Here is another video of that storm on May 8, 2009, that caused widespread damage and killed several people. It formed strong bow echos and with derechos that caused much of the damage along with some small tornados.
Yes, it was a while ago, but it was an unusual event, and its radar image and progression are interesting. This is a close-up that follows this one storm system from its genesis over Kansas until it crosses over Virginia into the Atlantic. For this animation I used NOAA composite radar images stored at mesonet.agron.iastate.edu on the suggestion of a reader. Thanks! Some technical discussion and production notes after the break.
First note that the time on the clock is in UTC, which is 5 hours ahead of Central Daylight Time (CDT), and 4 hours ahead of Eastern. Time was taken from the timestamp in the radar image file names. Sorry, I didn’t get an indication of a.m. or p.m. into the clock display, nor the date. The video actually spans 48 hours, from early May 8 (0:00 UTC, or 7 p.m. CDT May 7) until late May 9 (23:55 UTC, or 6:55 p.m. CDT). It’s encoded at 10 frames (50 minutes) per second.
Iowa State archives the national composite radar images every 5 minutes, whereas I am archiving them every 10 minutes. So more details are visible in the evolution of the storm over time, and I’m able to slow down the animation enough to let more details be noticed without it becoming a too jerky stop-motion animation. Those iastate.edu archived images do not have any transparency, though, so I modified this script to allow me to specify a color to turn transparent, so I was able to easily recreate the transparency in areas with no radar return.
You’ll doubtless notice the circular halo effects around certain fixed points. Iowa State applies a noise-reduction algorithm, and apparently it is applied just around the radar sites where ground reflection is most troublesome. But there are some frames here where conditions were such that the anomalous propagation overshot the circular areas that the algorithm was applied to. So you see relatively clean reflectance immediately around the radar sites, but arcs of noise extending beyond that clean area. Fortunately the effect occurs mostly just at the beginning of the animation, and it does not detract too much from video as a whole.
The radar images are superimposed on a map of the US that I created for this purpose with data from the National Atlas. It uses a polyconic projection that is sometimes described as obsolete, but I thought it fit the purpose here well. The projection retains true scale along all parallels of latitude and along its prime meridian. Because the storm moves almost constantly along the same latitude, west to east, this projections causes its forward speed to be accurately depicted without distortion. I set the meridian at 90° west (just east of St. Louis), and the prime latitude for the projection at 38° north, which is near the center here. For the record, the projection parameters in proj4 format are “+proj=poly +lat_0=38 +lon_0=-90 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs”. The map was made with the great (and improving) open source program Quantum GIS.
I managed to get YouTube to make it HD (720p), though it’s still not as good as the original. (How can I encode a video so YouTube won’t degrade the quality?) As a note to myself, here is the command I used to encode this video:
ffmpeg -r 10 -i ${framed}/t-%d.jpg -i v3.wav -vcodec mpeg2video -acodec mp2 -ar 44100 -ab 192k -f mpeg -s 960×720 -r 30 -b 8000k -aspect 4:3 -y v3.mpg
Credits
- The video image without sound is available for anyone to copy and reuse for an purpose, so long as they credit EGB13.net, under the terms of the Creative Commons Attribution 3.0 unported license.
- The music track is by Gurdonark , with samples from Brilliant Orange Object and onlymeith, which can be found on ccMixter.org. It is licensed for reuse under the terms of the Creative Common Attribution-NonCommercial 3.0 Unported license. That means that in the unlikely case you want to reuse the video in a commercial manner, you must remove the music.
- The thunder and other storm sound effects are by Charel Sytze and can be found on FreeSound.org. It is licensed under a Creative Commons Sampling Plus 1.0 License,
