Transcript of An Investigation of Null-Event Severe Convective Watches in the WFO Sterling Forecast Area Lee...
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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