Forecasting Significant Tornadoes: A case-study evaluation of the Storm Prediction Center
By
John F Gray
An Undergraduate Thesis
Submitted in Partial Fulfillment for the Requirements of
Bachelor of Arts
in Geography and Earth Science
Carthage College
Kenosha, WI
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Table of Contents
Abstract
3
Introduction
3
Literature Review
5
Description of Data Set
7
Methodology
8
Results
10
Discussion
18
Conclusion
20
Bibliography
21
2
Abstract
The purpose of this experiment was to evaluate the effectiveness of the risk assessment put out
by the Storm Prediction Center (SPC) during significant tornado outbreaks. A significant tornado
outbreak, for the purposes of this experiment, is defined as a tornado outbreak that results in multiple
EF-2 or greater tornadoes, resulting in a life-threatening situation for anyone in the path of these
storms.
For this experiment, two outbreaks were selected: the May 3, 1999 outbreak in Oklahoma and
the April 27, 2011 outbreak in Alabama. These outbreaks were chosen because of their historical
significance in meteorology and the fact that they both produced long-lived powerful tornadoes. A
target area was selected for each of the events and a time window was chosen. Tornado paths were
gathered and were compared to the areas of risk and were also compared by EF type.
One hypothesis was studied. The null hypothesis states that the intensity of tornadoes is not
related to risk zone, with the alternative stating that intensity is related to risk zone.
Introduction
Each day, the Storm Prediction Center, or SPC, a division of the National Weather Service,
releases a forecast for the day or days ahead. This forecast includes the probability of severe weather in
certain areas of the country. The SPC issues forecasts for Day 1, Day 2, Day 3, and Days 4-8. As with
all weather forecasts, the most short-term forecast, in this case the Day 1 forecast, active for the day on
which the forecast was issued, is usually the most accurate. That is the reason why the Day 1 forecasts
were chosen here. Another reason is the fact that forecasts change daily and rarely do the areas of risk
set by the SPC hold from day to day.
A major part of the SPC forecast is to determine where the “High Risk” areas are on that day.
High Risk days are very rare, occurring a few times every year. However, these High Risk days usually
coincide with some of the largest tornado outbreaks in recorded history. Two outbreaks that have
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occurred over the last 14 years produced some of the most powerful and long-lived tornadoes in
recorded history. These two outbreaks, twelve years apart, are reminders that these SPC forecasts are
very important and that all people, especially those in High Risk Zones, should pay attention to these
outlooks.
The first of these outbreaks took place on May 3, 1999 in the Central Plains of the United
States. Most of the tornadoes in this outbreak took place in the state of Oklahoma, including a few
powerful tornadoes that approached the city limits of Oklahoma City. One tornado in particular came
so close to entering the city that the National Weather Service had to use stronger-than-usual language
in its tornado warning. For this first time in history, a Tornado Emergency was issued (NWS, 2011).
Tornado Emergencies are still used to this day, and are issued when large, damaging tornadoes
approach populated areas such as cities or large towns. That same tornado that warranted the first-ever
Tornado Emergency set the record for recorded wind speed in a tornado at over 300 mph (NWS, 2011).
This would classify the tornado as an EF-5 by today’s standards, a tornado capable of damaging large
structures and completely destroying homes and smaller buildings. Overall, there were multiple
tornadoes that were EF-2 or greater, including 5 tornadoes that fell into the EF-4 or EF-5 class. Since
there were many long-lived, powerful and devastating tornadoes, May 3, 1999 is an important outbreak
to study.
