Sunday, September 14, 2008

Tornadoes on 9/12/08 near Kansas City

A somewhat rare September tornado episode occured on Friday 9/12/08 in the Kansas City area. While visually impressive (see photos above near Eudora & Desoto, Kansas), the tornadoes were weak, with damage only in the EF0-EF1 range (the National Weather Service has officially rated the tornadoes near DeSoto, Kansas and Sedalia, Missouri EF0 in intensity).

Shawna and I had a birthday celebration planned in north KC for her son Zach on Friday evening, so we missed alot of the excitement. When picking up Zach from school in Plattsburg, Missouri at mid-afternoon, Shawna and I noticed SE to NW bands of low dark clouds with rapid motion, suggesting strong low-level wind shear. With rain all day and a cool surface air mass north of a boundary to our south (see surface map above), I didn't give the setting much thought until Shawna saw what looked like a wispy funnel that lasted a few seconds under a cloud band in the distance. I looked at some maps online, and began to wonder a bit what might happen. SPC issued a tornado watch around 4:30 pm with tornado reports coming in to the southwest of Kansas City near the boundary, but we were committed to our celebration. Watching TV later at Shawna's parent's house, we saw how large the tornado near DeSoto was, and heard about reports of damage. Thankfully, the tornado warned cells that went over the KC metro area did not produce any damaging tornadoes.

The DeSoto, Kansas tornado was a classic case of a supercell crossing a boundary where wind shear increased rapidly, with easterly surface winds. The surface map above shows the stationary front in the Topeka-Olathe area, and the SPC maps above show the large increase in low-level wind shear (storm-relative helicity or SRH) along and north of the front, with CAPE near and south of the front. The best combination of CAPE-SRH was right along the front just south and southwest of the KC metro area. Deep-layer shear through 6 km above ground (not shown) was also on the order of 40 kts along and north of the front, suggesting good support for tornadic supercells. On radar images above, notice how the DeSoto cell (indicated by white arrow) crossed the boundary moving northeastward, which is about the time it produced a tornado. As this cell moved deeper into the cool surface air north of the front, it stopped producing tornadoes.

The RUC analysis profile at Olathe (OJC, also shown above) is interesting, showing a large veering wind profile in the lowest 1-2 km, and around 900 J/kg of CAPE. While the instability wasn't super-impressive, notice that it was bunched low in the profile, with the "fattest" CAPE roughly 13,000 ft above ground. Most warm-season tornadic thunderstorms average larger CAPE, with the "fattest" CAPE up around 24,000-30,000 ft above ground. With the CAPE in the Olathe profile "squeezed" down fairly low, that suggests more rapid movement of air accelerating upward, like a hot air balloon encountering cold air aloft and moving upward faster. This could help with upward stretching, and combined with the wind shear, might be another factor to help increase potential for a tornado in a "smaller CAPE" environment.

- Jon Davies 9-14-08


Anonymous said...
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DM said...

Great analysis as always, Jon. I put up some pictures of the supercell that was to the south of the "Eudora" one on my blog (stacked plates) I never saw that boundary while chasing, but I would have never thought that supercell would produce...

On May 22nd/23rd the supercells that fired on the dryline moving towards the WF would constantly produce, even after crossing the boundary as well...I guess 2008 was the boundary cross year!

Tell Shawna hello and I hope you guys are doing well!


LDCT said...

Looks like I missed some crazy stuff while I was chasing Hurricane Ike... I echo what Dick said, GREAT ANALYSIS!!! Justyn told me you said Hi and I just wanted to make sure I said Hi back... I saw that you got married this summer so Congratulation Jon!!! Getting married after a storm chase as to be one of the coolest things I have ever heard of!!! She must be a great lady!

I got back a couple of days ago from Galveston Island... It was a crazy experience and I have to say the island got rocked very hard... There were a few areas that looked similar to Katrina... However, we got very lucky that the eye went directly over us because if Ike would have gone 40 miles west I doubt there would still be a Galveston Island!!! As bad as it was, we got very lucky!

I hope all is well with you buddy and Congrats again on the marriage! Hopefully someday you and your wife can make it down to Starkville for a symposium... We would love to have you, and as always it would be great to see you again! You can't beat the BBQ!!! Talk to you again soon Jon!