Sent to you by Jon via Google Reader:
Last month meteorologists with the Weather Channel, the National Storm Prediction Center and other entities announced that this year may see an "above-average number of tornadoes."
Growing up in the Midwest and living a good portion of my life in the South Central Plains, violent spring and summer storms have just been part of the package. While I have not had a first-hand experience of a tornado, I have had enough of a dose of green skies, mega-hail, torrents of rain, and straight-line winds to know that these are phenomena not to be taken lightly in Tornado Alley…or anywhere else.
While we don't have a picture of specific location predictions for these mesocyclones, it is possible to look at the locational history of these events. Using data sets found on ArcGIS Online (AGOL) and mapping them with ArcGIS Explorer (AGX) we can examine past patterns, and possibly make some predictions about the future.
Launching AGX, I keep my map in 2D mode and focus on the contiguous 48 states. I also change my Basemap to "World Streets."
From the "Add Content" menu I select "ArcGIS Online" which opens my Web browser. In the "Search" window, I enter "tornadoes" which returns five items covering tornado activity and intensity from 1950 to 2008. The data are from an AGOL contributor from the University of Tennessee.
Clicking on an item's name gives me metadata about it.
From the search list or from the metadata page for an item, I select "Open in ArcGIS" and when prompted, I choose to "save" the LPK to my hard drive. (NOTE: The overall package for each is named ITEM.PKINFO. As I save each item I rename it, e.g., TORNADOTRACKS.PKINFO, to identify it for addition to my map.)
Back in AGX, I add the layers (Add Content > ArcGIS Layers > XXXX.PKINFO). Upon inspection, I discover that some layers are best used at a near or local scale (Tornadoes, 1950-2008) and others work well on a continental basis (Tornado Track Density…). At first glance, the general tornado 1950-2008 layer leads me to believe that tornadoes are generally widespread across the continent.
However, zooming into an area like Oklahoma City, I see that while active, the pattern seems random across the city.
More telling is the tornado tracks layer. I see more tight patterns, strength, and directionality. Also note that mountainous areas (the Rockies, Ozarks, and Appalachians seem less inclined to experience these events.
However, the most dramatic additions are surface density layers—tornado density and tornado track density with Fuijta weighting. These layers provide a "hot spot" picture of events over the nearly 60 year period. To help position these layers I add the "World Boundaries and Places Overlay" and to peer between them I use the transparency and swipe tools.
While I see that tornadoes can and do happen across a wide swath of the US, there are a number of areas that have in the past experienced these terrible storms more than others.
I wish you a safe spring and summer and again say thank you to the University of Tennessee Extension Office for helping us better understand this imposing weather phenomenon.
George Dailey, ESRI Education Program Manager
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