situation where a tornado is imminent, occurring, or likely to occur. PDS warnings are issued only when specific dangerous conditions are met and are meant to prompt immediate action from people in the affected area.

Main Criteria for Issuing PDS Warnings

The NWS issues a PDS tornado warning when radar data shows strong and persistent rotation in a storm, especially a tight mesocyclone or a hook echo with a debris signature. A debris signature indicates that a tornado is actually on the ground and picking up material. Additionally, if damage is already being reported from a strong tornado, this triggers a PDS warning. The presence of both strong radar rotation and damage reports makes the situation particularly dangerous.

Radar Indicators

Meteorologists look at Doppler radar for specific features that suggest a violent tornado. A mesocyclone is a rotating column of air within a thunderstorm visible on radar. A debris signature shows up as a concentrated area of reflectivity near the ground, indicating that a tornado is picking up debris. These radar signatures, especially when they persist and intensify, are key reasons for issuing a PDS warning.

Damage Reports as Evidence

When the National Weather Service receives reports of structural damage, injuries, or fatalities from a tornado, this is strong evidence that a particularly dangerous situation exists. Real-time reports from storm spotters, law enforcement, emergency responders, or the public help meteorologists confirm that a violent tornado is on the ground and warrant an immediate PDS warning.

Difference from Regular Tornado Warnings

A regular tornado warning indicates that a tornado has been sighted or indicated by weather radar, but a PDS warning indicates an even more severe and dangerous situation. While all tornado warnings require immediate action, a PDS warning emphasizes the extreme danger and the potential for significant loss of life and widespread destruction. The PDS designation helps communicate the highest level of urgency to the public.

Sources

  1. weather.gov (weather.gov)
  2. noaa.gov (noaa.gov)
  3. nws.noaa.gov (nws.noaa.gov)