Hank Pogorzelski, IBHS Applied Statistician

1. Mapping Tornado and Hail
Frequency in the Lower 48
A Spatial Analysis of Tornado and Hail
Reports from the National Climatic Data
Center

2. Tornado Report Data
• The original source of the data is the Storm
Prediction Center’s (SPC) Storm Data.
• Through the SVRGIS project at Ball State
University, the SPC data set was converted
into a shapefile format compatible with the
mapping software ArcGIS.
• This also involved concatenating multiple path
segments and removing reports with no liftoff
coordinates.

3. Tornado Report Data
• Date Range: 50 year period 1957 – 2006
• Includes tornadoes of F2 or greater strength
• 5,884 reports fit this criteria with no pattern of
increasing activity over the reporting period.
Number of Reported Tornadoes per Year
1957 - 2006 F2 and Greater
250
200
150
100
50
0
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005

4. Tornado Report Data
• Tornado reports are represented spatially
by a pair of coordinates representing
touchdown and liftoff.
• This implies a straight line path for all
tornado reports.

5. The Dataset Plotted:
1957 – 2006 Reports, F2 or Greater

6. Methods for Representing and
Quantifying Tornado Frequency
Common Method: create a grid and count the
occurrences of tornadoes within each grid cell
Small Grid Cells:
• Though some regions are more tornado prone than
others, precise touchdown and liftoff locations are
random.
• Using small grid cells can result in cells within tornado
prone areas with few or no reported tornadoes.

7. Methods for Representing and
Quantifying Tornado Frequency
Large Grid Cells
• Large grid cells in effect cast a wider net and
therefore are less likely to end up with “donut
holes” of low or no activity within larger areas that
are tornado prone.
• However, the use of large cells may over
generalize frequency, and result in a more
coarsely pixilated depiction of tornado frequency.
• Large cells are less sensitive to path length than
small grid cells.

8. Methods for Representing and
Quantifying Tornado Frequency
Approach of the current map:
• Begin with small grid cell: 10 x 10 mile, or
100 sq. miles.
• The tornado count is taken for each cell.
• These counts are used to calculate the
average of each cell and it’s nearest
neighboring cells.
• An interpolation technique is used to
smooth the transitions between cell
values.

9. The Process up Close
Reported F2 and greater Tornadoes reported in the
Dallas/Ft. Worth area 1957 - 2006

10. The Process up Close
Dallas/Ft. Worth with grid cells color coded by
tornado count

11. The Process up Close
Average of each grid cell with its nearest
neighbors: 3 cells in each direction or a 7 x 7 cell
area

12. The Process up Close
Interpolated frequency values for the 1 mile by 1
mile grid cells delineated in to frequency ranges

13. 10 by 10 Mile Grid Cell Frequency
Values

14. 7 by 7 Grid Cell Averages

15. Average Tornado Frequency
per 100 Square Miles, 1957-2006

16. Tornado Activity by Month
• 42% of tornado reports in this analysis
occurred in April and May, 66% between
March and June.
Tornado Reports by Month
1957-2006 Reports, F2 or Greater
1400
1200
1000
800
600
400
200
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

17. Tornado Frequency by Month

18. Tornado Frequency by Month

19. Hail Report Data
• The hail data also comes from the
SVRGIS project which converted reports
from the SPC hail database into
shapefiles.
• The entire SVRGIS data set includes
reports from 1955 through 2009.

20. Hail Reports Have Increased
Dramatically Over Time
Reports of Hail 1" or Greater by Year
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
1955
1957
1959
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009

21. Possible Explanations for the
Increase
• Population growth in areas that previously
had few or no people present to observe
an event, and more trained observers.
• Improved radar technology that can
identify weather conditions likely to
produce hail with increasing certainty.
• Increasing use of multiple reports to
describe what may have been contained in
one report in prior periods.

22. Consequences for Spatial
Analysis
• To the extent it exists, population bias will
deemphasize the threat of hail in rural
areas relative to urban areas of similar
risk.
• The presence of multiple reports, if not
distributed evenly, will result in similar
distortions.

23. Remedies
• Since some of the increase in the total
number of reports is attributed to improved
reporting in rural or previously rural areas,
only the most recent 10 years of data is
used in the current analysis (2000-2009).
• Any reports with coordinates that were
within 0.2 degrees of each other, and
within 30 minutes of each other were
combined into 1 report.

24. Hail Report Data
• There are 65,591 reports of hail 1” or larger in
the data set between 2000 and 2009.
• After consolidating multiple reports that were
very close to each other with respect to time
and space, 53,028 hail reports remained.
Reports of Hail 1" or Greater by
10,000 Year
8,000
6,000
4,000
2,000
0
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
Total Reports Consolidated Reports

25. Distribution of Hail Reports Over
the Conterminous U.S.

26. Methods for Representing and
Quantifying Hail Frequency
The approach of the hail map is the same as
the tornado map:
• Begin with small grid cells: 10 x 10 mile, or
100 sq. miles.
• Calculate the average of each cell and it’s
nearest neighboring cells.
• Use an interpolation technique to smooth
the transitions between cell values.

27. 10 x 10 Mile Grid Cell
Frequencies

28. 5 x 5 Grid Cell Averages

29. Average Number of Hail
Reports
per 100 square miles, 2000-2009

30. Hail Activity by Month
• 45% of tornado reports in this analysis
occurred in May and June, 75% between April
and July.
Hail Reports by Month
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

31. Hail Frequency by Month

32. Hail Frequency by Month

33. Questions?