A Climatology of Polygon Warnings in New England

Chris KimbleNWS Gray, ME

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

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

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

Methodology

SVR WarningsGYX - 2008

Methodology

GYX “Fishnet”0.25 degree

Methodology

SVR WarningsGYX - 2008

Min – 0

Max – 18

1 18

Methodology

SVR WarningsGYX – 2008 to 2014

Min < 3

Max – 64

Fewer at upstream CWA borders

1 64

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

Methodology

SVR WarningsNew England2008 to 2014

Min < 5

Max > 95

3 100

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

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

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

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

Results: FFW WarningsNew EnglandTopographyFFW WarningsNew England2008 to 2014

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

Results: TOR WarningsTOR WarningsNew England 2008 to 2014

Max – 8

1 8

Results: TOR Warnings

• Rarity makes climatology difficult• Maximum in south central Massachusetts

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

Overlapping PolygonsCWA Border

Which area never has overlap?

Upstream CWA border!

Premature Storm Death

Overwarned area

Fortunate Storm Death

No warning issued

Upstream CWA Downstream CWA

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

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

Downstream County RemovalCWA Border

Overlap is minimizedPremature Storm Death minimized

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

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

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

Acknowledgements/Questions?

• Justin Arnott – SOO at GYX• Mike Kistner at GYX

• Questions?