Particularly Dangerous Situation

A Particularly Dangerous Situation (PDS) is a type of enhanced wording first used by the Storm Prediction Center (a national guidance center of the National Weather Service, the meteorological agency of the United States) on certain severe weather watches. It is issued at the discretion of the forecaster composing the watch and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or (much less often) a long-lived, extreme derecho event, but possibly another weather hazard.[1]

PDS watches are quite uncommon; less than 3% of watches issued by the SPC from 1996 to 2005 were PDS watches, or an average of 24 each year.[2] When a PDS watch is issued, there are often more PDS watches issued for the same weather system, even on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower.

Background

The first PDS tornado watch was issued by Robert H. Johns for the April 2, 1982 tornado outbreak across the southern and central Great Plains.[3] While historically applied only to severe thunderstorm, tornado and flash flood watches (i.e., Severe Local Storm "polygonal" events), PDS wording could theoretically be applied to other types of weather watches (such as winter storm, high wind, hurricane, or fire weather watches) when an enhanced threat for such conditions exists. These watches have generally (but not always) been issued during a high risk or an upper-end moderate risk either of severe storms from the SPC's convective outlooks or of flash flooding from the Weather Prediction Center's Excessive Rainfall Outlooks.

On April 24, 2011, the National Weather Service Weather Forecast Office in Memphis, Tennessee issued the first PDS flash flood watch to highlight the threat for widespread, significant and potentially life-threatening flash flooding due to repeated rounds of severe thunderstorms.

Issuance

PDS Tornado Watch

PDS tornado watches are issued when there is a higher than normal risk of multiple strong to violent tornadoes – especially those that are predicted to be long-track in nature, with path lengths of more than 20 miles – in the watch area (usually amounting to damage consistent with EF4 or EF5 tornadoes), in addition to including significant wind and hail damage. While there are no set criteria for a PDS watch to be issued, they are usually issued when the potential exists for a major tornado outbreak. These types of tornado watches represent about 90% of PDS watches issued by the Storm Prediction Center.[2]

The PDS Tornado Watch shown below was issued covering Alabama during the early stages of the extreme tornado outbreak that killed over 300 people (which clearly verified) as explained above.

NOTE: The color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was rendered in red (not bolded) in the actual alert.[4]

PDS Severe Thunderstorm Watch

PDS severe thunderstorm watches are issued when there is a higher than normal risk of severe thunderstorm winds capable of major structural damage (in addition to large hail and perhaps a few isolated tornadoes), usually due to a strong and persistent derecho. These watches are very rare (accounting an average of only two each year), as the risk for tornadoes must remain low enough to not warrant a tornado watch (a normal tornado watch would be issued if the tornado risk is significant alongside the extreme wind threat).[2]

The PDS Severe Thunderstorm Watch shown below was from May 30, 2011:

NOTE: The color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was red (not bolded) in the actual alert.[5]

PDS Flash Flood Watch

PDS flash flood watches are issued when there is a higher than normal risk of widespread, life-threatening flash flooding. These watches are issued by local NWS Weather Forecast Offices, not the Storm Prediction Center.

This watch shown here, was the first PDS Flash Flood Watch and was issued by the National Weather Service forecast office in Memphis, Tennessee on April 24, 2011, as mentioned above.[6]

NOTE: The color of all text in the original alert issued was black, including the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was neither rendered in red nor bolded in the actual alert.

PDS Tornado Warning

PDS tornado warnings are currently issued on an experimental basis by the 38 National Weather Service Weather Forecast Offices within the Central Region.[7] The criteria for a PDS warning is when a tornado on the ground has been spotted or confirmed, or a significant tornado is expected (due to radar signatures). While the intention of this experimental warning may be to replace the loosely defined tornado emergency, PDS tornado warnings are structured as the second highest level of tornado warning within the Impact Based Warning system (an experiment – which also includes tags within warning products illustrating radar indications or physical observations of tornadoes, and damage potential – participated by the 33 Weather Forecast Offices within the Central Region, and will expand to eight additional offices within the Western, Eastern and Southern regions in the spring of 2014[8]); a tornado emergency, the highest warning level, is used within the United States for destructive tornadoes approaching more densely populated areas. These are the first warnings issued with PDS wording, and like PDS flash flood watches, are issued by local forecast offices.[9][10]

NOTE: The color of all text in the original alert issued was black, including the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was neither rendered in red nor bolded in the actual alert.[11]

PDS Wind Chill Warning

PDS wind chill warnings are issued when there is an enhanced risk of frost bite, hypothermia, and eventually death due to extremely low wind chills. These warnings are issued by the National Weather Service Weather Forecast Offices rather than the Storm Prediction Center.

The PDS Wind Chill Warning shown below was issued by the National Weather Service in the Twin Cities on January 5, 2014.

NOTE: The color of all text in the original alert issued was black, except for the text "THIS IS A PARTICULARLY DANGEROUS SITUATION", which was neither rendered in red nor bolded in the actual alert.[12]

Other watches and warnings

While the use of PDS wording for other types of watches and warnings has not been used, PDS verbiage could theoretically be applied to any kind of watch or warning to alert the public to weather events where there exists an increased risk of loss of life or widespread damage to property. Some such situations could include PDS watches or warnings for blizzards, ice storms, high winds, extreme heat, or fire danger.

References

  1. ↑ NOAA (2004-04-18). "Storm Prediction Center Frequently Asked Questions (FAQ)". Retrieved 2008-05-23.
  2. ↑ 2.0 2.1 2.2 Dean, Andrew R. (2006-11-07). "PDS watches: how dangerous are these "particularly dangerous situations?" (2006 - 23SLS_23sls)". American Meteorological Society. Retrieved 2008-05-23.
  3. ↑ Lewis, John (2007-11-03). "A Forecaster's Story: Robert H. Johns". Electronic Journal of Severe Storms Meteorology 2 (7).
  4. ↑ NOAA (2011-04-27). "Storm Prediction Center: Particularly Dangerous Situation (PDS) Tornado Watch 235". Retrieved 2011-04-27.
  5. ↑ NOAA (2011-05-30). "Storm Prediction Center: Particularly Dangerous Situation (PDS) Severe Thunderstorm Watch 405".
  6. ↑ "IEM Valid Time Extent Code (VTEC) App". Mesonet.agron.iastate.edu. Retrieved 2014-01-22.
  7. ↑ "Impact Based Warning Experimental Product". Crh.noaa.gov. Retrieved 2014-01-22.
  8. ↑ National Weather Service (2014). "Impact Based Warnings". National Oceanic and Atmospheric Administration. Retrieved March 12, 2014.
  9. ↑ "NWS Expirements" (PDF). NWS. Retrieved 15 April 2012.
  10. ↑ "'CATASTROPHIC': Experimental Tornado Warnings to be Explicit". AccuWeather. 2012-04-05. Retrieved 2014-03-12.
  11. ↑ http://forecast.weather.gov/wwamap/wwatxtget.php?cwa=usa&wwa=Tornado%20Warning|accessdate=26 April 2015
  12. ↑ National Weather Service (2014-01-05). "Iowa Environmental Mesonet NWS Product Archive". Retrieved 2014-01-06.

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