W. Scott Lincoln, GISP Senior Hydrologist, Cartographer, Environmental Scientist M.S./B.S. Environmental Science
By: ScottLincoln, 4:34 AM GMT on November 29, 2013
We've heard it before: "you won't survive this tornado
if you don't get below ground." We've also seen the controversy and
further exaggerations (eg. "drive south out of town") from those same
people making the claim. Violent (EF4/5) tornadoes, accounting for only ~2% of the total, cause approximately 67% of the fatalities (Ashley, 2007). By the time damage reaches EF4 severity to a single family residence, no walls are left completely standing, with maybe a few small "pockets" remaining in the rubble. At EF5 damage, all the rubble is swept clean away (interior hallways, bathrooms, and all), leaving nothing behind that isn't reinforced such as a safe room. After seeing pictures of the aftermath of violent tornadoes, it can lead one to wonder how anyone could survive such damage. From that, a myth is born... you won't survive this tornado if you don't get below ground or evacuate.
The science, however, does not agree.
The peer-reviewed literature already contains
some studies discussing fatality rates from violent tornadoes. In “Low-Level Winds in Tornadoes and Potential Catastrophic Tornado Impacts in Urban Areas” (Wurman et al.,2007), the authors estimated the mortality rate at 10% for persons sheltering in a residence that experiences a violent tornado strike. The authors then used this crude estimate as a basis for a hypothetical tornado which moved through densely populated
sections of Chicago and resulted in 13,000 or more fatalities. In a comment to Wurman et al. (2007), Brooks et al. (2008) questioned the core assumptions used in the former study, in particular the 10% mortality rate and the uniformly large area impacted by EF4/5 winds in the hypothetical tornado. Using data from the 1998 Brimingham, AL, F5 tornado and the 1999 Moore/OKC, OK, F5 tornado, the authors estimated the the mortality rate from a violent tornado strike as between 0.1-1.9% (Table, below) depending on the grouping of building damage.
2011 Joplin EF5 Tornado
In May of 2011, the deadliest tornado in the period of modern tornado
records hit Joplin, MO, causing 161 fatalities (including persons
seriously injured that died at a later date). Although tornado warnings
had been in effect for the Joplin area for 20-30 minutes prior to the
tornado, the actual large and violent tornado that hit Joplin formed
right on the outskirts of the city, rapidly intensified, and was mostly
wrapped in curtains of rain. Earlier warnings for tornadoes impacting
areas to the north of Joplin proper precluded the sheltering of most
impacted residents, and the formation just on the edge of town limited
live television and radio coverage as known during several historic
violent tornado strikes to major cities.
Several studies and assessments have already reported on the Joplin tornado. These include the NWS WFO Springfield damage survey and the NWS Central Region Service Assessment. The American Society of Civil Engineers did a comprehensive study of construction techniques of affected structures; the Structural Engineers Association of Kansas and Missouri did a study of their own. Another study, "Technical Investigation of the May 22, 2011, Tornado in Joplin, Missouri" (NIST, 2013)
was released by the National Institude for Standards and Technology
(NIST) in early November, 2013, for public comment. Of particular
interest was the summary of interviews, media reports, and other
documents dealing with the fatalities from the tornado. The location of
almost every fatality was documented. When possible, an estimate of
the number of survivors at a given location was also indicated, which
allows us to conclude on the mortality rate for different structures.
In Brooks et al. (2008), the fatality rate was estimated by dividing the
number of fatalities by an estimate of total persons residing in the
number of homes impacted. Using demographic data, the authors estimated
2.7 persons per detached home. For example, in the Moore/OKC tornado,
17 fatalities were listed as occurring in the F4/5 damage area which
also destroyed 334 homes (first row of Table 1). 17 fatalities / (334
homes) x (2.7 persons per home) yields a fatality rate of 1.9%. We can also apply this methodology to the Joplin tornado.
