Saturday, August 17, 2019

Tornadoes in Kansas in mid-August! 8/15/19 in northeast Kansas

Supercell tornadoes undoubtedly can occur in Kansas in early to mid-August, but they don't happen that often, and even as a native Kansan, I've never seen one.  That changed on Thursday, August 15 when a well-established supercell (see Shawna's photo above) in northwest flow produced tornadoes southeast of Manhattan and southwest of Topeka.

Here's a map showing the approximate location of three tornadoes that occurred with the supercell pictured above in the 7:00-9:00 pm CDT time frame:
(A fourth brief tornado occurred with a separate trailing supercell after 9:00 pm CDT.)

I was chasing with James Skivers and my wife, following the southernmost supercell that developed northwest of Manhattan and moved sharply southeast across Wabaunsee County, Kansas (KS).  We missed the first brief tornado (EF0) from this supercell near the western Wabaunsee County line south of I-70 around 8:12 pm CDT when we were driving south to stay ahead of the storm through a low area with trees.  Here's Brian Miner's cool photo of the full supercell base and this first tornado, all in the same shot:

The second tornado (rated EF1 by Topeka NWS) was longer-tracked near dark, southwest of Alma, KS and northeast of Alta Vista.  The image of this tornado below is from Shawna's video, backlit by lightning looking east-northeast, with the rear-flank downdraft (RFD) and gust front visible:

A third brief tornado (EF0, below, looking north and also backlit by lightning) occurred near the time the second tornado was ending, back to its northwest under an occluded mesocyclone behind the one that generated the EF1 tornado:

A separate supercell back farther to the northwest produced a brief EF0 tornado after 9:00 pm CDT east of Alta Vista (not shown).

The satellite image below, with relevant features superimposed, shows the setting just before 7:00 pm CDT (0000 UTC 8/16/19):

Note the stationary front and outflow boundary (from earlier storms that moved into northern Missouri) close together over northeast Kansas near Manhattan and Topeka. Winds from the low-level jet (LLJ, around 850 mb or 5000 ft MSL) were impinging on and overrunning these boundaries, helping to initiate storms northeast of a "cap" that was inhibiting convection to the southwest.

Three different supercells are visible in the image above (black arrows show their southeastward motion), but only the southernmost cell near Manhattan was able to access the most unstable air along and south of the boundaries by virtue of its location and motion, probably a big factor in its ability to produce tornadoes about 90 minutes after the time of the satellite image.  The low-level jet was also increasing near dark, as is typical, enhancing low-level shear supportive of tornadoes.

The SPC mesoanalysis depiction of the effective-layer significant tornado parameter (STP) at 8:00 pm CDT (shortly before the tornadoes) showed large values supportive of tornadoes along and near the aforementioned boundaries in northeast Kansas (the enhanced energy-helicity index, an experimental parameter, is also shown):

Notice that, even though these tornado forecasting parameters appeared "favorable" over a rather large area extending north of the boundaries, the tornadoes were limited to the air mass south of and near the boundaries.  This suggests the importance of following the location and evolution of relevant boundaries in forecasting/nowcasting tornado potential, rather than just relying on parameter "bulls-eyes".

Also, here's the NAM model 500 mb forecast (winds at roughly 18,000 ft MSL) showing the northwest flow driving into Kansas, a little unusual that far south in mid-August, and the reason the supercells moved southeastward to the right of the midlevel flow:

Two shortwave troughs are marked on the forecast above (thick dashed red lines) showing a lead shortwave associated with the morning and early afternoon storms that moved from northeast Kansas into northern Missouri, and a fairly strong trailing shortwave not far behind crossing Nebraska.  Often, morning and early afternoon storms leave subsidence (sinking motion) and a "worked-over" air mass in their wake, reducing the chances of severe storms behind them.  But that was not the case here, with another shortwave immediately behind generating upward motion and intensifying low-level winds converging on and overrunning the surface boundaries.

All in all, an extremely interesting case for mid-August in Kansas!

