Sunday, August 2, 2009

A surprise tornado in a very subtle setting - northeast Kansas on July 28, 2009.






Weak tornadoes probably occur more often than we think. If there's no one to see a tornado and it doesn't hit anything over open country, it doesn't get reported. A weak, brief, but interesting tornado was photographed in northeast Kansas near Centralia KS last Tuesday 7/28/09. The Topeka NWS office has a story about it on their web page here.

Veteran storm chaser Doug Nelson of Seneca KS happened to notice the tornado form from a rapidly rotating cloud base under a developing "shower" southwest of his shop at Centralia, and took the photos above around 1:20 pm CDT (1820 UTC). The tornado was brief and probably wouldn't have been noticed if not for Doug's observations.

In and of itself, this tornado isn't important... it was weak and no damage reported. But such tornadoes in subtle settings can help meteorologists study and become aware of ingredients that, when more pronounced, might lead to other more significant tornadoes in atypical settings.

Above, the first of two low-level radar base reflectivity images (about 1830 UTC) showed a subtle boundary as a hard-to-see ragged "fine line" oriented WSW to ENE (indicated by dashed white line). The tornado occurred with a small unimpressive echo (indicated by the white arrow) on this boundary. In the 2nd radar image above, the boundary was more evident as a line of storms fired along it, but this was well after the tornado occurred. Even at 1845 UTC on satellite (3rd graphic above), it was hard to see either the boundary or the "shower" (arrow), although local clearing was evident behind the morning storms over Missouri and extreme northeast Kansas, providing a little heat and air mass recovery.

The boundary was also hard to pick out on the 18 UTC surface map (see heavy dashed line on 4th graphic above), apart from the ESE wind at Manhattan KS and southwest wind at St. Joseph, with winds northerly at stations to the north. But, in fact, the boundary was probably a weak cool front moving slowly southeastward under an unseasonably deep trough at 500 mb (shown in the same graphic), obscured by the morning clusters of storms and outflows well in advance.

The NAM/WRF analysis sounding at Holton KS, located north of Topeka and southeast of Centralia KS, is shown in the last graphic above. This local environment estimate showed not a lot of CAPE (400-500 J/kg), but notice that a well defined "fat" area of CAPE was evident between 700 and 600 mb (about 3.5 km above ground). With this CAPE low to the ground (typical spring/summer thunderstorm soundings have the "fattest" CAPE much higher, around 6-7 km above ground), this suggests potential for rapid acceleration of air parcels in developing local updrafts, resulting in strong stretching. With the boundary, vertical CAPE distribution, and the shower right over the boundary with stretching, those ingredients are what probably came together as a local "mesoscale accident" to spin up a brief surprise tornado. Certainly not an event that could be forecast or even nowcast... a very subtle setting.

Nature never ceases to surprise!

- Jon Davies 8/3/09

5 comments:

Chad Cowan said...

Very informative, thanks for sharing.

In your write ups I've noticed the reoccurring theme of good CAPE distribution playing a key role in tornadogenesis. Have you done much research on this?

Chris said...

I've seen a number of funnel clouds and weak tornadoes here in central/northern Virginia that are a result of subtle boundaries: things like outflow boundaries, river/bay breezes, and weak fronts. Several times I've had the local NWS forecast office express serious doubts at my reports of such features, so I dutifully send them a map, pictures, and video (if available). Guess when they're not expecting spinups they don't see them!
Chris W., Stormsrus.blogspot.com

Capt. Geoffrey Spaulding said...

Great analysis- thanks!

-Groucho

Jon Davies said...

Chad:

I'm working on some database research about vertical CAPE distribution and tornado occurrence in smaller total CAPE situations, but struggling with how best to capture and express differences. Nothing close to formal yet... we'll see what comes of it.

Jon

Jon Davies said...

Chris & Geoffrey:

Thanks for your comments!

Jon