a climatology of polygon warnings in new england chris kimble nws gray, me
TRANSCRIPT
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A Climatology of Polygon Warnings in New England
Chris KimbleNWS Gray, ME
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Purpose
• Use GIS software to map a climatology of severe weather– Where are the severe weather hot spots?– First in GYX Forecast Area– Expanded to all of New England
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Problem
• Reports of severe weather do not fullyrepresent the spatial extent of severe weather
• Population density a significant factor
• Warnings issued regardless of population
2014 Severe WX ReportsWind Damage/GustHail
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Methodology
• Obtain polygon warnings from 2008-2014– http://mesonet.agron.iastate.edu/lsr
• Use GIS tool “fishnet” to create grid boxes– 0.25 degree spacing
• Count number of warnings per box– Must repeat for each CWA
• Repeat for SVR, TOR, FFW, SMW
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Methodology
SVR WarningsGYX - 2008
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Methodology
GYX “Fishnet”0.25 degree
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Methodology
SVR WarningsGYX - 2008
Min – 0
Max – 18
1 18
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Methodology
SVR WarningsGYX – 2008 to 2014
Min < 3
Max – 64
Fewer at upstream CWA borders
1 64
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Methodology
• Are observed trends more a result of climatology or artifacts of CWA boundaries?– MUST EXPAND!– All of New England: 2008 to 2014• Repeat process for each CWA• Stitch it back together for regional perspective
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Methodology
SVR WarningsNew England2008 to 2014
Min < 5
Max > 95
3 100
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Limitations
• Polygon warnings only go back to 2008• Warnings are not entirely storm based, some
political considerations are involved– County boundaries• Forecaster may consciously choose which to include
– CWA borders• Forecaster may NEVER issue warnings across CWA lines• Significant effect on frequency of warnings
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Results: SVR WarningsSVR WarningsNew England2008 to 2014
More warnings in south and west
storm motion
surfa
ce w
ind
Fewer warnings in east and coast
3 100
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Results: SVR Warnings
• General reduction in frequency west to east• General increase in frequency north to south• Coastal influences cause local minimums• Most frequent severe thunderstorm warnings:– Western Massachusetts
• Least frequent severe thunderstorm warnings:– Cape Cod and islands– Downeast Maine
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Results: FFW WarningsFFW WarningsNew England2008 to 2014
Max > 25
Min – 0
Max in hilly terrain
23
20
18
13
18 Max in inland urban areas
3 28
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Results: FFW WarningsNew EnglandTopographyFFW WarningsNew England2008 to 2014
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Results: FFW Warnings
• Northern and Western New England:– More FFW in mountainous areas
• Southern and Eastern New England– More FFW in urban areas
• Minimum in Cape Cod, islands, and Midcoast• Some CWA disparity– OKX and ALY issue more frequent FFW
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Results: TOR WarningsTOR WarningsNew England 2008 to 2014
Max – 8
1 8
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Results: TOR Warnings
• Rarity makes climatology difficult• Maximum in south central Massachusetts
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CWA Boundaries
• Local minimum at upstream CWA border– Only upstream border suggests storm motion
factor– Noticeable in all polygon warnings• Especially higher frequency warnings (SVR)
• Typical warning process results in overwarning– Overlapping polygons– Premature storm death
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Overlapping PolygonsCWA Border
Which area never has overlap?
Upstream CWA border!
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Premature Storm Death
Overwarned area
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Fortunate Storm Death
No warning issued
Upstream CWA Downstream CWA
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CWA Boundaries
• Upstream CWA border minimizes overwarning– Areas downstream of a CWA border have fewer
false alarms• At the possible expense of lead time
• Within CWA overwarning occurs more often– Overlap– Premature storm death
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Solutions
• Upstream CWA issues warning into downstream CWA– Minimizes the effect of CWA borders– Not realistic in current era
• Minimize overwarning within CWA– Avoid large warnings in pulse environments– Make use of county borders• Downstream County Removal (Kevin Laws – NWS BMX)• http://
www.wdtb.noaa.gov/courses/SOTM/001-Nov14/player.html
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Downstream County RemovalCWA Border
Overlap is minimizedPremature Storm Death minimized
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Downstream County Removal
• Makes strategic use of county borders– Avoids overlap– Reduces premature storm death– Avoids alerting counties multiple times• EAS / NOAA Weather Radio• NAWAS
– Reduces overall false alarm area• Warning duration should be long enough for
storm to fully exit the county
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Conclusions
• Polygon warning frequency can provide insights into climatology– Reduces population bias– CWA boundaries cause artificial minimums• Result of natural overwarning elsewhere• Can be reduced by new warning techniques
• Severe thunderstorms most common away from coastal influences
• Flash floods most common in hills and cities
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Future Work
• Expand to broader area– Do observed trends continue to appear?– Do new trends show up?• Maximum within ideal radar range?
• Statistical adjustments to blend CWA borders– More representative climatology
• Effects of county boundaries– EAS frequency
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Acknowledgements/Questions?
• Justin Arnott – SOO at GYX• Mike Kistner at GYX
• Questions?