- Jon Davies  8-17-19

Thursday, August 8, 2019

Updated map and images of May 28, 2019 EF4 tornado in northeast Kansas

It's been over 2 months since the long-track EF4 tornado in northeast Kansas (KS) on May 28 that just missed the main population of towns like Lawrence, Linwood, and Bonner Springs KS.

Some interesting video has surfaced since May 28.  I thought it would be useful to correlate some images of the tornado with a newer map I put together (above) that shows the relative width of the tornado path (> 1 mile near Eudora & Linwood) at various points from my own survey.  The smaller tornado that occurred earlier and merged with the larger one as it formed southwest of Lawrence is also shown.

The map above shows image locations (in red) by image number as discussed below, along with direction of view.

Image 1 below (from Quincy Vagell's video) shows the wet portion of the storm in southwest  Douglas County looking west around 5:57 pm CDT where a small EF1/EF2 tornado was embedded that struck Silver Lining Tours (SLT).   The new mesocyclone from which the large EF3-EF4 tornado developed about 10 minutes later is also visible north of the rain-wrapped area concealing the smaller tornado:

Image 2 below (from video by Robert Reynolds) shows this smaller EF1/EF2 tornado emerging out of the rain near Lone Star Lake around 6:05 pm CDT, looking east about 1/2 mile away.  This is the same tornado that struck SLT 2 to 3 minutes earlier:

Image 3 below (from video by Dalton Coody) shows the large tornado shortly after forming southeast of the town of Lone Star (in Coody's video, it is only visible briefly before rain wraps around it).  This was at roughly 6:10 or 6:11 pm CDT, after the merger of the smaller tornado with this larger one as it was developing:

Image 4 below (from video by Jack Miller) shows the HP supercell at about 6:12 pm CDT looking southwest from the south side of Lawrence.  The tornado, visible in the Coody video, is hidden by rain wrapping in from the southeast (a wet rear-flank downdraft or RFD):
Image 5 below (from video by Matt Grantham) jumps ahead to near Linwood KS around 6:35-6:40 pm CDT, looking west along Highway 32 west of Linwood.  The tornado was about a mile wide (!) at this point, but somewhat visible even though still wrapped up in rain:
Image 6 (from video by Cybil Walters) is from a housing development north or northeast of Linwood, and shows suction vortices (smaller intense "swirls" within a larger tornado) clearly visible.  This was around the time of EF4 damage near Linwood, and suggests why some houses north of Linwood were totally demolished, while others were still standing with mainly roof and outer wall damage:

Image 7 (from video by Matt Grantham again), looking southwest from northeast of Linwood around 6:40-6:45 pm CDT, also shows the tornado at about the time of EF4 intensity, although it is still somewhat difficult to see from some rain-wrapping:
The more I look at the path of this large tornado, the more I'm struck by how very fortunate it was that it did not directly hit any population centers, including the Kansas City metro area!

The images in this post also show how difficult it was to see this tornado (and the smaller one before it), as they were both associated with a large high-precipitation (HP) supercell.

Spotters and chasers BEWARE such storms, especially when they are moving faster than 30 mph... give them a very wide berth!

Thanks to Robert Reynolds, Quincy Vagell, Dalton Coody, Jack Miller, Matt Grantham, and Cybil Walters for their video images, and thanks to Rick Schmidt and Eric Lawson for pointing me to additional documentation and information.

- Jon Davies  8-8-19

Monday, July 1, 2019

A surprise photogenic tornado in southwest South Dakota on June 29, 2019

Similar to the unexpected and stunning Laramie tornado last year (see here), June 29 saw a striking and widely-photogrpahed tornado (photos above) that lasted around 40 minutes west of Martin, South Dakota.  It occurred mainly over open fields, and so was rated only EF1, although it may have been more intense.

