Over the weekend, Utah had not one but two tornadoes touchdown in far SE reaches of the state. The tornadoes developed near the UT-CO border SE of the Monticello and Blanding area on the afternoon of 9/13, touching down and dissipating multiple times. It seems mostly scrub and sagebrush were causalities of these storms, but there were some reports of damage to homes. I am not sure if these were true supercell tornadoes or the estranged sibling of supercell tornadoes - landspouts.
Many of the tornadoes in the western U.S can be classified as landspouts, which differ from the typical type of supercell tornadoes in one major way. Landspouts form from the ground up when an area of rotation/spin at the surface is stretched and pulled into the updraft of a developing non-supercell thunderstorm. Conditions favorable for landspout formation include: developing storms in proximity of a surface boundary with low-level instability (high CAPE and steep lapse rates). Supercell tornadoes develop when the the rotating updraft of a storm extends to the surface. Conditions favorable for supercell formation include: deep layer shear, unstable environment, and low-level jet/strong inflow. Typically, landspouts are weaker than supercells, topping out at an EF2 in terms of strength.
So were the tornadoes in SE Utah on 9/13 supercell or landspout tornadoes? There was a surface boundary and increased surface spin noted in the area along with relatively steep low-level lapse rates and instability present in the vicinity of storm formation, which is just the type of environment capable of producing a landspout.
On the flip side, there was a bit of deep layer shear (~25-30 kts), helicity, CAPE, and unobstructed southwesterly inflow (SW surface winds), which were all within the low-end ranges for supercell development. Additionally, the storm cell that produced the tornado/landspout was to the SE of the main surface boundary and not aligned with the region of surface spin. This lone cell could have tapped into the right environment for supercell development, or tapped into another smaller surface boundary created by outflow interactions or terrain.
Since, that area of the state is rugged and rural, radar coverage and surface observations are sparse. A composite and velocity radar image from KGJX is shown. Looking at the Grand Junction radar on the afternoon of 9/13, the cell that produced the tornado is located on the edge of the radar's range and in an area where the radar beam is scanning high into the atmosphere. Confidence in reflectivities is decreased at this range and noise and radar artifacts become a problem. Nonetheless, the radar imagery reveals a well defined cell with what looks like a notch-ish/kinked shape, as well as small but present velocity couplet coinciding with the kink. It's hard to say if this is a true velocity couplet/storm rotation or just noise, so I can't say with certainty if the radar was actually seeing the rotation associated with a supercell storm.
So was this just another landspout or a big daddy tornado? Even with all this analysis, I have a difficult time deciding with confidence, but I do feel pieces of information align more closely with a supercell tornado. In summary, my best informed guess was this was an actual supercell tornado or potentially some hybrid...
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