Tornado survey summary
Posted April 25, 2011
Although we're nine days past the tornado outbreak of April 16th, some of the specifics regarding the swarm of twisters remain in preliminary form. Just the same, the information collected by survey teams from six different weather service offices serving parts of our state has settled enough that the Raleigh office has put together what should be a reasonably stable map of the tracks and intensity estimates associated with each tornado that has been confirmed to this point. That map, the first attached image, shows that this outbreak resulted in at least 28 tornadoes in North Carolina, seven of those producing damage that only rated EF0 on the Fujita scale, but also five tornadoes that produced EF3 level destruction.
One characteristic of the map that probably jumps right out at you is the southwest to northeast orientation of all these tracks. This is a common result for tornadoes in the United States, especially those in large outbreaks, due to the fact that the appropriate combination of moisture, lift, instability and wind shear for producing multiple, long-track tornadoes usually involves the presence of a deep trough in the middle and upper atmosphere to the west of the storms, and the steering winds associated with this trough blow from southwest to northeast and carry the supercell thunderstorms along in that direction, often at high speed (the storms in this event were frequently moving along at 50-60 mph).
The second image is one you've seen us use on TV and elsewhere on our web page to illustrate the way in which this system was organized. It shows a measure of horizontal wind shear that is maximized where you see the brighter pink and orange colors. The shear results from the wind blowing toward and way from the Doppler radar because of horizontal circulations within the supercell storms called "mesocyclones" that have a diameter of anywhere from about 2-6 miles (the circulations associated with the individual tornadoes themselves are usually too small for radar to resolve, so we have to rely on the larger mesocyclones as a radar indicator that a tornado may be present). It is the path of these mesocyclones that is shown on this radar map, but by flipping between the confirmed tornado track map and the radar-based rotation path map you can certainly see that the two are related. Of course, on maps of these scale the areas of rotation and even the tornado paths appear to cover more area than they do in reality. For a couple of examples, I made a rough calculation, based on storm surveys with path length and some limited damage width information, that in Lee County the tornado damage path covered about 1.8% of the area of the county, while the same tornado, which weakened and narrowed somewhat as it moved northeast, had a damage path across Wake County that covered around 0.3% of that county's area.
The final image is one I adapted from a National Weather Service event summary covering the March 28, 1984 tornado outbreak, which was previously considered the largest such outbreak to affect our state. It is a similar map to the one shown for this year's storm, but note that the tornadoes are rated on the old Fujita scale rather than the more recent Enhanced Fujita (EF) scale, and the that older map also has the number of fatalities marked in counties where lives were lost. In addition, in researching the two storms I found that there was one very short F1 tornado track from the 1984 outbreak that occurred in Union County but was left off of the older map, so I added a marking to include that one here.
Comparing swarms of tornadoes like these two is subject to some difficulties, in that increasing population, much more capable radar systems, cell phone and internet communications, GPS location techniques, and other issues probably make some of the smaller tornadoes easier to find and confirm than they used to be. With that caveat, the two storms compare roughly as follows: In total number of tornadoes, the recent outbreak currently doubles the previous record with 28 tornadoes over our state as compared to 14 in 1984, four of which produced F4-level damage. Tornado track segments affected as many as 33 counties in North Carolina this time, spread over the eastern 2/3rds or so of the state, while the 1984 event was much more confined to eastern NC, with path segments in at least 18 counties. So far, this year's storm has taken 24 lives in our state, while 42 were lost here in the 1984 outbreak. A comparison of economic damages will only be meaningful after more of the loss figures are collected and analyzed from this year's event.
I've included links to the 1984 event summary, a link to the NWS track map from this year (in case it is updated later), and to a couple of good sites for researching and visualizing historic severe weather information. One of those is the Storm Prediction Center's online Severe Plot mapping system, and the other is the National Climatic Data Center's Storm Events Database. Note that in the case of the NCDC system, when you search for tornadoes it will list them by county segment, so that a single tornado crossing county lines can show up multiple times in the results. As an example, if you search for the tornadoes from March 28, 1984, you will get a list of 25 "tornadoes" that results from the 14 tracks seen on the map for that outbreak.