NWA 2006 PDS

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PDS Watches: How Dangerous are these “Particularly Dangerous Situations?” Joseph T. Schaefer NOAA/NWS Storm Prediction Center and Andrew R. Dean CIMMS/University of Oklahoma NOAA/NWS Storm Prediction Center

“Particularly Dangerous Situation” Watches: 

“Particularly Dangerous Situation” Watches Tornado Watches: Multiple strong or violent (F2 – F5 damage) events Severe Thunderstorm Watches: Long lived wind events (derechoes)

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SEL6 URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 776 NWS STORM PREDICTION CENTER NORMAN OK 510 PM CDT SAT SEP 16 2006 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF NORTHWEST IOWA CENTRAL AND SOUTHERN MINNESOTA EFFECTIVE THIS SATURDAY AFTERNOON AND SUNDAY MORNING FROM 510 PM UNTIL 100 AM 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. … PDS WATCH

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Frequency 1996-2005 total PDS Severe Thunderstorm Watches 5827 22 0.4% Tornado Watches 3058 216 7.1%

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Strong and Violent Tornadoes: Axis from Central Arkansas to Central Kentucky PDS Tornado Watches: Axis from Southern Arkansas to Northern Alabama

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Coverage of Watches by Reports (%) The decision to issue a watch is made before the decision to go PDS is made Portion of Watch impacted by Severe Weather is Greater when a Watch is PDS

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The Probability of a Tornado being in a Watch increases with its intensity Since only 7% of Tornado Watches are PDS Watches, more Strong Tornadoes occur in non-PDS Tornado Watches

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The percent of watches containing tornadoes, clustered tornadoes, strong tornadoes and clustered strong tornadoes increases from Severe thunderstorm Watches, to Tornado Watches, to PDS Tornado Watches.

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“Particularly Dangerous Situation” tornado watches portray an increased risk to life and property from strong and violent tornadoes All though tornado watches are classified as SEV, PDS SEV, TOR, and PDS TOR, there is a continuum of risk associated with watches … the tornado risk in a SEV watch is not zero, and the threat for non-tornadic severe events in TOR watches can be high. INFERENCES

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Storm Prediction Center