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An Investigation of Null-Event Severe Convective Watches in the WFO Sterling Forecast Area Lee Picard Student Volunteer, WFO LWX University of Miami, Coral Gables, FL Matthew Kramar Senior Forecaster, WFO LWX

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Definitions Severe thunderstorm watch o From SPC: outlines an area where an organized episode of hail 1 inch diameter or larger and/or damaging thunderstorm winds are expected during a three to eight hour period. Tornado watch o From SPC: includes the large hail and damaging wind threats, as well as the possibility of multiple tornadoes.

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Definitions Null watch o A watch, either severe thunderstorm or tornado, where the expected coverage and intensity of thunderstorms does not materialize

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Motivation Provide guidance to forecasters as to synoptic patterns that would typically result in a null watch Reduce the frequency of null watches and thereby decrease public desensitization

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Motivation

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Methodology Assembled a database of convective watches issued in the WFO Sterling CWA from 2005-10 Sorted through archived products to determine if severe weather occurred during watches in CWA Eliminated null cases from list subject to: o Outside typical severe season (May-Sept.) o Small part of CWA affected o Thunderstorms occurred, but below severe threshold

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Methodology Constructed synoptic composites using data from the NCEP/NCAR and NARR Reanalyses Used 6-hour synoptic time immediately preceding watch initiation Variables: Geopotential height Air temperature Vector wind Zonal wind Meridional wind Relative humidity Precipitable water Levels: 100 mb 250 mb 500 mb 700 mb 850 mb 925 mb Surface

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Methodology Chose 18 cases of very active severe convective weather watches from 2005-10 Created a second set of composites to use as a baseline to contrast with null cases

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Methodology Synoptic composites o NCEP/NCAR Reanalysis o Lower resolution o 2.5 x 2.5 lat./lon. grid o 6-hourly data o NCEP North American Regional Reanalysis (NARR) o Higher resolution o 1 x 1 lat./lon. grid o 3-hourly data

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500-millibar variablesComparisons Null eventsVery active events

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500-millibar variables

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700-millibar variables

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850-millibar variables

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925-millibar variables

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Surface variables

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Precipitable Water

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Pattern Analogs Analog searches provided by Greg Carbin (SPC) o Dates since 1979 with the patterns closest to those of null watches o RMS errors for geopotential height at 500 and 850 millibars o Grid difference for precipitable water Selected dates that occurred as matches to both the 500- and 850-mb patterns Ruled out dates before 1990 and those dissimilar to composite patterns based on manual inspection Determined if severe weather occurred on analog dates

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Pattern Analogs Of 6 national domain matches, three produced severe weather Of 6 local domain matches, zero resulted in severe weather National matches could be tied more heavily to the ridge/trough patterns across the country and could undervaluate local details that result in null events

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Results Null events Upper level o Less amplified ridge and trough o Relative westward wind maximum (southern Ontario) o Stronger wind maximum Severe events Upper level o More amplified ridge and trough o Relative eastward wind maximum (upstate NY/Lake Ontario) o Weaker wind maximum o Dipole in meridional wind

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Results Null events Middle level o Less amplified ridge and trough o Less amplified 700-mb temperatures o Stronger wind maximum o Additional wind maximum over Atlantic Ocean Severe events Middle level o More amplified ridge and trough o More amplified 700-mb temperatures o Weaker wind maximum

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Results Null events Lower level o Relative northward surface low pressure center (northern Quebec) o Strong wind maximum off northeast seaboard o Relative warmer temperatures o Relative higher PW Severe events Lower level o Relative southward surface low pressure located (southern Ontario) o Weak wind maximum off northeast seaboard o Relative cooler temperatures o Relative lower PW

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Local Domain 500 & 850 mb Geopotential Height Composites