The Storm Prediction Center (SPC) had outlooked a 5-10 % chance of tornadoes over parts of North Dakota on Saturday, but nothing over southwest South Dakota (SD).  Why?  The reason is that this tornadic storm was not a typical tornadic supercell storm, and like the Laramie tornado, was difficult if not impossible to forecast.

Looking back at Saturday afternoon June 29, there were certainly doubts about whether storms would even develop over SD - see the model radar simulation forecasts for mid-afternoon below:

Given these model radar forecasts and very high cloud bases expected (around 2500-3000 meters above ground, not shown) along with little if any low-level wind shear, no forecaster would anticipate a long-lived tornado hours in advance within such a setting.  Tornadoes from purely supercell processes typically occur with cloud bases under 1500 m and at least some low-level wind shear; it is unusual for tornadoes to occur when temperatures are near 100 deg F, which guarantees high cloud bases.

How could such an environment support a long-lived tornado?  Some important ingredients can be seen coming together just before the tornado occurred. 

Here's the surface map at 3:00 pm CDT, roughly 30 minutes before the tornado.  Notice the northeast-southwest boundary/trough over southwest South Dakota:

A storm or two did begin to form on this boundary shortly before 3:00 pm CDT (see satellite image below) with strong surface forcing and convergence on the boundary.  The very hot temperatures (around 100 deg F, see heat axis on map above) also helped to break through a stout layer of warm air aloft (700 mb temperatures > 14 deg C, not shown) to initate a storm.

SPC mesoanalysis graphics at 3:00 pm CDT also showed heating and steep low-level lapse rates poking into southwest SD along the boundary, along with large surface vorticity (light blues lines) along the same boundary:

This combination of strong low-level lapse rates and vorticity (slowly "spinning" air) along the boundary apparently set the stage for rapid vertical stretching beneath any storm updraft that could form and establish itself directly over the sharp boundary.

Typical supercell tornadoes require significant low-level wind shear (a rapid change in wind speed and direction with height) in the lowest 1 or 2 km.  But this was definitely not a typical case, as there was very little low-level wind shear (storm-relative helicity, or SRH) over southwest SD on SPC mesoanalysis graphics at 3:00 pm CDT:

However, the storm updraft essentially "locked on" to the sharp wind shift boundary to provide the necessary vorticity ("spin") to stretch vertically into a tornado, in lieu of significant SRH/low-level wind shear (again, note the large surface vorticity along the boundary on the earlier SPC graphic).

In the 2nd panel of the graphic immediately above, also notice that there was just enough deep-layer vertical wind shear (25-30 kt) to support a supercell storm.  So, instead of only brief updraft stretching from steep lapse rates along a boundary, the storm became more organized and long-lived through interacting with this deep-layer shear.  Indeed, it took on supercell characteristics with a hook-shaped echo and clear rotation detectible from a distance on radar (see radar inset below):

Thus, the tornado was able to keep going for quite awhile.  With the contribution of non-supercell processes (stretching of boundary vorticity in a steep lapse rate setting) and not a lot of rain falling from the storm (see photos at top), the high cloud bases and potential evaporative cooling did not seem to be a negative factor.  The tornado was also quite visible from a distance due to these same high cloud bases.

Because this setting combined both non-supercell ("landspout") processes with supercell characteristics, I would call this a "hybrid" tornado event.  WIthout the boundary and the storm directly linked to it, the tornado probably would not have occurred in the absence of notable SRH.  But with enough deep-layer shear to support a supercell, that was also part of the "recipe".

Farther north (not shown), tornadoes were expected over North Dakota on June 29, bur did not occur.   This was probably due in part to the slight cooling influence of an outflow boundary from morning storms.   Without as much surface heating, a strong layer of warm air aloft (temperatures around 14 deg C at roughly 10,000 ft MSL) helped to "snuff out" evening storms that managed to form over south-central North Dakota before they could really mature and take advantage of increasing low-level shear/SRH as nighttime approached.

