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An Evaluation of Flash Flood Warnings and Flash Flood Reports for South Central TexasKevin Barrett
Texas State University-San MarcosAbstract
Flash flood warnings issued by the Austin/San Antonio forecast office of the National Weather Service from 1986 to 2006 were spatially evaluated to find possible trends related to population growth and change in government severe weather warning policy. It was found that almost three times the number of warnings were issued in the five year time period of 2001 to 2006 compared to the decadal time period of 1986 to 1995, with the highest number of warnings trending to center on urban areas and the location of the National Weather Service office. When the warnings are compared to actual flash flood reports, a strong statistical correlation exists suggesting a low false alarm rate for the area. Results indicate that National Weather Service modernization, technological upgrades in flood monitoring, public awareness of flood dangers and population growth may be factors contributing to the spatial distribution of warnings.
AP Photo/ Waco Tribune Herald, Jerry Larson
Comparison R² P-ValueWarnings 86-95 & Population 0.39 0.025
Warnings 96-05 & Population 0.17 0.344
Reports 86-95 & Population 0.63 < 0.001
Reports 96-05 & Population 0.33 0.061
Warnings 95-00 & Population 0.16 0.374
Warnings 01-06 & Population 0.19 0.290
Reports 95-00 & Population 0.17 0.344
Reports 01-06 & Population 0.43 0.013
Warnings 86-95 & Reports 86-95 0.82 < 0.001
Warnings 96-05 & Reports 96-05 0.83 < 0.001
Warnings 95-00 & Reports 95-00 0.85 < 0.001
Warnings 01-06 & Reports 01-06 0.73 < 0.001
Comparison Bexar County R² P-Value
Warnings 93-06 & Reports 93-06 0.87 < 0.001
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Warnings 1986-2006 Bexar County
Single County Analysis
National Weather Service Modernization
The above map comparison illustrates the drastic increase in the number of flash flood warnings issued after the National Weather Service modernization effort.
The most populated county in the study area is Bexar. The number of warnings for this county increased drastically after the National Weather Service modernization effort, starting in 1997. In 2002 and 2004, significant rain events caused major flooding for the county. The county shows a high statistical correlation for issued warnings and flash flood reports for the time period of 1993-2006 (1993 is the first year that accurate flash flood reports are available).
Statistical Analysis
Statistical comparisons show a very high correlation between warnings and reports.
Graph Comparison
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1986-1995 1996-2005
Warnings
More than four times the number of warnings were issued from 1996-2005 compared to 1986-1995. There is also an increase in the number of warnings issued between 2001-2006 compared to 1995-2000.
Post National Weather Service Modernization
The above maps compare warnings and reports for time periods after the National Weather Service modernization. The majority of warnings and reports are from the more populated counties in the study area.
Findings•National Weather Service modernization corresponds to an overall increase in the
number of flash flood warnings issued.
•The strong statistical correlation between flash flood warnings issued and flash
flood reports suggest a low false alarm rate.
•The spatial pattern of flash flood warnings suggests the warning forecast favor
counties with higher populations.
•Extreme rainfall events in 2002 and 2004 exaggerate the findings for the later
time period of this analysis.
Further Study•Compare flash flood warnings and reports to physiographic features known to have an
influence on flash flooding.
•Compare the spatial distribution of flood monitoring stations to flash flood warnings and
reports.
•Determine exact false alarm rates for each county in the study area.
•Test the theory that the population of certain counties can tolerate a higher number of
false alarms.
•Compare correlations of population demographics to flash flood warnings and reports.
•Build a warning systems computer model that can test various influences on flash flood
warning forecast decisions.
PurposeThe purpose of this study is to spatially analyze flash flood warnings issued
by the National Weather Service office in New Braunfels, Texas to determine if there are patterns related to population and government severe weather warning policy.
Study Area
The study area is the 33 county warning area (CWA) covered by the National Weather Service Office located at New Braunfels, Texas. The most populated counties are Bexar (1,392,931) and Travis (812,280). Population data are from the United States Census Bureau 2000 census.
San Antonio
Austin
Dallas
Houston
All flash flood warning data from the National Weather Service.All flash flood report data from the National Climatic Data Center. Contact: [email protected]
© 2009
