Thursday, May 3, 2018

Kansas tornado season finally begins: The May 1, 2018 Tescott-Minneapolis tornado

Wow... this is the first year since 1980 that a tornado hasn't occurred in Kansas before May 1 (and the first year ever that the same has happened in Oklahoma!).  But May 1 really kicked off the Kansas tornado season in a big way with the EF3 tornado on Tuesday northwest of Salina, among other weaker tornadoes across northern Kansas (KS) and southern Nebraska.

With so many storm chasers out on Tuesday, there are more photos and video online of the Tescott-Minneapolis KS tornado than I've seen since the Dodge City tornado day back in 2016.   Here are some more, but we'll use the images to look at the storm's evolution and, later on down, the change toward an environment more favorable for tornadoes during the evening hours before dark.

My wife Shawna and I left Kansas CIty around 12:30 pm and approached one of the first storms near Russell around 4:00 pm CDT (2100 UTC).  The messy training of other storms into the cells north of Russell led us to move southward to intercept a more isolated storm northwest of Great Bend around 5:00 pm, which we followed for the next 3 hours or so until it became tornadic.

Here's an image of this initially non-tornadic supercell in "HP" (high-precipitation) mode north of Hoisington, KS around 5:30 pm (2230 UTC):

We left the storm for a period to navigate around an area lacking roads southeast of Wilson, and then re-intercepted the supercell near I-70 north of Ellsworth around 6:50 pm (2350 UTC). At this time, it was developing a new mesocyclone on its southwest flank (notice the inflow cloud bands converging into this new circulation):

For reference, here's a couple radar reflectivity images roughly an hour apart showing the supercell prior to and during the large tornado west and northwest of Salina:

Moving eastward to a hill north of I-70 and Glendale, KS (west-northwest of Salina), we watched the supercell evolve into a tornado-producer:

As the mesocyclone moved northwest of us, the wall cloud began to consolidate, and dirt began to rise off the ground amidst wrapping rain streaks:

Then, this circulation began to quickly "occlude" and weaken, while a new circulation formed immediately to its east:

As this new circulation moved across to our north, a "cone" tornado touched down just after 7:45 pm (0045 UTC) in reduced contrast to the north-northeast of our location:

A lightning flash behind the tornado made it a little easier to see, contrast-wise:

Shawna's photo image shows the whole structure of the storm at this point, looking toward the north-northeast:

The tornado then widened into a dusty wedge shape around 7:50 pm, and rapid horizontal motion became visible in the ragged condensation tags around the edge of the tornado:

A wider image shows the tornado becoming darker as dirt fills it, behind the widening rear-flank downdraft (RFD):

Shawna's striking image here shows the full storm structure again, with a striated "collar" cloud visible just above the wedge tornado to our northeast:

As often happens, as the large tornado matured, it began wrapping precipitation around it's back side (the radar "hook") as we drove north toward Tescott, making it less visible:

Around 8:00 pm (0100 UTC), the tornado became invisible to us as more curtains of rain wrapped around its backside to our northeast:

Although we could not see it from our position, the half-mile wide tornado was on the ground for at least 10-15 minutes more as it moved northeast to the south edge of Minneapolis, Kansas.  I've not heard of any injuries, although several farms/homes were damaged along its 14 mile path.

As a severe weather meteorologist, it was very informative being able to watch a storm go from an environment that was not very supportive of tornadoes into one that became quite supportive during the evening.  What follows is a brief analysis of the meteorological setting and its changes.

Here's the large-scale setting from the evening 500 mb forecast (roughly 18,000 ft MSL) and 7:00 pm CDT (0000 UTC) surface map on May 1:

The tornado threat area was, as is quite typical, between "splitting" jets, suggesting an area of upward forcing in the midlevels of the atmosphere east of a large trough or "dip" in the jet stream, and north of a "capped"/subsident environment over most of Oklahoma near and south of the southern midlevel wind branching.

As was detailed in the morning Storm Prediction Center (SPC) outlook discussions, the prime environment for tornadoes was expected to be in the evening before or near dark when low-level winds just above ground increased, and before nighttime cooling lessened surface-based instability.  This can be seen in the wind profiles (hodographs) below.  The first hodograph is a short-term model forecast for Great Bend KS valid at 5:00 pm CDT (2200 UTC) just south of the supercell in its earlier non-tornadic stage:

Notice how the low-level wind shear (storm-relative helicity or SRH; see yellow areas above) changed dramatically by the time of the model forecast hodograph for Salina (2nd hodograph) during the large tornado at 8:00 pm CDT (0100 UTC).  The SRH values actually quadrupled (!) as the low-level jet intensified diurnally, approaching dusk.  Given that instability (CAPE) was about the same from afternoon to evening (not shown), it was this low-level wind shear that made the big difference in rotational support for the supercell as it moved northwest of Salina to produce the large EF3 tornado.  The lack of any interfering storms to the south of the Salina supercell (see earlier radar images) also helped this storm utilize the increasing low-level wind shear flowing into it from the south. 

Accordingly, the RAP forecast of the energy-helicity index (EHI; a composite of instability and low-level wind shear helpful in identifying areas potentially more supportive of supercell tornadoes) suggested that, compared to the afternoon environment, the evening setting would be more supportive (larger EHI values, in red) for tornadoes over central Kansas:

This was confirmed by the large significant tornado parameter and EHI values south of the Salina supercell on the SPC mesoanalysis at 8:00 pm (0100 UTC) during the large tornado:

Another factor was probably cloud base heights (lifting condensation levels, or LCL heights).  During the afternoon with initial storms westward closer to the dryline, LCL heights were much higher (mixed-layer LCL heights > 1500 m AGL, not shown) than during the evening in the more deeply-mixed moisture enviroment near Salina (LCL heights < 1000 m AGL, not shown).  This would make a difference in reducing low-level evaporative cooling that could be detrimental to tornado formation. 

Unlike some recent posts where I've talked about the role of boundaries in helping to produce tornadoes, the Tescott-Minneapolis KS tornado formed from a supercell in the warm sector away from any pre-existing boundaries.  It appears that the unobstructed evening combinations of wind shear and instability over central Kansas flowing into this supercell were simply large enough to support a strong tornado without the help of any detectable boundaries. 

The end result was one of only three EF3 tornadoes in the United States so far in 2018, and Kansas' first tornado day of the year.

- Jon Davies - 5/3/18