Sadly, one of yesterday's cold-core tornadoes at midday over southern Illinois killed a woman in a double-wide mobile home near the small town of Opdyke (see photos above). It's rare that someone dies in a tornado related to a nearby cold-core mid-level low, as such tornadoes are usually weak and short-lived. This tornado did not appear particularly strong or long-lived, but hit in the wrong place at the wrong time, and will probably be rated either EF1 or low-end EF2. (As I write this, there's no information yet regarding tornado intensity from the NWS).
The same system produced many cold air funnels over Oklahoma on Wednesday March 21, and again over Kansas on March 22 (not shown). The main reason there were no actual tornadoes on those days had to do with the Pacific cool front surging far eastward into the Mississippi Valley, taking the potential surface boundary focus and true surface warm sector well east of the mid-level low spinning over the OK/KS area.
However, on Friday March 23, the mid-level low (see the NAM model 500 mb panel and satellite image above) was moving east again, moving closer to the surface Pacific front that had stalled near the Mississippi River with a surge/spoke of energy aloft. As a result, a new surface low was forming over the mid-Mississippi Valley (see surface map above), providing a surface boundary focus for vorticity and convergence where thunderstorms were building at midday over southern Illinois just ahead of the closed low and cold pool aloft.
One cell generated intermittent tornadoes from near Cape Girardeau MO to north of Carbondale IL. Another cell developed rapidly north of Marion IL and produced the killer tornado near Opdyke shortly before 1:30 pm CDT (1830 UTC). An estimated sounding from the new Rapid Refresh model (set to replace the current RUC model in April 2012) showed sizable CAPE at Marion IL over an hour before the Opdyke tornado, with nearly all instability located below 26000 ft (see the sounding diagram above).
While such a CAPE environment by itself won't support tornadoes with thunderstorms, when coupled with a good surface boundary focus that mazimizes available vorticity as on Friday in southern IL, CAPE "squeezed" into the lower part of the atmosphere can accelerate air parcels upward in storm updrafts to provide rapid stretching needed for tornadoes. There doesn't need to be a bunch of storm-relative helicity or SRH (notice that SRH on the sounding above wasn't large in yesterday's environment); the enhanced stretching with the cold air aloft over the northwest edge of the surface warm sector often seems to be able to do the trick with what otherwise looks like relatively weak SRH/low-level shear. (Note: When both SRH and CAPE are strong in cold-core settings, a somewhat rare occurrence, the associated tornadoes can be stronger and larger as well; see this blog post from last year.)
So, again, the surface boundary focus and proximity of the mid-level low (as well as surface heating from the sun) appear to be very important in producing tornadoes at the ground with cold-core systems. (For additional information, my peer-reviewed paper about "cold-core" tornadoes is here.)
- Jon Davies 3/24/12