One of the most exciting new features of the site (see Major Upgrade to Tornado History Project – 7/19/2008) is the inclusion of enhanced path data derived from the National Climatic Data Center (NCDC) historical archive (thanks to Stuart Hinson for providing access to the archive!) While there are many differences between the Storm Prediction Center (SPC) historical archive (the primary source for the site) and the NCDC archive, the most significant difference is the method used to give coordinates.
In a nutshell, for tornadoes in which path data is available, the NCDC archive gives latitude and longitude coordinates when a tornado enters another county, while the SPC archive only gives coordinates when a tornado enters another state. This means that multi-county tornadoes in the NCDC database may have more data points available to draw a more detailed approximate path on the map. Thus, instead of the straight lines generated for SPC derived paths, NCDC derived paths generally have more turns and zig-zags.
How big of a difference does the inclusion of NCDC data make? Well, that depends on the number of NCDC derived data points given for any specific tornado, but let’s look at an example. The screenshot below shows a few F5 tornado paths using SPC data. As you can see, the approximate paths are straight lines, except for where a tornado crosses a state line.
The screenshot below shows the same area using NCDC derived data, when available. As you can see, some of the paths show a greater amount of detail. Compare the apparent path near Jackson, Mississippi, for example. With the enhanced path data below, the path passes the city from the South rather than the North. And the more you zoom in, the more the difference will become apparent.
Displaying enhanced path data was not an easy task and took me almost a year to complete. The main issue was that there was no easy way to match up the tornado records between the two historical archives. I had to do quite a bit of manual tweaking to get it to work. Nevertheless, I think it was well worth the effort. I haven’t done an exact count, but I estimate that there are roughly 2,000-3000 tornadoes with enhanced path data, out of the 50,000 tornadoes in the database (the vast majority of tornado paths are short and only affect a single county.)
Please note that you can only see enhanced path data when you are logged in.