Believe it or not, another tornado from processes similar to those on Saturday formed again on Sunday afternoon (June 30) over south-central South Dakota near the town of Burke, along yet another sharp boundary with strong heating and ingredients lining up enough to support a tornado:

But to reiterate, such tornadoes are almost impossible to forecast hours in advance because everything has to come together just right, and typical supercell tornado forecasting factors (such as SRH and lower LCL heights) don't work with these type of "hybrid" tornadoes.  Yet when the proper ingredients do come together, these "hybrid" tornadoes can be quite spectcular visually!

- Jon Davies  7/1/19

Wednesday, June 5, 2019

A merger of two separate long-track tornadoes in northeast Kansas on May 28, 2019 !

I wasn't planning on posting anything further about the Lawrence-Linwood tornado and the Silver Lining Tours (SLT) encounter with a separate rain-wrapped tornado, thinking enough has been said already.

However, the National Weather Service in Topeka today released an addendum to their original survey identifying and adding a new tornado (EF2) that was previously unreported in southwest Douglas County, Kansas.  This was the rain-wrapped tornado that struck SLT, rolling two of their vans, and was located at the time of the incident within the rain-filled rear flank downdraft (RFD) of the developing mesocyclone to its north that gave birth to the large Lawrence-Linwood tornado.

As I suggested in my prior post, this additional tornado was not a satellite tornado, but a separate tornado that had not yet been confirmed or reported previously.  It ended up merging with the developing large Lawrence-Linwood tornado, a complex and fascinating evolution and interaction.

Here's a map I constructed by putting together NWS Topeka and NWS Kansas City survey maps of the two tornadoes.  This shows the full tracks (nearly 43 miles total) and where the two tornadoes merged:

Roger Hill was kind enough to share his video with me privately, shot while SLT was retreating to the south away from the developing Lawrence-Linwood mesocyclone and what they thought was away from danger.  With Roger's permission, here's a panoramic image I put together from his video as they were pulling away to move south:

You can clearly see the visual mesocyclone to their west-northwest (at right), also the focus of several other chaser videos online.  But also notice the advancing rain-filled RFD coming up from their south (center of the image) that unbeknownst to them contained a hidden and unreported tornado that no one yet knew about.

Most spotters and chasers are taught to stay southeast of northeastward-moving mesocyclones (areas of organized rotation in supercell storms) to stay safe.  So, SLT thought they were doing the careful thing to head back south to Highway 56 and follow the primary mesocyclone from a safe distance.

Complaints and accusations online (Facebook, Storm Track, etc.) have focused on the question, "What was SLT even doing in the bear's cage?"  Well, that question kind of misses the point when the room becomes dark and you don't even know an additional bear's cage is there after you're already steering clear of the main bear's cage that you know is behind you.

The tornado merger in this case was very similar to the Hesston-Goessel, Kansas tornado merger I studied on March 13, 1990 that led to an EF5 tornado:

The difference is that the 5/28/19 merger took place hidden in rain within an HP storm, while the Hesston-Goessel supercell was a classic non-HP storm and very visible.  In fact, we might not have even known about this merger had SLT not had their encounter!

To my knowledge, this is the first time a merger of two separate medium or long track tornadoes has actually been documented within an HP supercell, although similar mergers have undoubtedly occurred but not been documented.  This case certainly deserves further study from several perspectives.

Thanks to meteorologist and severe storms expert Greg Stumpf for being astute enough to recognize the similarity to the 1990 Hesston-Goessel merger/interaction, and mentioning it on Facebook already several days ago.

- Jon Davies  6/5/19

Sunday, June 2, 2019

Radar data and the Silver Lining Tours incident 5/28/19 south of Lawrence, Kansas

Roger Hill and Silver Lining Tours (SLT) have been getting a bunch of heat on social and other media for two of their tour vans getting rolled during last Tuesday's tornadic storm south of Lawrence (see my post about the weather setting here).

