Particularly dangerous situation

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A particularly dangerous situation (PDS) is a type of enhanced wording used by the National Weather Service's Storm Prediction Center in Norman, Oklahoma on certain weather watches. It is issued at the discretion of the forecaster writing the watch and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or an extreme derecho event.[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] There often are multiple PDS watches issued for the same weather system and on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower. 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]

PDS tornado watches are issued when there is a greater risk of strong or violent tornadoes in the watch area, in addition to significant wind and hail damage. While there is no hard criteria for a PDS watch to be issued, they are usually issued when the potential exists for a major tornado outbreak. They represent over 90% of PDS watches issued.[2]

PDS severe thunderstorm watches are issued when there is a greater risk of severe wind damage capable of major structural damage (in addition to large hail and perhaps a few isolated tornadoes), usually due to a strong derecho. They are very rare (an average of only two each year) as the tornado risk has to remain low enough to only warrant a severe thunderstorm watch (a tornado watch would be issued if the tornado risk is significant alongside the wind threat).[2]

These watches are generally issued after a high risk or an upper-end moderate risk of severe storms is issued by the SPC in the convective outlooks.

[edit] Sample PDS Tornado Watch

This watch was issused for the storm system that later produced the Greensburg, Kansas tornado.

   URGENT - IMMEDIATE BROADCAST REQUESTED
  TORNADO WATCH NUMBER 232
  NWS STORM PREDICTION CENTER NORMAN OK
  955 AM CDT SAT MAY 5 2007
  
  THE NWS STORM PREDICTION CENTER HAS ISSUED A
  TORNADO WATCH FOR PORTIONS OF 
  
         PARTS OF WESTERN AND CENTRAL KANSAS
         PARTS OF SOUTHWEST AND CENTRAL NEBRASKA
  
  EFFECTIVE THIS SATURDAY MORNING AND EVENING FROM 955 AM UNTIL
  1000 PM CDT.
  
  ...THIS IS A PARTICULARLY DANGEROUS SITUATION...
  
  DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER...
  THUNDERSTORM WIND GUSTS TO 90 MPH...AND DANGEROUS LIGHTNING ARE
  POSSIBLE IN THESE AREAS.
  
  THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE
  MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH NORTHWEST OF
  BROKEN BOW NEBRASKA TO 55 MILES SOUTHWEST OF RUSSELL KANSAS.  FOR
  A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
  OUTLINE UPDATE (WOUS64 KWNS WOU2).
  
  REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
  TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
  AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
  THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
  AND POSSIBLE WARNINGS.
  
  DISCUSSION...VERY POTENT TORNADIC SUPERCELL PATTERN IN PLACE ACROSS
  WATCH AREA AS AIR MASS IS EXTREMELY UNSTABLE WITH VERY FAVORABLE
  SHEAR PROFILES.  WITH LITTLE INHIBITION REMAINING ALONG E OF DRY
 LINE...STORMS WILL RAPIDLY BECOME SEVERE BY EARLY THIS AFTERNOON WRN
  KS INTO SWRN NEB.  TORNADIC SUPERCELLS WILL DEVELOP WITH POTENTIAL
  FOR LONG TRACK/VIOLENT TORNADOS.  AS DRY LINE REMAINS WRN KS THRU
  THE AFTERNOON...ADDITIONAL DEVELOPMENT OF TORNADIC SUPERCELLS ARE
  LIKELY  OFF THE DRY LINE THRU THE EVENING HOURS.
  
  AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
  SURFACE AND ALOFT TO 4 INCHES. EXTREME TURBULENCE AND SURFACE
  WIND GUSTS TO 80 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
  600. MEAN STORM MOTION VECTOR 22040.
  
  
  ...HALES

[edit] References

  1. ^ NOAA (2004-04-18). Storm Prediction Center Frequently Asked Questions (FAQ). Retrieved on 2008-05-23.
  2. ^ a b c Dean, Andrew R. (2006-11-07). PDS watches: how dangerous are these "particularly dangerous situations?" (2006 - 23SLS_23sls). American Meteorological Society. Retrieved on 2008-05-23.
  3. ^ Lewis, John (2007-11-03). "A Forecaster's Story: Robert H. Johns". Electronic Journal of Severe Storms Meteorology 2 (7). 

[edit] External links

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Severe Weather Watches and Warnings

Tornado: Watch (PDS) / Warning / Emergency   •   Severe Thunderstorm: Watch (PDS) / Warning   •   Flash Flood: Watch / Warning / Emergency   •   Winter Storm: Watch / Warning   •   Blizzard: Watch / Warning