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Recent Trends in Tornado Statistics and Associated Warnings. Perceptions (and Misperceptions) Alan Gerard NWS Jackson Sixth Annual Southeast Severe Storms Symposium Mississippi State University. Premise.

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Recent trends in tornado statistics and associated warnings

Recent Trends in Tornado Statistics and Associated Warnings

Perceptions (and Misperceptions)

Alan Gerard

NWS Jackson

Sixth Annual Southeast Severe Storms Symposium

Mississippi State University


  • There are many “perceptions” in the public and in the meteorological community regarding tornadoes

  • My question:

    • Do recent statistical trends support these often long held beliefs?

      Important note: These analyses are informal. No detailed formal statistical analyses have been conducted.

Perception 1
Perception #1

  • The large majority of fatalities associated with tornadoes are caused by violent tornadoes.

    • This statement can be found in many books and all over the web.

    • The most commonly quoted statistic is that 67% of fatalities occur with F4-F5 tornadoes.

From Tornado Project website

Do recent trends support this
Do Recent Trends Support This?

  • No!

  • During the most recently completed decade, only 39% of the tornado related fatalities were caused by F4+ tornadoes.

    • That number decreases to 14% for 2001-2005.

    • Since January 1 2006, there have been 110 tornado related fatalities nationwide. Only 10 were caused by a violent (F4+) tornado. (Note: This figure was 0 until Miller’s Ferry and Enterprise, AL tornadoes were upgraded to EF-4 this week.)

Additional thoughts
Additional Thoughts

  • Fujita scale was operationally implemented in the NWS around the mid-1970s

    • Numbers since that time have never supported a value over 50% for fatalities associated with violent tornadoes

    • Statistic likely based on data including the ‘50s and ‘60s, which were “post-analyzed”

  • A value closer to 40% is probably more reasonable

    • Recent trends are even lower?! 2001 to today: 11%!

Possible explanations
Possible Explanations

  • “Post-analysis” ratings for the 1950s and ’60s were based more on fatalities than damage descriptions

  • Ratings have trended more “conservative” as more has been learned about relationship between wind and damage

  • Impact of QRT and E-F Scale??

Perception 2
Perception #2

  • The number of tornado related fatalities is decreasing due to improved warnings and technology (e.g., Doppler radar).

  • Do recent statistical trends support this perception?

  • Maybe.

Long term trends
Long Term Trends

  • The very long term trend is unmistakable.

    • Tornado related fatalities have decreased dramatically since around 1925.

  • Attributable to education, preparedness, warning program, etc.

U.S. tornado deaths per million of people since 1875. From Brooks and Doswell 2001

Short term trends
Short Term Trends

  • In the shorter term, things are not as clear.

    • Very little trend since 1975.

    • Annual number of fatalities since 1996 averaging around 55.

      • Compared to 42 in 1986-95 period.

Possible explanations1
Possible Explanations

  • Some obvious possibilities

    • 1996-2005 a more active period, so higher number of fatalities in spite of better warnings.

      • 22 tornadoes with 5+ fatalities compared to 12 from 1986 to 1995.

    • Population has also increased.

  • Another potential explanation:

    • Mobile homes

Mobile homes
Mobile Homes

Tornado death rate extrapolated out to 2020. Solid line is straight extrapolation – dashed line is based on a model incorporating mobile home residential trends. From Brooks & Doswell 2001

Percentage of people residing in mobile homes in U.S. Note that rate increased by sixfold in Southeast U.S. since 1960 to 1990.

Mobile homes1
Mobile Homes

There remains one particularly vulnerable group of people in the U.S., residents of mobile homes. The rate of death is relatively close to the pre-1925 values in the U.S. The increase in use of mobile homes for housing has meant that they are an increasingly large component of the overall death toll. Half of all fatalities from 1996 to 2000 occurred in mobile homes, more than twice the fraction twenty years earlier. It seems likely that the fraction of deaths in mobile homes will continue to increase. Efforts to improve safety practices and communication of forecast and warning information for this group will be especially important in the future if the long-term decrease in tornado fatalities in the U.S. is going to continue.

From Brooks and Doswell 2001

Perception 3
Perception #3

  • The false alarm rate for tornado warnings is so high that it could cause a “cry wolf” desensitization among the public.

  • Do recent statistical trends support this perception?

One possible measure
One Possible Measure

  • How many tornado warnings are people actually being put under?

    • Look at a core region of the country most susceptible to tornadoes.