I'm certainly no radar expert, but based on radar data I went through carefully on Saturday, I do think everyone needs to chill a bit before passing judgement about Roger and the tour group being "too close".
A discussion on Storm Track forum has been looking at the early stage of the Lawrence tornado's track via Topeka (TWX) radar, and there was definitely some complex evolution going on with the storm.   This data does suggest that the main tornado was developing a couple miles to the north of SLT's position around the time they were struck by strong winds or some sort of 2nd rain-wrapped circulation coming up from the southwest in an unusual location, which matches statements by Roger.

I also looked at radar data saved from the Kansas City radar (EAX, a little farther away), and can make out two separate circulations at 6:00 pm CDT in the storm-relative velocity field:

This was just before the large tornado developed at 6:05 pm CDT, with the 2nd smaller circulation visible closer to Highway 56.  This 2nd circulation was located about 2 miles (check the scale on the graphic) to the south or south-southeast of the developing large tornado.

Base reflectivity at the same time (below) showed the large developing tornado circulation to be within some rain, with the 2nd circulation located southward within the wet rear flank downdraft (RFD) that was beginning to surge east with this large HP supercell:

Discussion on Storm Track has highlighted this additional circulation to the south of the axis along which the large tornado formed, based on radar images and other information.  Here are radial velocity images from TWX (courtesy of Jeff Snyder on Storm Track) at 5:56 pm, 6:00 pm, and 6:04 pm CDT.  I've marked the locations and tracks of the two circulations.  Notice that it appears the southern circulation moved right over the location where the SLT incident occurred, which lends credence to Roger's description of the incident (click on the image to view it larger):

Dan Robinson on Storm Track put together a graphic suggesting the evolution of the two circulations discussed above.  Here it is, a rather complex evolution prior to the start of the large tornado, with a red "X" marking SLT's location:

Quincy Vagell was shooting video at about 6:00 pm CDT from near the location of the SLT incident,
looking toward the north-northwest:

Notice the lowering visible to the northwest (in spite of rain), which is probably the developing circulation and feature that spawned the large tornado at 6:05 pm, matching what Roger and SLT say they were watching from the southeast.

Quincy describes on Storm Track a "bluish" rain curtain that was moving up from the southwest behind him and to his left.  This was probably the wet RFD containing the small embedded circulation closer to Highway 56 that was not visible, and may have been what hit SLT.

To give some scale and context, I'll mention that my wife Shawna and I watched last year's Tescott, Kansas EF3 tornado on May 1, 2018 (a "classic" non-HP supercell) develop from a distance of about 2 miles to our northwest and north:

This was a similar distance and position to what Roger and SLT had on 5/28 relative to the lowering that they could see to their northwest, which appeared to spawn the large Lawrence tornado a few minutes later.  But, with the wet RFD curtain coming up from the southwest, there was no way to see the 2nd circulation coming with it (from an unusual location within the storm) as they drove back south toward Highway 56 in order to head east and follow the storm from what they thought was a safe distance.

As I mentioned in my prior post, my wife Shawna and I got caught in a somewhat similar surprise situation (strong surging wet RFD winds from the southwest, south of a large tornado) with the El Reno storm in 2013, even though I have years of experience chasing storms as a meteorologist. Luckily, we managed to drive out of the RFD winds without incident.

I think what has sparked accusations of being "too close" in this situation is the characterization of the 2nd / southern circulation shown in the radar images above as a "satellite" tornado, something that implies being really close to a larger tornado.  In fact, this 2nd circulation was separated from the developing main tornadic circulation by at least 2 miles or more, until it moved toward the developing large tornado after striking SLT.  As such, it seems part of a complex evolution that gave birth to the tornado, and not a true "satellite" tornado.

I talked to Roger briefly on Saturday, and know that he is very shaken and more than a little confused by the evolution of what happened last Tuesday.  I think we should all give him the benefit of the doubt on this event.

He and I also talked about swearing off HP storms in the future.... they often surprise and do the dangerously unexpected.