As reported by the NIST report, there were 161 fatalities in the Joplin
tornado, of these 58% (93) were in areas impacted by EF4/5 winds. It
was also noted that 2063 structures classified as "demolished"
(equivalent of approximately EF3 winds or higher). A better estimate of
residences impacted by EF4/5 winds can be found in "Damage Survey of
the Joplin Tornado: 22 May 2011" (Marshall et al., 2012).
The authors surveyed over 7000 residences that were damaged by winds
ranging from EF0 to EF5 strength; of those, they concluded that 511
sustained EF4 damage and 22 sustained EF5 damage (533 total). Using the
Brooks methodology, the 93 fatalities yields a mortality rate of 6.5%,
substantially higher than their summary (but still low). As mentioned
by Blumenfeld (2008), this method is rather crude by assuming that all
persons impacted were in detached homes, and also assumes that the
average home occupancy applies to all areas impacted in Joplin.
Detailed information from the NIST report allows us to break down the fatalities by location. In the table below, the location of each fatality is
listed, along with information on total building occupancy, number of
injuries, and estimate wind speed, when available.
In the locations listed in the above, there were 161 fatalities at 28
different locations. The summary also includes the generalized
locations of "outside," "vehicle," and "single family residences" and 10
locations where no fatalities occurred. At these same locations, there
were 41 known serious injuries. The total occupancy of each location
was estimated as being at least 571 (actual occupancy is very
likely higher, but not enough information was provided to provide a
reasonable estimate). This suggests that the chance of dying and the
chance of being seriously injured or dying in the Joplin tornado was less than
28.2% (161/571) and 35.4% (202/571), respectively, for all locations.
These values are subject to large uncertainty because not all buildings
had estimates for occupancy available. Looking only at locations with
both a number of fatalities and an estimate of total occupancy,
the estimated mortality rate would be 10.3%; for only areas that
experienced EF4/5 winds, 16.8%. The sample size for individual shelter
classifications was substantially smaller, but the trend was toward much
lower fatality rates for persons classified as "safest" compared to
Past studies have suggested that the majority of persons will survive a
direct strike by the strongest portion of a violent tornado if they
follow the standard tornado safety advice. The chance of survival
increases as the quality of shelter improves. Past tornadoes have
demonstrated that being outdoors or in vehicles during a tornado strike
is very dangerous. This is especially true when roadways are busy, such
as during rush hour periods or during an evacuation. Precise forecasts
of a tornado's exact track and intensity or simply not possible, even
on very short lead times when evacuations are impractical. Sheltering-in-place remains the best options in the overwhelming majority of tornado threats.
Review of information on fatalities that occurred from the Joplin, MO, tornado
of 2011 seems to corroborate conclusions from previous studies.
Complicating this type of analysis, however, is the lack of data on most
of the detached homes impacted by the tornado, which dwarf commercial
buildings in number and likely had a higher occupancy than the other
locations listed. Virtually no information is available to indicate the
total number of persons in the various commercial districts impacted
because we know little about structures where fatalities did not
occur. One estimate quoted in the NIST report suggested that over
20,000 persons may have been in the area impacted by the tornado in
Joplin. Although limited analysis can be done with that information,
the fact that only 161 persons were killed by a tornado with maximum
intensity of EF5 in an urban area with 10s of thousands of persons
present strongly suggests that the chance of survival in a violent
tornado is high, even in areas where no basement is available.
See the expanded blog post for Figures, Tables, and References
Important Blog Note
Information presented in a blog post is accurate to the best of my knowledge but has not gone through a rigorous review process typical of articles in a peer reviewed journal. Technical information and or
analysis provided is not necessarily intended to supersede
conclusions made in the peer reviewed literature. As specified by the NOAA Scientific Integrity Policy, comments I post online through my Weather Underground account, personal website, or any
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professional opinion, but may not reflect the official opinion of NOAA
and National Weather Service.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.