Does Virginia Have a Tornado Alley?
A Spatial and Temporal Analysis of Tornado Tracks from 1950 - 2018
What is a "Tornado Alley?"
While not a term with a scientific definition or description, Tornado Alley is used mostly in reference to the Southern Plains/Upper Midwest areas of the United States where tornadoes are the most prevalent during the spring and early summer months.
The EF5 2013 Moore tornado as it passed through south Oklahoma City, OK. Credit: Wikipedia user Ks0stm
Another part of the United States also has a Tornado Alley...
Dixie Alley
In this graphic depicting Tornado Warning Frequency from 2008-2016, areas of the southeast United States experience more frequent tornado warnings than other parts of the country.
Image credit: www.ustornadoes.com
This part of the country is referred to as Dixie Alley and is a completely separate tornado alley from the one in the Great Plains.
Image of a strong tornado near Arab, Alabama, during the outbreak of April 27, 2011. Credit: Charles Whisenant
Is There a Tornado Alley in Virginia?
With knowledge that parts of the United States experience a high frequency of tornadoes, we were curious whether Virginia has been experiencing similar tornadic behavior. By analyzing Virginia's tornado history, could we identify an area of Virginia that exhibited characteristics of a "tornado alley?" Using Esri's ArcGIS Pro software and a tornado database (1950 - 2020) from NOAA's Storm Prediction Center (SPC), we began to investigate this important question through mapping, visualization, and closer examination of Virginia's tornado characteristics. Space Time Cube development and subsequent Space Time Pattern Mining were performed to determine if parts of Virginia were experiencing an increasing frequency of tornadoes.
Virginia Tornado Locations (1950-2020)
The first step in determining if Virginia has a tornado alley is knowing where tornadoes have occurred.
By filtering the tornado tracks from SPC's dataset to Virginia-only tornado tracks, we can visually identify that there appear to be numerous tornadoes occurring in the eastern and northern parts of the state.
Location of tornado tracks provides a major clue, but what about tornado strength? Might that shed more light on whether Virginia has a tornado alley?
Virginia Tornado Strength (1950-2020)
The next phase involved creating a Proportional Symbol map of the tornado tracks by strength to determine where the strongest tornadoes in the state have occurred. Virginia has only experienced two EF 4 tornadoes since 1950 and no EF 5 tornado in recorded history.
This Proportional Symbol map provides another clue regarding where a tornado alley might exist in Virginia. However, the strength of individual storms isn't necessarily the best indicator of a broader regional pattern.
What if we looked at areas in Virginia that had a higher density of more intense tornadoes?
Virginia Tornado Strength Density (1950-2020)
Kernel Density analysis using a Magnitude attribute (tornado intensity rating on the Enhanced Fujita Scale) as our population variable produced a 100-meter grid cell density surface depicting areas of Virginia that have experienced the strongest tornadoes. The darker red shading indicates areas where tornado intensity has been the highest since 1950.
This density surface provides a better idea of regional trends in Virginia tornado strength than the depiction of individual tornado strength by a proportional symbol map.
Now that we've identified where the strongest tornadoes in the state have tracked, let's find out if areas of Virginia are experiencing an increasing frequency of tornadoes.
Performing a Space Time Cube Analysis
In order to ascertain whether parts of Virginia have experienced an increase in tornado activity since 1950, we performed a Space Time Cube Analysis using our Virginia Tornado dataset using the following tools in the Space Time Pattern Mining Toolbox:
- Create Space Time Cube by Aggregating Points
- Emerging Hot Spot Analysis
- Visualize Space Time Cube in 2D
Create Space Time Cube by Aggregating Points
In order to use the Create Space Time Cube by Aggregating Points tool, we converted our linear tracks into discrete points.
A point spacing of 5 kilometers was determined to be optimal for mimicking the linear nature of a tornado.
A netCDF cube was created with a Time Step Interval of 10 years, a Hexagon grid Aggregation Shape Type, a 250 kilometer Distance Interval.
