Friday, May 27, 2011
The Joplin tornado environment - poor visibility with large shear & instability
The death toll from last Sunday's horrendous and tragic EF5 tornado in Joplin, Missouri (see my prior blog discussion) is now up to 132 people, according to reports on Friday 5/27/11. As the deadliest single U.S. tornado in over 60 years, it is worth taking a brief look at the storm environment that helped generate it.
The RUC analysis sounding for Joplin at 2200 UTC (5:00 pm CDT, see 2nd image above), an estimate of the environment roughly 40-45 minutes before the tornado struck, showed very large instability (MLCAPE > 4000 J/kg) and quite sizable low-level wind shear, with 0-1 km storm-relative helicity (SRH) near 300 m2/s2. This matched values shown on SPC mesoanalysis maps at 2200 UTC (not shown). On the same SRH-CAPE diagram where I plotted the Tuscaloosa AL tornado environment in a blog post a few weeks back (see 3rd image above), this combination of SRH and CAPE places the Joplin tornado environment in the same general area and magnitude on the diagram as the Moore OK tornado in 1999 and the Greensburg KS tornado in 2007, both rated EF-5 as well. Although the NAM 12-hr model forecast under-represented the energy-helicity index values (EHI, combinations of SRH & CAPE; see 4th image above), it did forecast a good estimate of the pattern, with an EHI maximum indicated over northwest AR and southwest MO. So it is not a total surprise that the tornado was so strong and deadly.
What is a surprise is the huge death toll, which was partly a result of the tornado being rain-wrapped and very difficult to see (see my photo at top above), and the size of the tornado (around 3/4 mi wide) going down through a very populated area. Going back to the RUC sounding above, relatively weak winds of 40 kts or less at storm anvil level (around 300 mb or 30,000 ft and above) probably contributed to the high-precipitation (HP) rain-wrapped nature of the storm, allowing significant precipitation to fall in and around the updraft/mesocyclone area where the tornado was located, rather than being blown downwind away from the updraft. This seems a little unusual for an EF5 tornado, as most such environments I have examined (e.g., Greensburg KS and Moore OK) have had much stronger winds at that level, instead of the hodograph doubling back on itself with weaker winds in the 9-12 km AGL elevation range. This exacerbated the visibility issue on a day that was already hazy and murky with moisture and humidity, and may have fed into the number of deaths as residents could not get a visual sense of urgency until the tornado was almost right on them.
Another possible ingredient that may have helped contribute to the tornado was a subtle WSW to ENE boundary that appeared visible in the low-level cumulus field on satellite at early afternoon across the Joplin area (see 4th image above). As the Joplin storm complex moved across or "phased" with this subtle feature, it might have provided some focus for the low-level wind shear and SRH to help spin up the tornadic circulation. But this is only speculation.
The last point I'll make is the complex evolution of the Joplin storm, as can be seen in the radar images in the last graphic above. The original supercell and mesocyclone formed near Parsons KS, but began to fall apart shortly before 5:00 pm CDT over northern Cherokee County KS. At this time, three new cells were rapidly developing on the original storm's right/southern flank; two of these cells ("A" and "B" in the images above) quickly merged, while the southern-most of these newer cells ("C") generated a new mesocyclone around 5:15 pm CDT that prompted an NWS tornado warning for Joplin proper at 5:17 pm CDT. This new mesocyclone produced the Joplin tornado about 20-25 minutes later as it merged into the complex that had been cells A and B. The relevance, if any, of these complicated mergers and rapid evolutions to the intensity of the Joplin tornado is not immediately clear. It can be said that, with each new right flank cell raining into the one to its northeast and merging into the complex, this probably further created visibility problems that helped make the monster Joplin tornado even more deadly.
This intensely tragic event has turned literally thousands of lives upside down. Please consider making a donation to one of several organizations assisting in the Joplin area. See http://www.cnn.com/2011/US/05/23/joplin.how.to.help.
- Jon Davies 5/27/11