2018-06-06 Event: WY

[Dan Robinson]

A remarkable long-lived photogenic tornado occurred near Laramie, Wyoming yesterday (Wednesday the 6th). The tornado was produced by a low-precipitation (LP) supercell, was highly visible and was in progress for an hour. It also occurred outside of all SPC tornado risk areas. I would place this tornado's quality as on par with Campo, maybe better.

A post-event analysis from this case would be interesting. So far, the SPC Event page data has not yet completely populated:
http://www.spc.noaa.gov/exper/archive/event.php?date=20180606

Looking at model charts for yesterday, the event occurred with southwesterly flow aloft associated with a trough moving in from the Pacific (500mb winds of 20 to 30kt). Dewpoints ranged from the high 40s to low 50s, with a good veering wind profile in the low levels, around 1,100j/kg of MLCAPE, 30kt of 0-6km shear and a plume of steep midlevel lapse rates moving overhead. Other than the lapse rates chart, I don't see anything that particularly stands out in the environment that would have inclined me to target that area.

Any other thoughts?

Some videos:

 

[Jeremy Perez]

Something I noticed orographically is the plateau/range to the east of Laramie. Surface winds out of the SE would be running up along that range before feeding into the storm as it moved straight into that. Seems like that arrangement would've helped strengthen streamwise vorticity before it hopped the ridge. Could be completely mistaken, but it's an idea I'm stashing away for future terrain/surface flow alignments.

[edit: attempting to correct for massive image size]

 

[Dan Robinson]

Using http://www.tornadohistoryproject.com/tornado/Wyoming/map there are two past examples of long-track tornadoes within that geographic feature, August 27, 2002 and May 22, 2008. Jonathan Finch has a great write-up on the 2008 event here:

http://bangladeshtornadoes.org/UScases/052208/22may2008terrain.html

 

[Jesse Risley]

I'll see if I can find it, but I read a paper about the Cheyenne tornado back in the 1970s. There was reference to the orographic enhancement to low-level shear, essentially exacerbating the tornado potential, due to the orographic nature of the terrain immediately off of the front range. With that having been said, there are of course so many other storms during upslope season that just don't ever go on to produce tornadoes like that. I definitely think the orographic effect is a strong component, but there has to be something else at play that causes setups like yesterday to perform as it did, whereas storm in a similar area in sometimes more synoptically favorable environments don't really end up doing much more than producing some marginally severe hail.

http://www.ejssm.org/ojs/index.php/ejssm/article/viewArticle/59/80

Sent from my Pixel XL using Stormtrack mobile app

 

[Taylor Wright]

Around 7 AM morning of, I messaged a couple Facebook chats indicating I'd target Laramie if I were chasing. Perhaps dumb luck, but it burns more knowing I would've been there if I would've been chasing.

My forecast was based off of turning wind profiles with ambient effective SRH >250 and effective shear ~40 kts, easterly surface winds rapidly advecting moisture upslope with a weak, well-timed mid level shortwave ejecting over Wyoming there, and some great lapse rates. NAM also highlighted an area of weaker LSI there around 21z. Pattern recognition told me this favored a couple tornadoes in the area noting similarities to June 17, 2017 and the cyclical Cheyenne tornado producer from earlier this year. As the day went on it looked as if the cap may prevail. HRRR was forecasting only 64 surface temps over upper 40 dews, with some sort of cold pooling at the surface creating an inversion. While not obvious, a boundary appeared evident on GOES early in the morning.Surface temps were warming faster than HRRR had forecast on the cool side of the boundary. This is all based on memory so I'll need to review the surface pattern to remember what was going on here, but it appeared there was some sort of baroclinic zone sitting in southeast WY maybe due to terrain? The cap aided in holding off storms until better moisture into the lower 50s arrived. I think this all panned out perfectly for a tornadic supercell. Why it went demon and produced hour long Wray-Campo hybrids baffles me, but perhaps it interacted with that boundary nearby, and the storm was moving into rapidly improving moisture as it moved east. I hate writing everything off as "boundary magic" because there are boundaries present all the time near storms and most of them don't cause cyclical tornado producers.

Will be anxiously awaiting this to pop up on SPC event archive so I can re-analyze the surface pattern that was present and see if it indeed interacted with some sort of boundary. If so, I'd love to see some more research on the subject as these boundaries seem to be driving a majority of significant tornadoes these days in otherwise "marginal" environments.

SRH plot from NAM 3km below, which is similar to what I saw on HRRR:

 

[Jeff Duda]

Definitely a lot going on in this case. I certainly noticed multiple boundaries from KCYS well before the storm even fired. Looked like they intersected near Douglas, which is where the first few storms popped and persisted. I also noted the presence of either an outflow-induced gravity wave/bore or an OFB itself (although there was no fine-line on radar to mark an OFB if there was one) output by these initial storms that flew south and intersected the vicinity of the Laramie storm pretty close to when it fired. The signature dissipated on vis satellite before the tornado appeared, so I'm not sure if that feature had any direct impact on the tornadic nature of the storm.

The main problem with an event in this region is the paucity of observations. There just aren't many surface stations to provide actual measurements out there, and there was some discussion on Discord about the strange dewpoint obs from KLAR (a sudden massive drop in dewpoint follwed by a sudden rise into the low 50s accompanied by a burst of SEly winds). The 23Z RTMA from GSD (below)


suggests a surge of quite high moisture for that altitude (mid-to-perhaps-upper 50s at 7000+ feet) accompanied by that surge of sudden easterly winds. The winds and moisture surely could have enhanced buoyancy and shear locally, although in that particular spot, easterly winds would actually have some downslope component east of Laramie.

