Technical Investigation

of the May 22, 2011,

Tornado in Joplin, MO

December 10, 2013

NCST Advisory 

Committee Meeting

Tornado Hazard 

Characteristics

Franklin T. Lombardo

Investigation Task Co-Leader

Research Engineer, Engineering Laboratory
Objective #1

• Determine the tornado hazard characteristics and 

associated wind fields in the context of historical 

data 

Presentation Outline

1. Objective Summary

2. Findings 

2
1. Objective Summary

3
Summary: Meteorological Conditions

• Conditions favorable for the development of tornado-producing 

thunderstorms in the Joplin area were well forecast. 

• Two tornado warnings issued that included all or part of Joplin. 

Tornado touched down at 5:34 CDT and was on ground for 15 

minutes in Joplin.

• Radar was able to identify possible tornado, but the distance 

from the storm precluded detailed near-surface information

4

Tornado affects 

Joplin East 

Middle School

Tornado 

affects 

Joplin 

High 

School

Second 3-minute siren 

sounded as tornado 

enters the City of Joplin

Initial tornado 

touchdown 

(Spotter reports)

Second tornado 

warning issued 

(TO.W 31)

Initial 3-minute 

siren sounded

First tornado 

warning issued 

(TO.W 30)

Moderate risk of severe 

storms issued by SPC

Tornado watch 

issued by SPC2:00

3:00

4:00

5:10

5:20

5:30

5:40

5:50

1:30 pm

3:00 pm

5:09 pm

5:11 pm

5:17 pm 5:34 pm

5:38 pm

5:42 pm

5:48 pm

May 22, 2011

Watches and warnings timeline

Tornado timeline

Siren timeline
Summary: Near-Surface Wind Environment

5

• Indirect Method: EF-scale

– Rated NIST-surveyed structures using EF-scale and compared 

with ratings of others

– Variability in ratings increase with large structures

© 2011 GeoEye.

Used with permission.
Summary: Near-Surface Wind Environment

6

• Indirect Method: EF-scale (cont.)

– Random sample of 10 residential structures within each general 

damage class rated

• Samples were assumed to be 

representative of the entire 

population of structures in the 

damage class.

Data Source: Pictometry®. 

Used with permission.

Analysis by NIST
Summary: Near-Surface Wind Environment

7

• Indirect Method: Tornado Wind Field Model (Rankine vortex)

– Thousands of trees felled in Joplin

– Initialize tornado wind field model in simulations to “re-create” observed 

tree fall

– Grid system was created throughout Joplin and wind field model 

translated through it (250 ft or 80 m spacing)
Summary: Near-Surface Wind Environment

8

• Indirect Method: Tornado Wind Field Model (cont.)

• Model Assumptions/Limitations:

– Average critical tree fall wind speed (see range below) used due to relatively unknown 

variation in critical speed between tree types, urban v. rural, tree spacing, age, etc..

– Rankine vortex (RV) is a simplification of flow fields – multiple vortices present in early 

stages and tornado wind speeds known to exhibit variations in time and space

– Terrain corrections, debris effects not accounted for

– Uniform profile assumed, no vertical wind speeds, representative of “3-second gust”

• Model Parameters:

Parameter Initial Range Description

VT 30 ± 5 mph (radar, video) Translation speed

θT Damage centerline ± one grid point Translation direction – fixed in model

α 0 – 90 deg (other studies) Angle to determine wind components

φ 0.4 – 1.0 (other studies) Decay exponent

Gmmm 3.0 – 6.0 (other studies) Ratio of VRR to VT

RRR Estimated damage width ± one grid point Radius of maximum wind

Vcccc 70 – 110 mph (EF-scale, studies) Critical tree fall speed

V� Gmmm + 1 VT Maximum wind speed – derived
Summary: Near-Surface Wind Environment

9

Damage Width (DW): Width of tree fall 

Damage Ratio (DR): Ratio of tree fall width on either side of 

“convergence line” (180 deg to VT)

Tree Fall Direction (β): Distance from convergence line where 

tree fall directions were 90 and 180 degrees

• Indirect Method: Tornado Wind Field Model (cont.)

– Match to observed tree fall dimensions and directions

– Used factorial design -- 36 (729) model runs using ranges on previous slide
Summary: Near-Surface Wind Environment

10

• Indirect Method: Tornado Wind Field Model (cont.)