Looking at the bigger picture, maybe spotter and chaser training should start focusing more on large HP storms as a special case with elevated dangers, emphasizing the possibility of wet RFD surges and dangerous winds as an additional hazard south and some distance away from a rain-wrapped tornado.  And, maybe training should teach and emphasize, for safety's sake, that spotters and chasers give large HP storms a much wider berth than what would seem necessary with storms that are more "classic" in nature.  Large tornadic HP supercells are complex and visually challenging, even for experienced spotters and chasers.

Shawna and I are incredibly glad this did not turn out to be another deadly situation, and that everyone with SLT survived.  That’s what's most important.  Just my two cents worth…

- Jon Davies  6/2/19

Thursday, May 30, 2019

A violent tornado close to home: The May 28, 2019 EF4 Lawrence-Linwood, Kansas tornado

The third violent (EF4 intensity) tornado in the U.S. so far this year occurred on Tuesday May 28 near Lawrence, Kansas (KS) and west of Kansas City, Missouri (MO), not far from my home.  It was the first violent tornado to affect the Kansas City metro area since 2003, and sent my wife Shawna and our cats to the basement for the first time ever.

We were on a 3-day Memorial weekend trip to the High Plains of Colorado (our first multiple day chase trip in over 3 years), but awoke Tuesday morning in North Platte, Nebraska to find Kansas CIty under the gun.  So, instead of chasing north-central Kansas, we rushed back to Kansas City to check on family and cats, and found ourselves in our basement taking shelter not long after we got home.

The track map below shows how the tornado would have come close to our house had it not dissipated near Bonner Springs, KS.  We're extremely grateful that the tornado did not directly affect us or family members, but are sad for those near Lawrence and Linwood, KS (see photos of the tornado near those locations above) who lost property and homes.  Good warnings and media coverage undoubtedly saved lives, but 18 people were injured.

I've had several people ask me why the tornado was violent and long-lived (nearly 32 miles and just short of an hour on the ground).  The surface map at 4:00 pm CDT showed a stationary front (reinforced by earlier storms) draped acoss northeast KS and northwest MO, with the Lawrence-Linwood supercell taking shape near Emporia, KS (circled "S"  on map below), in the warm sector well south of the front:

Computer model forecasts from the morning suggested that low-level winds and the low-level jet would strengthen over eastern KS during the day, nearly doubling in speed between noon and 7:00 pm CDT on NAM forecast graphics for 850 mb (roughly 5000 ft MSL):

When we arrived home at late afternoon after passing through the front from north to south, we noticed that winds were strong from the southeast, which tended to confirm the model forecasts.  This in turn increased low-level shear, which combined with instability resulted in increasing energy-helicity index (EHI) values during the afternoon, supportive of significant tornadoes (compare the RAP model EHI forecasts for noon and 5:00 pm CDT below):

Typically, the low-level jet is at a diurnal minimum in speed at late afternoon in late spring. But on May 28, a strong upper system (see the NAM 500 mb upper air forecast in midlevels for 4:00 pm CDT below) was lifting northeastward through the Plains.  The increasing low-level jet and 850 mb flow was in response to this strong dynamic system:

Going back to the surface map earlier, notice the backed southeast winds at Topeka (TOP) and Olathe, KS (IXD).  That further increased the low-level wind shear ahead of the soon-to-be tornadic supercell, and likely helped it to produce the large tornado long before it interacted with the front farther north.

The HRRR model forecast of the fixed-layer significant tornado parameter (STP) also suggested strong support for tornadoes at late afternoon over northeast KS:

A dangerous aspect of this violent tornadic supercell was that it was very HP (high-precipitation) in nature, wrapping the large tornado in rain, making it difficult if not impossible to see.  The views of the tornado at the top of this blog post are from the north or northwest looking toward the south or southeast, where the back edge of the tornado was sometimes visible.  However, from the east, northeast, or southeast, a large rain-wrapping shield hid the tornado, as in this view toward the west or southwest from Highway K-10 east of Lawrence:

The experience Shawna and I had with the El Reno tornadic supercell back in 2013 has forever made me very leary of HP supercells, so much so that I now always put lots of distance between me and such storms.  In fact, I tend to avoid chasing them.  That's because HP supercells are so difficult to view and assess both storm movement and structure, not to mention any tornado, which is truly frightening.

