Particularly dangerous situation

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.cite web |url=http://www.spc.noaa.gov/faq |title=Storm Prediction Center Frequently Asked Questions (FAQ) |date=2004-04-18 |accessdate=2008-05-23 |author=NOAA]

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.cite web |author=Dean, Andrew R. |url=http://ams.confex.com/ams/23SLS/techprogram/paper_115252.htm |title=PDS watches: how dangerous are these "particularly dangerous situations?" (2006 - 23SLS_23sls) |publisher=American Meteorological Society |date=2006-11-07 |accessdate=2008-05-23] 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.cite journal |last=Lewis |first=John |authorlink= |title=A Forecaster's Story: Robert H. Johns |journal=Electronic Journal of Severe Storms Meteorology |volume=2 |issue=7 |date=2007-11-03 |url=http://www.ejssm.org/ojs/index.php/ejssm/article/view/29 ]

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.

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).

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.

Sample PDS Tornado Watch

This watch was issued for the tornado outbreak the day after the Greensburg, Kansas tornado (which clearly verified). 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

ample PDS Severe Thunderstorm Watch

Although, as mentioned, PDS Severe Thunderstorm Watches are very rare, they are issued when the tornado threat is low and when the forecast wind gusts are very high. Here is one such PDS Severe Thunderstorm Watch (which did not verify):

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

References

External links

* [http://www.spc.noaa.gov NOAA-NWS Storm Prediction Center]