– Ranges of parameters narrowed, used for final uncertainty

– Estimated maximum wind speed 175 ± 35 mph; ~80% of uncertainty due to VT

Mile Point (X)

Radius of Maximum Wind (RMW) 

(miles) Phi Gmax Alpha (degrees) VT (mph)

1.22 0.12–0.14 0.6–0.7 4.5–5.0 15–25 25-35

2.10 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

2.68 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

3.14 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

3.69 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

5.18 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

6.01 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

6.47 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

6.77 0.14–0.18 0.6–0.7 4.5–5.0 15–25 25-35

0 100 200 300 400 500 600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

160

180

200

220

Time From Tornado Start (sec)

Wind Speed (mph)

0 200 400 600 800 1000 1200

100

150

200

250

300

350

Time From Tornado Start (sec)

Wind Direction (deg)
Summary: Near-Surface Wind Environment

11

• Grid-based simulation – time histories and spatial estimations

Left: Estimated wind speed and direction 

time history for a specific grid-point

Below: Estimated maximum wind speed 

associated with EF-number in Joplin

© 2011 GeoEye. Used with permission. Enhancements by NIST
Summary: U.S. Tornado Hazard

12

• Current “point-based” (tornado-area based) analysis – low 

probability of occurrence – precludes use in codes and standards

• Risk underestimated for large structures (Twisdale and Dunn, 1983)

“Region X”

Region Area = 1000 km2

Total Tornado Area = 50 km2

Time = 50 years

EF-4 Tornado

Tornado Area (EF-2 Area) = 40 km2 (4 km2)

EF-1 Tornado

Tornado Area = 2 km2

EF-2 Tornado

Tornado Area (EF-2 Area) = 4 km2(1 km2)

EF-1 Tornado

Tornado Area = 2 km2

EF-0 Tornado

Tornado Area = 1 km2

EF-0 Tornado

Tornado Area = 1 km2
Summary: U.S. Tornado Hazard (Cont.)

13

• Observations in Joplin:

– 28% ≥ EF-3 damage for residential structures – 12% in NRC (2007) for rated 

EF-5

– 43% ≥ EF-2 damage for residential structures – 24% in NRC (2007) for rated 

EF-5

– Similar observations in other tornadoes striking populated areas; could 

underestimate risk for these areas

• “Spatially-based” analysis

• Forego tornado areal 

distribution; use a 

community-based 

area

• Account for tornado 

frequency 

“Region X”

Region Area = 1000 km2

Total Tornado Area = 50 km2

Time = 50 years

EF-4 Tornado

Tornado Area (EF-2 Area) = 40 km2 (4 km2)

EF-1 Tornado

Tornado Area = 2 km2

EF-2 Tornado

Tornado Area (EF-2 Area) = 4 km2(1 km2)

EF-1 Tornado

Tornado Area = 2 km2

EF-0 Tornado

Tornado Area = 1 km2

EF-0 Tornado

Tornado Area = 1 km2

C

Area = 40 km2
Summary: U.S. Tornado Hazard (Cont.)

14

25

25

25

25

25

50

50

50

50

50

300

300

300

300

500

500

500

500

700

700

700

700

1700

1700

1700

1700

2500

2500

2500

5000

5000

5000

10000

10000

1000

100000

Regional Tornado Rate (tornadoes/year)

Area (sq. miles)

Wichita, KS

Joplin, MO

Point Based (EF-2; Joplin Region)

New York, NY

Lima, OH

Boston, MA

Oklahoma City, OK

0.1 0.5 1 2 5 10

10-1

100

101

102

103

Figure: Mean recurrence interval of 

tornado rated EF-2 or greater striking 

any portion of a given city
2. Findings Related to Tornado 

Hazard Characteristics

15
• F1: Current radar technology is incapable of determining 

tornado occurrence and intensity at heights above ground 

that are relevant to structural engineering design. Closest 

radar to Joplin was 60 miles (100 km away)

• F2: Reliable measurements of tornadic wind speeds, 

especially in the most intense tornadoes are lacking or 

non-existent. 

Findings – Measurements of the NearSurface Wind Field in Tornadoes(< 20 m) 

16
• F3: NIST estimated the maximum wind speeds in the 

Joplin tornado to be 175 mph with an upper bound of 210 

mph. Existing indirect methods have considerable 

uncertainty in estimating wind speeds for structural 

design. 

• F4: The spatial extent of damaging winds in the Joplin 

tornado were significantly greater than those expected. 

For example, EF-3 or greater wind speeds in the Joplin 

tornado were estimated to account for approximately 

twice the area than was expected for an EF–5 rated 

tornado. 

Findings – Measurements of the NearSurface Wind Field in Tornadoes(< 20 m) 

17
• F5: The risk from tornadoes is underestimated using pointbased methodology. Actual damage for Joplin and other 

communities affected by damaging tornadoes were greater 

than predicted in the point-based methodology. 

• F6: In tornadoes rated higher than EF-3, large amount of 

damaged area experiences wind speeds lower than EF-3. 

In Joplin, approximately 40 percent of the fatalities and as 

much as 90 percent of the tornado area were associated 

with EF–3 or lower wind speeds. 

Findings – Assessment of Tornado Climatology, 

Hazard and Risk for Structural Design

18
• F7: The Enhanced Fujita scale lacks adequate damage 

indicators (DIs) and corresponding degrees of damage 

(DODs) for distinguishing among the most intense 

tornado events. The lack of DIs and DODs and overall 

nature of the EF-scale results in subjective, nonquantitative assessment of tornado damage. 

Findings – Limitations of the Enhanced 

Fujita Scale

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