Patrick Marsh:
One of the most frequently asked maps is for all plot of all the tornado warnings. Here are the tornado warnings issued by year. Each warning was color-filled red with an alpha value of 0.1. Thus, it would take 10 overlaps for the color to display as true red. Thus, the colors are shaded from light pink (no overlaps) to red (at least 10 overlaps). 1 October 2007 the tornado warnings transitioned from county-based warnings to storm-based warnings.
As you can see, the number of warnings increased up through the start of storm-based warnings. After that time period, the number of warnings increased, but the overlap has decreased. One thing that is very apparent is that the number of warnings across the southeast United States has certainly been on the increase…
Pretty cool data mining/visualization efforts from Patrick. Check out his other graphics for tornado warnings by county warning areas, counties warned for tornadoes by county warning areas, tornado warning frequency, and tornado emergencies. If you’re not following Patrick, shame on you.