A related dangerous factor of HP storms is the rain-filled rear flank downdraft (RFD) that often surges and wraps around the tornado from the south and southeast.  On the radar reflectivity images below just after 6:00 pm CDT, notice how the wet RFD surges eastward on the south and east side of the tornadic circulation.  If trying to stay with the storm by stair-stepping eastward and northward, it is not unusual to have this surge overtake you, as can be seen along north-south Highway 59 south of Lawrence in the images below:

My experience in the field is that rain-filled RFD surges are often strong wind producers (apart from any nearby tornado), as with the 2013 El Reno tornadic storm where we nearly got blown off the road retreating south of the intensifying and rapidly expanding tornado.

I've seen a couple videos of chasers following the May 28 Lawrence-Linwood tornadic storm way too close, hooping and hollering while surrounded in blinding rain and seemingly oblivious to the danger near them.  I used to do stupid stuff when I was younger 25-30 years ago, but experience and maturity in recent years has taught me to give these storms a very wide berth, even if it sacrifices the chance to view a large tornado.  I fear that more chasers will get killed in future years on HP-type storms of this nature.

It is truly miraculous and awesome that no one got killed with the May 28, 2019 Lawrence-Linwood tornado.  That is something to celebrate.

- Jon Davies 5/30/19

Monday, May 13, 2019

Boundaries aiding or inhibiting tornado potential: Examples on May 5, May 6, and May 7, 2019 in the Plains

A week ago, May 5 - May 7, 2019 saw several supercells and tornadoes over the central Plains (for example, see images above by Tony Laubach on May 6 after dark in central Kansas ).  Thankfully, there were no serious injuries from any of the storms.  But boundaries on these days certainly had some influence on whether particular storms did or did not produce tornadoes.

This post will look at boundary details that likely contributed to certain storms being tornadic or non-tornadic on these three days, using surface, satellite, and radar data.

First, on Sunday May 5, an outflow boundary was leftover from morning convective showers over central and southern Kansas, seen on this noontime visible satellite image:
By mid-afternoon, this boundary was still visible over central Kansas, along with a dryline wind shift boundary over southwest Kansas that was marked by newly-developing cumulus (also see the 4:00 pm CDT surface map below):
By 5:00 to 6:00 pm CDT, storms had developed from Kansas to west Texas:
It's notable that the storm/storm cluster in central Kansas near the old outflow boundary on May 5 produced tornadoes off and on for at least a couple hours as the cluster moved southeast near this boundary, including this EF1 tornado northwest of Hutchinson, Kansas:
The SPC mesoanalysis depiction of the significant tornado parameter (STP) at 5:00 pm CDT suggested that factors supporting supercell tornadoes (such as wind shear and CAPE) were maximized somewhat in central Kansas, probably due in part to this boundary:
It's interesting that the morning HRRR model forecasts (not shown) did not show any storms developing along this boundary.  The tornadic storms on May 5 in central Kansas near Great Bend and Hutchinson probably would not have occurred without the remnants of the morning outflow boundary enhancing convergence and local wind shear.

A landspout also occurred on May 5 northwest of Dodge City between 5:00 and 6:00 pm CDT, as storms developed along the sharp wind shift boundary in southwest Kansas seen on the satellite photos earlier, where surface heating and CAPE intensified along the vorticity-rich boundary.
A large dusty tornado (EF2) also occurred with a supercell in west Texas south of Lubbock (image below), within the warm sector and not associated directly with a boundary.  This was the strongest and longest-lived tornado of the day, and formed ahead of  a dryline bulge within an environment of enhanced STP values in that area (see earlier STP graphic).