Emerging Hot Spot Analysis
Using our netCDF cube as the input for the Emerging Hot Spot Analysis tool, we chose COUNT as the Analysis Variable in order to see if there were areas of hot spot activity in Virginia.
The results of the Emerging Hot Spot Analysis indicated that a large section of eastern Virginia had an Oscillating Hot Spot characteristic, the majority of northern Virginia had a Consecutive Hot Spot characteristic, Hampton Roads had a Sporadic Hot Spot characteristic, and parts of south-central Virginia exhibited New Hot Spot characteristics.
See https://pro.arcgis.com/en/pro-app/tool-reference/space-time-pattern-mining/learnmoreemerging.htm for esri's definitions for Emerging Hot Spot Analysis tool outputs.
The Emerging Hot Spot Analysis results gave us a great indication of where Virginia's tornado alley may be located, but we wanted to look at one more variable: tornado trends.
Visualize Space Time Cube in 2D
For the Visualize Space Time Cub in 2D tool we again used the netCDF cube as the input. The Cube Variable remained COUNT and we chose Trends for the Display Theme.
The result of the tool indicated areas of Virginia where tornado frequency has been increasing since 1950. We speculate the absence of any down trend may be a result of the infrequency of tornadoes in areas with no data.
Now that we have explored Virginia tornado characteristics spatially, we are ready to conclude whether Virginia has a tornado alley.
Virginia's Tornado Alley
Virginia's Tornado Alley
After performing our analyses on Virginia's tornado database, we concluded that Virginia does have a tornado alley that covers that majority of the eastern section of the state. We arrived at this conclusion due to:
- Our Proportional Symbol map and Kernel Density analysis of tornado intensity revealed that the most intense tornadoes have occurred in northern Virginia, just south of Richmond, and in southeastern Virginia (Hampton Roads).
- Our Emerging Hot Spot Analysis that identified areas of oscillating, sporadic, and new hot spots in eastern Virginia.
- Our Visualize Space Time Cube in 2D Analysis exposed trends in increasing tornado frequency in many areas of eastern Virginia.
Conclusions: Does Virginia Have a Tornado Alley?
The combination of proportional symbol mapping, kernel density analysis, and Space Time Pattern Mining tools allowed us to determine areas of Virginia that had experienced the strongest tornadoes, detect areas of the state where hot spots in activity have occurred, and frequency of storms has increased. This evidence led us to conclude that eastern Virginia exhibits the characteristics of a Tornado Alley.
This analysis did have some limitations, chief among these was the nature of tornado track frequency over the 10-year time step interval chosen for our netCDF cube. The result of the netCDF analysis only indicated areas of hot spots and increasing frequency with no evidence of any areas of decreasing trend. It would be interesting to run the same analysis for a region that experiences a higher frequency and total number of tornadoes than Virginia.
Future work for this project will involve the exploration of additional spatial analysis tools to examine other tornado characteristics and behavior, narrowing down the Tornado Alley area for Virginia to a more local level, and developing more tools to communicate tornado risk to Virginians (e.g. Virginia Tornado Mobile Home Risk Index pictured below).
Virginia Tornado Mobile Home Risk Index
Top 15 Areas of Virginia with increasing tornado frequency and mobile home percentage.
We took the top 15 areas that were identified as having an increase in tornado frequency by our Visualize Space Time Cube in 2D analysis and used ArcGIS Pro's Enrich tool to combine those areas with their Mobile Home Percentage from the 2014 - 2018 American Community Survey. We chose to Enrich the our Visualize Space Time Cube in 2D analysis with mobile home percentage because mobile homes are more susceptible to tornado damage than homes with a strong foundation.
This mash-up of data allowed us to create a Virginia Tornado Mobile Home Risk Index to highlight the most vulnerable areas of tornado risk in Virginia.
Old Dominion University / Center for Geospatial Science, Education, and Analytics.
