Thursday, May 8, 2008

Western Kansas landspout tornado setting 5/8/08

My friend photographer Jim Reed got some good images of a landspout (nonsupercell) tornado in Western Kansas between Tribune and Leoti near the town of Selkirk on 5/8/08 (see one of Jim's pretty shots above, along with a couple other local images courtesy of KAKE-TV). Thankfully, the tornado stayed in open country with little or no damage. The setting this day affords a good opportunity to look a typical high plains landspout setting.

Above is a surface map at about 2 p.m. CDT (1900z) showing a surface low near the KS/CO border, and a dryline boundary north-south through the low where some surface convergence was taking place. Notice, too, the surface heating axis that pointed into this boundary area northeast of the surface low, with temperatures mid-upper 70s F impinging on dew points in the low-mid 50s F east of the dryline. This type of setting with heat axis intruding on a moisture axis suggests potential for rapid storm development.

Satellite images shown above at 1:45 p.m. (1845z) and 2:15 p.m. CDT (1915z) show sunny skies and strong surface heating over western Kansas helping to explode towering cumulus along the boundary northeast of the surface low into a thunderstorm within 30 minutes or less (very rapid development). Shortly after the image at 2:15 p.m., the landspout tornado formed and stayed on the ground for about 20 minutes.

A graphic from SPC's mesoanalysis at 2 p.m. CDT (1900z, also shown above) depicts surface vorticity ("spin") and low-level CAPE northeast of the surface low in the area where the storm was forming. This graphic suggests that good vertical "spin" was available along the boundary where the storm formed, as well as a surface-baced environment with strong heating (notice I also drew in the heat axis in red from the earlier surface map). These ingredients are typical building blocks for landspout tornadoes: steep near-surface lapse rates helping enhance low-level stretching with developing updrafts directly over a surface boundary that has some good "spin" within a surface-based environment.

A modified analysis profile from the NAM/WRF model an hour or so before the storm formed is also shown above. This profile, modified temperature-wise in the lowest 1-2 km based on observed surface temperatures near the KS/CO border in the mid 70s F, suggests an excellent environment for supporting landspout tornadoes, if a storm or storms were to develop directly over a windshift boundary, as happened in this case. Notice that there was good instability (CAPE), little if any convective inhibition (CIN), and that the lapse rate in the lowest 2 km was very steep (around 9 deg C per km), similar to soundings associated with dust devils in elevated terrain on sunny days. This would allow for rapidly rising air parcels beneath the storm updraft on the boundary, providing focused stretching of vertical spin for a possible nonsupercell tornado.

A somewhat similar situation occurred in northwest Iowa on 5/2/08 with some of the same ingredients when a dusty EF-2 tornado occurred near Rock Valley. A big difference that day was the presence of strong low-level wind shear (absent on 5/8/08 in western KS at the time of the landspouts) that helped the Iowa tornado become a long track supercell tornado, in essence a kind of "hybrid" event.

Jon Davies - 5/9/08

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