The next day (Monday, May 6), the 7:00 pm CDT surface map showed a slow-moving east-west front over central Kansas that initiated storms during the late afternoon and evening:
Between 6:30 and 7:00 pm CDT on May 6, storms on radar were ongoing in central Kansas, including a cell that was tornado-warned near McPherson (MPR, south of Salina/SLN).  Note the blue "fine line" on the radar-image below, located just _south_ of McPherson and indicated by the small white arrows:
This strongly suggests that the tornado-warned storm was undercut by cold air north of the front, which is probably why the storm did not generate a tornado, although it did produce large hail (2"+).  A photo of the McPherson storm also visually suggests it was occurring atop cold surface air, with lines of scud clouds visible near the ground and a laminar shelf forming:
Another tornado-warned storm was located farther west, northeast of Dodge City (DDC), also just north of the surface front (dashed line below) with cold air likely undercutting it, as suggested by the frontal "fine line" position (not shown) from the Dodge City radar:
As the evening went on, the boundary slid southward through Dodge City (still visible as a "fine line"), but then slowed down as storms consolidated into a significant supercell just northeast of Kinsley, Kansas after 9:00 pm CDT:
Notice that the boundary east of Dodge City (as marked by the dashed line above) had gotten "pulled" back north into the strong supercell northeast of Kinsley so that the storm could access the unstable warm sector air mass, instead of being undercut like earlier with cool post-frontal air.  That likely had something to do with the supercell producing a tornado after dark east-northeast of Kinsley shortly before 10:00 pm CDT (see images at top of this post):

The SPC mesoanalysis depiction of STP at 9:00 pm CDT showed an environment supportive of tornadoes feeding into this supercell from south of the boundary:
It's also interesting that, farther west,  a tornado occurred around 9:20 pm CDT at the location where the aforementioned boundary hooked into the approaching squall line west of Dodge City (see the 9:16 pm CDT radar image up above).  This boundary intersection would be a favored location for a tornado from an embedded circulation at the intersection point.

So, it seems that the boundary positioning on May 6, depending on whether a particular storm had access to warm and unstable surface air or was located over the top of cold post-frontal surface air, had much to do with tornado potential.

I don't have much room here to go into Tuesday, May 7 in the Texas panhandle.  But it is worth noting that a stationary front oriented southwest to northeast appeared to have much to do with supercells' ability to produce or not produce tornadoes that Tuesday afternoon.  Storms immediately north of Amarillo at mid-afternoon appeared unable to produce tornadoes when they moved north of a fine line (not shown) marking the frontal position and colder surface air.  However, some storms located more to the northeast of Amarillo were able to access warm and unstable surface air due to their location just south and east of this same boundary:
SPC's own Roger Edwards was chasing near Amarillo that afternoon, and noted on his Twitter post to #txwx the cold air coming out of the storm he was chasing just north of the boundary, indicative of the coldness of the surface air associated with the boundary:
Here's the SPC depiction of STP at mid to late afternoon, suggesting a supportive environment for supercell tornadoes over much of the Texas panhandle, but not doing much to indicate where the surface frontal boundary and cold surface air were truly located: 
This is why it is so important to keep track as much as possible where relevant surface boundaries are located when considering which storms are more likely to produce tornadoes.

Finally, an aside on a different subject... images (below) of the EF2 tornado on the west and southwest edge of Lincoln, Nebraska on Sunday, May 5 show how difficult to classify some tornado events are.  Although radar images depicted a rotating supercell (below), images of the tornado (also below) look more like a gustnado along a storm gust front:
This high-based tornadic storm formed just ahead of a slow-moving east-west front where SPC mesoanalysis graphics indicated steep low-level lapse rates (below).  This suggests the tornado might have been some kind of a "hybrid event" where a front flank mesocyclone and the storm's gust front got together in some way to produce the tornado.

In summary, May 5, 6, and 7 had some very interesting settings for tornadoes in the Plains, and provided some excellent examples of how boundaries can sometimes help and other times hurt tornado production.

- Jon Davies  5/12/19