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

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.”

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

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

Motivation

Methodology Assembled a database of convective watches issued in the WFO Sterling CWA from 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

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

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

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

500-millibar variablesComparisons Null eventsVery active events

500-millibar variables

700-millibar variables

850-millibar variables

925-millibar variables

Surface variables

Precipitable Water

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

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

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

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

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

Local Domain 500 & 850 mb Geopotential Height Composites