Recent Trends in Tornado Statistics and Associated

Warnings

Perceptions (and Misperceptions)

Alan GerardNWS Jackson

Sixth Annual Southeast Severe Storms SymposiumMississippi State University

Premise

• 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

• 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?

• 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.)

Trends

Tornado Fatalities

934

1181

590

419

577

110

675

883

258200 209

073 75 44 48 36

00

200

400

600

800

1000

1200

1400

1956-65 1966-75 1976-85 1986-95 1996-05 2006-07

Decade

# o

f Dea

ths Tornado Fatalities

Violent Tornado Fatalities

% of Fatalities from F4+ Tors

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%!

Are Most “Significant” Killer Tornadoes Violent?

Years < F4 tornado w/ 10+ fatalities

F4+ tornado w/ 10+ fatalities

1950-1975 5 67

1976-1985 3 9

1986-1995 0 6

1996-2005 5 6

2006-Current 2 0

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

• 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

• 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

• 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.

Tornado Fatalities

934

1181

590

419

577

110

675

883

258200 209

073 75 44 48 36

00

200

400

600

800

1000

1200

1400

1956-65 1966-75 1976-85 1986-95 1996-05 2006-07

Decade

# o

f Dea

ths

Tornado Fatalities

Violent Tornado Fatalities

% of Fatalities from F4+ Tors

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

Percentage of people residing in mobile homes in U.S. Note that rate

increased by sixfold in Southeast U.S. since 1960 to 1990.

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

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

• 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

• 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.

• Core area looked at from central Plains into southeast U.S.

• 2000-2006 data, except:– HUN/BMX only

since HUN office opened (2003).

– Excluded JAN 2005 (polygon warning test)

• Findings– On average, a

county can expect just under two tornado warnings per year

Number of tornado warnings per county per yr

Mean: 1.93

Median: 1.68

• 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

• 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?

• 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• 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

• 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

• 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 #3• 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!

• 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

• Barnes, et. al 2006, “False Alarms and Close Calls: A Conceptual Model of Warning Accuracy” WAF paper on the web at: http://www.cimms.ou.edu/~schultz/papers/barnesetal.pdf

• 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