PDS Watches:How Dangerous are these “Particularly Dangerous

Situations?”Joseph T. SchaeferNOAA/NWS Storm Prediction Center

and Andrew R. Dean

CIMMS/University of Oklahoma NOAA/NWS Storm Prediction Center

“Particularly Dangerous Situation” Watches

Tornado Watches:Multiple strong or violent

(F2 – F5 damage) events

Severe Thunderstorm Watches: Long lived wind events (derechoes)

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

Fraction of Tor Reports and Harm by F-Scale 1996-2005

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F1+ F2+ F3+ F4+

F-Scale

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TorFracDeaths

InjuriesDamage

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

Strong and Violent Tornadoes: Axis from Central Arkansas to Central Kentucky

PDS Tornado Watches: Axis from Southern Arkansas to Northern Alabama

TYPE PDS? # RepCov TorCov F2+Cov F4+Cov WndCov Gust>64 kts Cov

SEV N 5805 34.3 2.0 0.2 0.01 22.4 2.6

SEV Y 22 60.5 3.2 0 0 54.8 9.1

SEV ALL 5827 34.4 2.0 0.2 0.01 22.6 2.6

TOR N 2842 36.6 8.4 1.6 0.12 21.6 3.0

TOR Y 216 52.0 17.0 5.9 0.56 31.2 5.4

TOR ALL 3058 38.0 9.2 2.0 0.16 22.5 3.2

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

PODs by F-Scale and Watch Type (1996-2005)

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InWatch

InTor

nonPDSTOR

PDSTOR

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

Report Occurrence in Watches 1996-2005

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>=1 tor >= 2 tor >= 3 tor >= 1 F2+ >= 2 F2+

# of reports in watch

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PDS TOR

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.

“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

02/02/04

“Where America’s Weather and Climate Services Begin”

Storm Prediction Center