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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 (much less often) a long-lived, extreme derecho event.



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.

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 about 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 and persistent 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 - generally not PDS - would be issued if the tornado risk is significant alongside the extreme wind threat).[2]

These watches are generally (but not always) issued after a high risk or an upper-end moderate risk of severe storms is issued by the SPC in the convective outlook

Tornado watch PDS samplesEdit

these samples are from the april 22-24 tornado outbreak


SEL1
   
   URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 91
   NWS STORM PREDICTION CENTER NORMAN OK
   600 AM CDT SAT APR 24 2010
   
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   TORNADO WATCH FOR PORTIONS OF 
   
          EASTERN ARKANSAS
          NORTHEAST LOUISIANA
          CENTRAL AND NORTHERN MISSISSIPPI
          SOUTHWEST TENNESSEE
   
   EFFECTIVE THIS SATURDAY MORNING AND AFTERNOON FROM 600 AM UNTIL
   100 PM CDT.
   
   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...
   
   DESTRUCTIVE TORNADOES...LARGE HAIL TO 3 INCHES IN DIAMETER...
   THUNDERSTORM WIND GUSTS TO 70 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 25 MILES NORTH NORTHEAST OF
   MEMPHIS TENNESSEE TO 55 MILES WEST OF PINE BELT MISSISSIPPI.  FOR
   A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
   OUTLINE UPDATE (WOUS64 KWNS WOU1).
   
   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.
   
   OTHER WATCH INFORMATION...CONTINUE...WW 89...WW 90...
   
   DISCUSSION...CLUSTERS OF SEVERE THUNDERSTORMS ARE EXPECTED TO
   INTENSIFY THIS MORNING AS LOW LEVEL JET STRENGTHENS AND UPPER TROUGH
   APPROACHES.  A COMBINATION OF HIGH MOISTURE VALUES...MODERATE
   CAPE...AND EXTREMELY STRONG SHEAR PROFILES WILL POSE A RISK OF
   TORNADIC SUPERCELLS THROUGH THE MORNING.  STRONG TORNADOES ARE
   POSSIBLE.
   
   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   500. MEAN STORM MOTION VECTOR 24040.


SEL3
   
   URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 93
   NWS STORM PREDICTION CENTER NORMAN OK
   800 AM CDT SAT APR 24 2010
   
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   TORNADO WATCH FOR PORTIONS OF 
   
          CENTRAL AND NORTHERN ALABAMA
          MIDDLE TENNESSEE
   
   EFFECTIVE THIS SATURDAY MORNING AND AFTERNOON FROM 800 AM UNTIL
   200 PM CDT.
   
   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...
   
   DESTRUCTIVE TORNADOES...LARGE HAIL TO 2 INCHES IN DIAMETER...
   THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE
   POSSIBLE IN THESE AREAS.
   
   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 70 STATUTE
   MILES EAST AND WEST OF A LINE FROM 20 MILES NORTH NORTHWEST OF
   NASHVILLE TENNESSEE TO 30 MILES SOUTH SOUTHWEST OF SELMA ALABAMA.
    FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
   OUTLINE UPDATE (WOUS64 KWNS WOU3).
   
   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.
   
   OTHER WATCH INFORMATION...CONTINUE...WW 89...WW 90...WW 91...[http://www.spc.noaa.gov/products/watch/ww0092.html WW
   92]...
   
   DISCUSSION...THUNDERSTORMS OVER MS WILL SPREAD EASTWARD INTO AL AND
   MIDDLE TN THROUGH THE MORNING...WITH SCATTERED SUPERCELLS POSSIBLE. 
   STORMS WILL BE INITIALLY ELEVATED...BUT RAPID DESTABILIZATION WILL
   INCREASE THE RISK OF TORNADOES BY LATE MORNING.  VERY STRONG SHEAR
   PROFILES ALSO SUPPORT A RISK OF SIGNIFICANT TORNADOES.
   
   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   500. MEAN STORM MOTION VECTOR 24035.

...HART

Sample PDS Severe t-storm watchEdit

these are rarely issued. here is a example


URGENT - IMMEDIATE BROADCAST REQUESTED
  SEVERE THUNDERSTORM WATCH NUMBER 539
  NWS STORM PREDICTION CENTER NORMAN OK
  910 AM CDT SUN JUN 15 2008
  
  THE NWS STORM PREDICTION CENTER HAS ISSUED A
  SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 
  
         MUCH OF CENTRAL AND EASTERN KANSAS
         MUCH OF NORTHERN MISSOURI
  
  EFFECTIVE THIS SUNDAY MORNING AND AFTERNOON FROM 910 AM UNTIL 500
  PM CDT.
  
  ...THIS IS A PARTICULARLY DANGEROUS SITUATION...
  
  EXTREMELY DAMAGING THUNDERSTORM WIND GUSTS TO 90 MPH...LARGE HAIL
  TO 3 INCHES IN DIAMETER...AND DANGEROUS LIGHTNING ARE POSSIBLE IN
  THESE AREAS.
  
  THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 70
  STATUTE MILES NORTH AND SOUTH OF A LINE FROM 60 MILES WEST OF
  SALINA KANSAS TO 40 MILES SOUTH SOUTHEAST OF KIRKSVILLE MISSOURI.
   FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
  OUTLINE UPDATE (WOUS64 KWNS WOU9).
  
  REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
  FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY
  DO PRODUCE TORNADOES.
  
  OTHER WATCH INFORMATION...CONTINUE...WW 536...WW 537...WW 538...
  
  DISCUSSION...RAPIDLY INCREASING SEVERE TSTMS SERN NEB WILL EVOLVE
  INTO A FORWARD PROPAGATING MCS ACCOMPANIED BY DAMAGING WIND AND
  LARGE HAIL AS IT MOVES SEWD INTO THE WATCH.  WITH VERY STEEP LAPSE
  RATES AND DRY MID LEVELS COUPLED WITH MUCAPE RISING TO AOA 3000
  J/KG...POTENTIAL EXISTS FOR VERY DAMAGING WIND GUSTS BY EARLY THIS
  AFTERNOON.
  
  AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
  TO 3 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 80
  KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 550. MEAN STORM
  MOTION VECTOR 31035.
  
  
  ...HALES

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