The second of these outbreaks took place on April 27, 2011. This outbreak consisted of
tornadoes all over the Eastern half of the United States, from Louisiana up to New York, but the main
events occurred in “Dixie Alley,” a region of the country that is susceptible to large and damaging
tornadoes. Due to the weather patterns in this area, storms generally moved faster than in the more
well known “Tornado Alley” in the Central Plains. This is due to the fact that the jet stream tends to be
more powerful in the spring in this region, forcing storms to move more quickly. This means that large
tornadoes in this area have the potential to be more devastating than tornadoes of similar strength that
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would occur in Tornado Alley, as the faster a storm moves, the less warning time people have. This
proved to be true as the events of that day killed over 300 people across Dixie Alley. This day was also
the largest tornado outbreak in recorded history, with 358 confirmed tornadoes over the 4 days in which
this outbreak took place (NWS, 2012). Most of these tornadoes, especially the very strong ones, took
place on April 27, 2011. Multiple Tornado Emergencies were issued on this day, as many powerful
tornadoes approached or went through heavily populated areas. One EF-4 tornado went all the way
from the city of Tuscaloosa, Alabama, where the University of Alabama is located, to Birmingham,
Alabama, the largest city in the state. Since this was the largest tornado outbreak in recorded history
and it produced many long-lived and powerful tornadoes, this was also an important day to study.
Since the SPC is the main resource used by television and other media outlets to find out when
and where severe weather will most likely strike, it is important to perform an assessment of the areas
of risk that have been put out by the SPC on these two days. Since both were High Risk days, it is
logical to compare them. They are also both in the active part of tornado season, and occurred less than
a week apart on the calendar. Chosen were two events from different years in order to determine if the
SPC has become more accurate with their placement of High Risk zones over the years, leading to the
following research question: By studying two historic tornado outbreaks, can a significant difference be
found in the accuracy of the SPC's forecasts for the 1999 outbreak and the 2011 outbreak? One
hypothesis was studied. The null hypothesis states that there tornado intensity is not related to risk
zone, while the alternative states that intensity is related to risk zone.
Literature Review
No studies were found comparing these two specific tornado outbreaks to one another.
However, there is a rich literature in the forecasting of tornadoes. For example, Rasmussen (2003)
investigated which atmospheric variables were most valuable when forecasting tornadoes. His research
examines three different variables that are commonly used to forecast tornadoes: SRH (Storm Relative
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Helicity), EHI (Energy Helicity Index), and CAPE (Convective Available Potential Energy).
Rasmussen (2003) discusses his methodology, which involves looking into past research done
on forecasting tornadoes. This is important because it is exactly what needs to be done for every study
that involves the forecasting of tornadoes. He used all tornadoes available to be studied, unlike in this
experiment, where two specific events will be analyzed.
Tornado forecasting was also the focus of Doswell (1993), who discusses the parameters used
by the National Weather Service and other agencies for the forecasting of tornado outbreaks. This is
further information about the forecasting of tornadoes, and even discusses what the National Weather
Service looks at when forecasting tornadoes. Doswell mentions upward motion (lifting), moisture, and
shear as important variables, but does not go into detail about which are the most important, as
Rasmussen does. Doswell (1993) did, however, provide valuable insight into the forecasting of the
National Weather Service, which tells what variables are important to them now.
Richard Thompson of the Storm Prediction Center discussed the Supercell Composite
Parameter, which aids the SPC when forecasting severe weather (2004). Thompson describes the
equation used by the SPC in numerical forecasting of storms that will turn out to be supercells, as
opposed to discrete non-supercell storms. Of particular importance to the research in this thesis,
Thompson also discusses the Significant Tornado Parameter (STP), which is used by the SPC when
forecasting significant tornado outbreaks. Since the STP was introduced between the two outbreaks, it
follows that the two outbreaks in this study were forecast quite differently, and it is likely it would have
aided the SPC in the forecasting of the 2011 outbreak.
These research articles were supplemented by the information released by NOAA on both
tornado outbreaks. NOAA has a very descriptive background for both events, and even discusses the
Tornado Emergency issued during the May 3, 1999 outbreak. Another source used was an article that
discussed the forecasting of the April 27, 2011 tornado outbreak. This article not only describes the
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events themselves, but also the conditions that caused the outbreak itself. It approaches the outbreak
from several different angles, including how the outbreak was forecast and even the use of trained
spotters before and during the storm.
