Jon's blog: Comments and observations about recent tornado/severe weather cases and issues.
Tuesday, April 29, 2008
4/28/08 Virginia tornadoes: Large low-level CAPE, and RUC profile problems
I won't post as often as I've been doing recently, but yesterday's Virginia tornado event was quite newsworthy, and had a very interesting environment worth discussing.
Above at top is the Suffolk, VA RUC profile at 20z (4 pm EDT) in close proximity to the tornadic supercell approaching Suffolk at the time (see radar image farther down, above). I've modified it to properly show the low-level moisture. Notice how the CAPE is essentially "squeezed" down below 500 mb, with the fattest CAPE at 700mb, and lots of CAPE in low-levels. Compare that profile to a nighttime tornadic profile from north-central Kansas on 4/25/08 at 05z (also shown above). On that profile, the low-level shear was larger, but the fattest CAPE was much higher up in the profile (around 400mb) and the total CAPE was distributed through a deeper layer, with not as much low-level CAPE as the Suffolk modified profile. This is more typical of tornado profiles in the plains, and contrasting this with the Suffolk profile at top, it is interesting to see how variable tornado environment profiles can be.
Yesterday's Virginia environment appeared to have CAPE and low-level shear (storm-relative helicity, or SRH) co-located down close to the ground, which may have facillitated or optimized tilting and stretching of low-level shear (SRH) for tornado development. I'm just speculating, but the contrasting 4/25/08 profile in Kansas may have required more low-level shear (SRH) because of less CAPE down low in the environment, and therefore less vertical stretching.
Yesterday's case also points out some problems with RUC profiles that I've talked about in recent presentations at NWS offices. In some cases with rapid moisture return and strong warm advection, the RUC does not moisten the low-levels enough. An extreme example was the RUC's handling of a tornado outbreak setting in Illinois on 4/20/04. Regarding 4/28/08 in Virginia, examine the unmodified Suffolk RUC analysis profile at 20z that I've also shown above, near tornado time. (Remember, the profile at top was modified by me to be more representative of the low-level moisture). The unmodified RUC profile has a dry "bubble" near the ground that appears erroneous and unrepresentative of the true environment, particularly if you compare that profile to the WRF analysis profile 2 hours earlier at Suffolk (also shown above) that seems much more reasonable in its low-level moisture depiction. Using mixed-layer lifted parcels (as most SPC mesoanalysis graphics do), the unmodified Suffolk profile yields about 50% less total CAPE than the modified profile, as well as less low-level CAPE, not a good representation of the setting.
This is significant, because SPC graphics are driven by the RUC. Notice how the Significant Tornado Parameter (STP) at 20z (shown above) using mixed-layer parcels and effective shear/SRH (that computation is supposed to be the "best" one, according to SPC research) showed no STP values whatsoever in this damaging tornado situation (at least EF-3 intensity in the Suffolk area). The main reason for this and problems with other SPC graphics yesterday was the erroneous low-level RUC profiles in the southeast Virginia area, similar to the unmodified Suffolk profile discussed above.
This is an error that forecasters need to be keenly aware of, and also addressed by SPC researchers regarding the SPC mesoanalysis output. The problematic graphics feeding off the RUC errors may have masked yesterday's environment and had something to do with SPC's slowness to issue a tornado watch in Virginia after tornado warnings were already being issued. Additional research also needs to focus on better operational highlighting of environments where CAPE is squeezed low in the environment, and co-located in the vertical with low-level shear/SRH, a situation not that easy to pick out using operational graphics of commonly-used supercell tornado parameters.
Jon Davies 4/29/08
Posted by Jon Davies at 7:55 AM
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The SPC mesoanalysis graphics do suffer from problems with the RUC moisture profiles above the ground. However, that's why we provide an alternate "fixed layer" version of the STP. It replaces MLCAPE with SBCAPE, effective shear/SRH with 0-1 km SRH and 0-6 km bulk shear, and SBLCL. Use of the surface parcels sidesteps some of the RUC analysis problems, though this version of STP performs worse than the effective layer version when the RUC profiles are accurate. We don't have any easy or consistent way to fix the graphics when the input data are questionable - garbage in, garbage out.
I'll try to send you a graphic of the corresponding fixed layer STP.
Thanks for the info and discussion. I completely missed this event though I was previously "on alert" as the trough approached. After looking at the data Sunday evening and Monday morning, I was not impressed and didn't take off from work. By the time I could see the parameters, it was too late.
The RUC giveth, and the RUC taketh away. As a follow-up to Rich's post, below are graphic links to the fixed layer/SBCAPE version of STP at 18z/20z. Using these calculations, there was a value of 1 since at least 17z across southeast VA/northeast NC. It's nice we still have this STP version at our disposal for a double check, especially in marginal CAPE and/or tropical scenarios etc.
STP Fixed Layer 18z
STP Fixed Layer 20z
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