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