Description of Data Set
For this experiment, two outbreaks were chosen because of their significance in the field of
meteorological studies. These outbreaks took place on May 3, 1999 and April 27, 2011. The May 3,
1999 outbreak resulted in the first ever Tornado Emergency, which is issued when a large and powerful
tornado is about to enter a populated area. The April 27, 2011 outbreak was the largest in recorded
history, with 229 tornadoes occurring in just one day. Both outbreaks had several long-lived powerful
tornadoes that came close to large populated areas, and also included rare “High Risk” forecasts from
the SPC.
The data was obtained from NOAA's WCM page. Some data were contained within Excel files,
which included start points and end points for each tornado, Fujita scale rating, and several other
categories, such as county, damage total, and NWS report number, that were not useful in this study.
Figure 1 is a screen shot of one of the files downloaded.
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Figure 1: Screen shot of raw data downloaded from NOAA
Also used in this study were maps from the SPC showing the areas of risk for each of the two
days. Figure 2 shows this for April 27, 2011 below. Because these were images, rather than spatially
referenced vector files, these maps were not able to be put into ArcMap. The maps were digitized by
hand using ArcMap's editing tool. After the maps were digitized and the polygons were created, the
risk zones and tornado tracks could be viewed on the same map at the same time.
Figure 2: April 27, 2011 risk zones
Methodology
These tornado outbreaks each occurred over several days, but because tornadoes were being
compared to risk areas, which are forecast by the SPC on a daily basis, only tornadoes that took place
on one day of each of the two outbreaks were considered. All other tornadoes that were still considered
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part of the outbreak, but took place on other days, were disregarded in this study.
The data sets were downloaded from NOAA's website. All tornadoes that took place on those
days were used. The first task in processing and analyzing these data sets was clean up: only the
location of start and end, the date, the time, and the rating on the Fujita scale were retained. The data
came with both start and end points on the same row; in order for mapping in ArcGIS, the data had to
be separated. It was arranged so that the first group of points was the start of each tornado, while the
second group was the end points. Then, the tornadoes were given a Polyline ID, with the starting and
ending point for each respective tornado given the same value in this category.
At this point, GIS processing began. The data sets were then imported into ArcMap. To display
them onto a US state boundaries base map, the XY to line tool was used to connect the start points to
end points, making a line segment. After the lines were made, the line table was joined with the
original data table, using the Polyline ID field as the field on which the join was based. From this, the
tracks could be sorted and queried by EF scale rating and were given different colors on the map. An
appropriate color scheme was chosen, as each level of intensity looked more intense on the map, with
EF-0 being light green and EF-5 being black.
After the tornado tracks had been placed onto the map, the areas of risk needed to be drawn as
well. Maps were taken from NOAA that depicted the areas of slight, moderate, and high risk for each
day. There was no way to get these maps over to ArcMap directly, so polygons had to be drawn by
hand. New personal geodatabases were created for each zone, slight, moderate, and high. Then the
editing tool was used to create the polygons for each area of risk.
After the lines and polygons were both on the map, the tornadoes in each area of risk were
counted, and the frequency of each intensity was evaluated by zone. The total number of tornadoes in
each zone was counted, and the percentage of tornadoes in each zone was calculated. This was done
with all tornadoes, and broken down by EF scale as well. These statistics from both outbreaks were
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then compared.
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Results
Figure 3: May 3, 1999 outbreak with all zones highlighted.
Figure 3 is the May 3, 1999 outbreak with all zones of risk and tornado tracks included. In all,
there are 71 total tornadoes, all of them lying within the slight or higher zone. 100 % of the total
tornadoes are in this zone. 100 % of the EF-0 and EF-1 tornadoes are in this zone. 100% of the EF-2
and EF-3 tornadoes are in this zone. 100% of the EF-4 and EF-5 tornadoes are in this zone.
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Figure 4: May 3, 1999 outbreak with slight risk zone highlighted.
Figure 4 is also the May 3, 199 outbreak with only the slight risk area being examined. In total,
there are 5 tornadoes, or 7%, in this zone. All 5 of the tornadoes in this zone are EF-0 or EF-1, out of a
total of 54 tornadoes in this EF class range. This means that 9.26% of all EF-0 or EF-1 tornadoes fall
within the slight risk zone. No tornadoes from other EF scale ranges fall within this zone (0% of EF-2
through EF-5 tornadoes).