    • Calculate how many times they are being put under a tornado warning in a given year.

    • Number of times people have to take action will give a subjective idea of how sensitive they may be to false alarms.

Recent trends in tornado statistics and associated warnings

Number of tornado warnings per county per yr

Mean: 1.93

Median: 1.68

Recent trends in tornado statistics and associated warnings

  • Further broke down area into “Plains Core” and “Southeast Core”

    • Did not include EAX, SGF or LZK as did not easily fit into one or the other

  • Expect on average about one more tornado warning per county per year in Southeast

    • Likely due to more extended season and much less favorable spotting environment

Plains Core

Southeast Core

Mean # of TOR Warnings

Plains Core 1.38

Southeast Core 2.64

Stats summary
Stats Summary “Southeast Core”

  • If you live in the most tornado prone parts of the county, expect about two tornado warnings per year for your spot.

    • A little less in the Plains, a little more in the Southeast.

  • If you want to get statistical with this, this boils down to:

    • 0.5% of days on which you will have a tornado warning

    • Based on an average length of 45 min per warning, this is 90 minutes out of 525,600 min in a year (.0017%)

Crying wolf
Crying Wolf? “Southeast Core”

  • Still important to note that about 75% of these warnings are false alarms.

    • However, even for those warnings which verify, they will be “false alarms” for the vast number of people warned.

  • Question becomes: overall, are two tornado warnings per year, most of which will be false alarms, enough to cause people to begin to ignore the warnings?

    • This is not a meteorological issue, but rather a sociological issue.

Studies “Southeast Core”

  • Sociological studies on this issue are relatively limited, but those that have been done suggest that the “cry wolf” possibility may be overstated:

    • Barnes’ (2006) study of false alarms with flash flood warnings in damaging flash flood prone areas indicated tremendous “forgiveness” in the public for false alarms

      • People would rather have more warnings with the possibility of a false alarm or close call

      • False alarms do not appear to reduce confidence in the warning process

    • Important part of Barnes’ results was that the false alarm/warned event concept is too simplistic

      • Many false alarms are “near misses,” e.g., tornadoes that pass nearby, funnel clouds, significant severe storms that aren’t tornadic, etc. Making public aware that a “near miss” occurred further strengthens the warning process and decreases the possibility of “cry wolf” phenomenon.

Studies “Southeast Core”

  • Dow and Cutter (1998) looked at hurricane evacuations and relations to false alarms

    • Public did not hesitate to evacuate even after recent false alarms

    • Reported they would be willing to evacuate again even after false alarms

    • Important: These findings are for the large majority of people. There is a small group (~10 to 15%) that will begin to alter their decision making due to false alarms.

Studies “Southeast Core”

  • From Barnes, et al, 2006

    • Conventional wisdom is that overwarning reduces the public’s willingness to respond to future warnings.

    • “In contrast, more recent research indicates the public may have a high tolerance for false alarms…Evidence for the cry-wolf effect in natural-hazards research has not been forthcoming.”

    • Looked at approximately 10 studies on the warning process – none indicated that false alarms had any significant “cry wolf” impact.

Perception 31
Perception #3 “Southeast Core”

  • The false alarm rate for tornado warnings is so high that it could cause a “cry wolf” scenario among the public.

  • Do recent statistical trends support this perception?

    Probably not for the large majority of people, based on number of warnings and sociological studies – but additional research is needed.

Important caveats
Important Caveats! “Southeast Core”

  • Even if the “crying wolf” scenario is not as feared as we might initially think, this is not “carte blanche” to issue low probability warnings!

    • Social studies are based on meteorologists issuing warnings based on the best science and technology available.

    • It is still incumbent upon us as a community to try to reduce false alarms as much as the science and technology will allow (without reducing the probability of detection).

  • Impact of storm based warnings?

References “Southeast Core”

  • Barnes, et. al 2006, “False Alarms and Close Calls: A Conceptual Model of Warning Accuracy” WAF paper on the web at:

  • Brooks and Doswell 2001, “A Brief History of Deaths from Tornadoes in the United States”

  • Barnes 2006, “False Alarms: Warning Research Project Findings and Warning Accuracy Conceptual Model”

  • Dow and Cutter 1998, “Cry Wolf: Repeat Response to Hurricane Evacuation Orders”

  • Hart, SVRPlot software for tornado statistics

  • Storm Prediction Center website for recent (2002-7) tornado statistics