It has been a slow start to tornado season in Oklahoma (OK) and Kansas (KS), very much like last year. Last year's first Kansas tornadoes were on May 1, and Oklahoma's were on May 2. But this year's first tornadoes in both states (see images above) were a week ago on April 17 near Shattuck, OK (top image) and Wellington, KS (bottom image). These appeared to be due to mainly "landspout" processes (storm updraft stretching directly over a pre-existing wind shift boundary), rather than last year's supercell tornadoes on May 1 and May 2 when plentiful low-level wind shear was present over large areas.
The surface map setting at 3:00 pm CDT on April 17 (below) showed a stationary front present as a sharp wind shift over northwest OK into the northeast Texas (TX) panhandle:
This sharp boundary could be seen on high resolution visible satellite images at early to mid afternoon:
Radar reflectivity images from Amarillo showed a new storm developing on this boundary at 3:00 pm CDT in the northeast corner of the TX panhandle (1st image below), north of other storms farther to the south:
A radar image less than an hour later (middle image above) showed the same cell exploding with a red core. At this time (3:52 pm CDT), a rope-like tornado (not pictured) was on the ground northeast of Canadian TX, while new cells began to develop northeastward into OK along the boundary. The last radar image above at 4:49 pm CDT showed a supercell storm had developed in northwest OK on the boundary near Shattuck, where two tornadoes were occurring simultaneously (see the photo at the top of this post).
Although at least a couple of the storms in the northeast TX panhandle and northwest OK were supercells (storm rotation indicated on radar, but not shown here), my opinion is that the actual tornadoes were due mainly to "non-mesocyclone" or "landspout" processes, or at least that the tornadoes were a "hybrid" event (combining non-supercell and supercell processes). This is suggested by the SPC mesoanalysis images below at 3:00 pm CDT before the tornadoes:
These graphics clearly show the stationary front wind shift boundary in the surface vorticity field (blue analysis lines and thick black dashed line on 1st image), steep low-level lapse rates from surface heating (axis of red dots, 2nd image), and no low-level wind shear (storm-relative helicity or SRH, 3rd image) over the northwest OK and the TX panhandle where the tornadoes occurred during the following couple hours. These are all factors that relate to landspout tornado formation.
It's also interesting that a supercell farther south that moved into west-central OK (visible on the last two panels of the radar images shown earlier) did not produce any tornadoes, although it was later tornado-warned based on radar. This storm was within the warm sector and away from the stationary front. But, without the sharp boundary wind shift, it had no source for low-level vorticity ("spin") as a result of the lack of SRH (low-level wind shear) over western OK during the afternoon. (Low-level wind shear helps generate low-level rotation and mesocyclones that can give birth to supercell tornadoes.)
Here are a couple more images of one of the two tornadoes near Shattuck, OK between 4:30 and 5:00 pm CDT along the boundary (probably the tornado at right in the earlier photo):
And, essentially the same boundary also extended into south-central KS as a pre-frontal trough, helping to generate the landspout tornado pictured earlier near Wellington around 6:40 pm CDT.
Again, this case seems to be a good example of how landspout processes can produce multiple tornadoes via strong low-level stretching directly over a sharp wind shift boundary with little immediate temperature contrast (see here for another more prolific case).
Jon Davies - 4/24/19
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