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Figure 5: May 3, 1999 outbreak with moderate risk zone highlighted.
Figure 5, from the May 3, 1999 outbreak, focuses on the moderate zone alone. Just three
tornadoes, or 4.23%, fell within this zone. All of these tornadoes were in the EF-0 to EF-1 range,
meaning 5.56% of those tornadoes were in the moderate risk zone. No tornadoes above EF-1 were
recorded in the moderate risk zone.
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Figure 6: May 3, 1999 outbreak with the high risk zone highlighted.
Figure 6, from the May 3, 1999 outbreak, focuses on the high risk zone. 63 of the 71, or 88.7%,
fall within this zone. Of the 54 EF-0 and EF-1 tornadoes, 46, or 85.2%, fell within the high risk zone.
All (100%) of the EF-2 and EF-3 tornadoes fell within this zone, as well as all 5 (100%) of the EF-4
and EF-5 tornadoes.
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Figure 7: April 27, 2011 outbreak with all zones highlighted.
Figure 7 of the April 27, 2011 outbreak looks at the outbreak as a whole, focusing on all zones
of risk. There are 229 tornadoes total in this outbreak, and each one fell within the overall zone of
concern. The slight or higher zone contained all tornadoes from each EF class range.
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Figure 8: April 27, 2011 outbreak with the slight risk zone highlighted
Figure 8, from the April 27, 2011 outbreak looks at the slight risk zone only. Out of the 229
total tornadoes, 24, or 10.48%, fell within this zone. 21 of these tornadoes, or 14.7%, were either EF-0
or EF-1. 3 of the 61 (4.9%) EF-2 or EF-3 tornadoes fell within this zone. No EF-4 or EF-5 tornadoes
fell within the slight risk zone.
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Figure 9: April 27, 2011 outbreak with the moderate zone highlighted.
Figure 9, from the April 27, 2011 outbreak takes a look at the moderate zone by itself. 72 of the
229 (31.4%) total tornadoes fall within this zone. 53 of the 143 (37%) EF-0 and EF-1 tornadoes fall
within this zone. 19 of the 61(31.2%) EF-2 and EF-3 tornadoes fell within this zone. One (4%) EF-4
tornado fell within this zone.
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Figure 10: April 27, 2011 outbreak with the high risk zone highlighted.
Figure 10 of the April 27, 2011 outbreak focuses on only the high risk zone. 133 of the 229
(58.1%) total tornadoes fell within this zone. 69 of the 143 (48.3%) EF-0 and EF-1 tornadoes fell
within this zone. 39 of the 61 (63.9%) EF-2 and EF-3 tornadoes fell within this zone. 24 of 25 (96%)
EF 4-5 tornadoes fell within this zone.
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Figure 11: May 3, 1999 table
Figure 11 shows the percentage of each EF class of tornadoes that fall within each risk zone on May 3,
1999.
Figure 12: April 27, 2011 table
Figure 12 shows the percentage of each EF class of tornadoes that fall within each risk zone on April
27, 2011.
Discussion
After completing the research, I found that the April 27, 2011 outbreak was forecasted slightly
more accurately than the May 3, 1999 outbreak. The percentage of EF 0-1 tornadoes that occurred
within the slight risk zone was higher in 2011 than it was in 1999. Also, the percentage of EF 2-3
tornadoes in the Moderate Risk Zone was higher in 2011 than it was in 1999. This indicates that the
SPC was more accurate, in their forecasting of the Slight Risk and Moderate Risk zones.
The only unexpected result was the fact that one EF-4 tornado fell in the Moderate Risk zone on
April 27, 2011. This does not mean that the May 3, 1999 outbreak was forecasted more accurately for
the High Risk zone only. However, the High Risk zone is the area where most of the tornadoes took
place in 1999, with only a handful taking place outside the High Risk Zone. Still, the numbers show
that the May 3, 1999 outbreak had 100 percent of the EF 4-5 tornadoes in the High Risk Zone, with
only 96 percent of the EF 4-5 tornadoes occurring in the High Risk Zone on April 27, 2011. Although
one rogue tornado did fall outside the High Risk zone on April 27, 2011, the tornado itself was just
outside of the High Risk Zone. Also, it is not a significant enough difference to classify the April 27,
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2011 outbreak as being less accurately forecast by the Storm Prediction Center. Instead, it is fair to say
that the Storm Prediction Center has improved in their forecasting abilities, but the one tornado that fell
outside the High Risk zone was due to other factors.
There is one possibility for the one EF-4 tornado that fell outside the High Risk zone on April
27, 2011. Dixie Alley is a very different geographic region from Tornado Alley. Storms in the
southeastern US often move faster than those in the Central Plains. Because of this, it is likely that
these long-lived supercells were still dropping high intensity tornadoes further in their tracks. Because
of the speed at which they were moving, it is likely that they moved outside of the High Risk zone
while they still had the instability to produce powerful tornadoes (Moore, 2012).
There were a few chances to make errors while this experiment was being conducted, and that
cannot be ignored in the discussion of the results. One possible error comes from the fact that the maps
of risk area did not come georeferenced from NOAA, and had to be done by hand using ArcMap's
editing tool. This means there could have been points where the zones of risk were misplaced,
meaning some tornadoes that were counted in one zone while analyzing the data could have actually
occurred in another zone.
Another error could have been in the actual counting process. Since the count was also done by
hand, there is a chance that some tornadoes in each zone could have been miscounted, whether by total
number of tornadoes, or by EF scale rating. I have since learned that ArcGIS is capable of
summarizing zonal statistics like this automatically, using the Spatial Join tool. The Spatial Join counts
the points, or lines in this case, within a polygon. For this experiment, it would have been wise to use
the Spatial Join in order to avoid potential errors. Both of these errors are important to note, as it could
slightly affect the numbers in the charts that were used to show the data.
Another thing to focus on would have not only been the accuracy of the forecast, but the
precision to which each outbreak was forecast by the SPC. It is not only important that the higher
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intensity tornadoes fall within the high risk zone, but that the tornadoes are forecast in a very precise
way, meaning that within a high risk zone, there should be a larger percentage of tornadoes.
In addition to the errors, this experiment is not meant to show the complete accuracy of the
forecasting ability of the SPC. Because of the limits of time and resources, I compared only two
carefully chosen outbreaks. However, to do a full analysis on the accuracy of the SPC, all High Risk
days would need to be studied.
Another thing to study would be the difference in the size of the risk zones, and if the size of the
zones themselves have anything to do with the accuracy of the forecast. For example, the SPC could
issue a High Risk for the entire country on a day where tornadoes are likely, and then every tornado
would fall inside the High Risk zone. However, it is just as important to be precise in a smaller zone as
it is to be accurate. Issuing smaller zones and also being more accurate would save people time from
having to prepare for tornadoes that might not come to them.
Conclusion
Tornadoes are one of the most destructive and powerful meteorological phenomena. Significant
tornado outbreaks are very important to understand because they produce powerful and destructive
tornadoes that can come close to large populated areas and even destroy lives.
The SPC issues daily outlooks every day, with the Day 1 Outlook usually being the most
accurate. These forecasts are very important, because they tell what dangers could be present during a
day of severe weather. The High Risk days are extremely important, because this means the SPC has
seen an area that has an increased risk in severe weather.
The expectations were that the April 27, 2011 outbreak would be accurate across the board, but
it turns out the May 3, 1999 outbreak was more accurate in the High Risk Zone. This is because one
EF-4 tornado fell within the Moderate Risk Zone on April 27, 2011. All EF 4-5 tornadoes on May 3,
1999 fell within the High Risk zone. However, given the methods of counting used in this experiment,
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it is clear that both outbreaks were forecast very well by the SPC.
This was a case study that examined two historic tornado outbreaks that were both classified as
High Risk Days. Further research is needed on this topic to more accurately assess the forecasts of the
SPC. If more time and resources were available, all High Risk days would have been studied, which
would have been a much more accurate assessment of the SPC.
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"The Historic Tornadoes of April 2011." National Weather Service. National Oceanic and Atmospheric
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