My guess is this event was indeed the result of local orographically induced vorticity in an environment favorable for deep, moist convection. It certainly wasn't anything that was going to show up on the synoptic scale, and I'd hesitate to even call it much of a mesoscale event (meso-beta or meso-gamma, yes, but not meso-alpha). This event was probably not very predictable, as there are likely many instances where similar conditions occur, but we don't see tornadoes like this.

Definitely a Campo-like event, but, as I recall discussing back in 2010, this was something of a 1/50 event (just because the tornado occurred outside the 2% tornado risk area in an SPC outlook doesn't necessarily imply a 0% chance of tornadoes; don't forget the general principle of continuous fields), so you'd generally have to target about 50 of these to get one of them to pay off. The last time this happened...you could argue was 31 May 2010...so give it another 8 years or so before the next such event occurs (yes, I am playing the sampling uncertainty pretty loosely here with that conclusion, so don't take it as gospel).

 

[Ethan Schisler]

Can we discuss what caused that particular supercell to look so anemic on reflectivity, but show a significant velocity couplet and even debris signature when viewing CC and SRV? I understand that it was a low precipitation supercell, but even looking at the reflectivity it was pretty far on the low end of what I would expect with an event of that magnitude.

Perhaps the terrain had something to do with blocking the radar beam and we weren't picking up as much of the precipitation? I know the radar is in Cheyenne and the tornado occurred NW of there. I remember seeing a storm in New Mexico a couple weeks back that did a similar thing, albeit not as impressive, where the reflectivity wasn't as pronounced due to terrain. Perhaps this is a similar case?

 

[Ethan Schisler]

NWS Cheyenne has completed their damage survey of yesterday's event. They rated the Larmie tornado an EF-3 and noted a satellite tornado touched down around the same time as the parent tornado's inception, rated EF-2.

Small bit of trivia: Last Friday there was an EF-3 tornado near Gilette which was the 10th one in the state on record at that intensity or stronger. This would be #11. To have 2 significant tornado events like that within a week of each other out there, just mind boggling for me.

Also of note, when looking at a couple photos on Twitter, which I won't link here because I don't own them, there appeared to be another tornado in the rear behind the main tornado. I'm wondering if this is the EF-2 they are referring to. Based on the location of it, it seems to fit the bill as they had it a couple miles closer to Laramie, than the parent (EF-3) tornado.

Here is the survey information on that: http://kamala.cod.edu/wy/latest.nous45.KCYS.html

 

[Jeff Duda]

To have 2 statistically significant events

What do you mean by that?

In response to your previous post, I looked at the other vertical radar slices and there's no reason to think beam blockage by terrain was responsible for the overall minimum in reflectivity around the time the tornado was becoming long-lived. The reasons for that include 1) there really isn't a pronounced increase in reflectivity at higher elevation angles in any part of the storm; and 2) even though there are parts of that mountain range that have terrain tops that probably skirt the bottom of the radar beam in standard atmospheric conditions, since this was happening during the warmest time of the day when the PBL was likely deep and well mixed and temperature decreased rapidly with height near the ground, the beam is more likely to be sub-refracted (i.e., bent upward more sharply than in standard conditions).

A few different perspectives on the structure of this storm during the big tornado:



Again, no strong indication of higher reflectivity higher up in the storm. Truly remarkable.

Velocity structure was also very impressive. Look how deep that column of inbounds is! Even if you examine SRV applied to the hook echo, the column persists, so it's not just translation (the storm wasn't moving that fast anyway).


Echo tops barely exceeded 40,000 ft, which is also remarkable.

 

[Ethan Schisler]

What do you mean by that?

In response to your previous post, I looked at the other vertical radar slices and there's no reason to think beam blockage by terrain was responsible for the overall minimum in reflectivity around the time the tornado was becoming long-lived. The reasons for that include 1) there really isn't a pronounced increase in reflectivity at higher elevation angles in any part of the storm; and 2) even though there are parts of that mountain range that have terrain tops that probably skirt the bottom of the radar beam in standard atmospheric conditions, since this was happening during the warmest time of the day when the PBL was likely deep and well mixed and temperature decreased rapidly with height near the ground, the beam is more likely to be sub-refracted (i.e., bent upward more sharply than in standard conditions).

A few different perspectives on the structure of this storm during the big tornado:
View attachment 17110
View attachment 17111
View attachment 17109
Again, no strong indication of higher reflectivity higher up in the storm. Truly remarkable.

Velocity structure was also very impressive. Look how deep that column of inbounds is! Even if you examine SRV applied to the hook echo, the column persists, so it's not just translation (the storm wasn't moving that fast anyway).
View attachment 17112

Echo tops barely exceeded 40,000 ft, which is also remarkable.

Sorry, I just merely meant based on the damage it created. I know that doesn't necessarily correlate to actual intensity all the time, especially considering the terrain and population out there. However I figured since that is all we have to go off of to rate intensity, it was a significant tornado event (EF2-EF5). I've edited my comment to further reflect what I meant. Sorry for any confusion

 

[Dan Robinson]

Jon Davies has posted an analysis of this event on his blog:

http://davieswx.blogspot.com/2018/06/the-awesomely-photogenic-laramie-wy.html

 

[Willoughby Owen]

Hi,

We were watching a nice dominant and anchored supercell (from a distance) in the mountains at Esterbrook / 6,516 ft ASL (west of Glendo) 90 minutes before the show started from Bosler on highway 30. Beautiful LP structure and notched a bit on doppler - would not have surprised me at all to hear of a tornado from there, but very remote mountainous region that way.

74/54 obs upstream at Esterbrook struck me as pretty amazing at 650 hPa level. Rich easterly upslope flow and timing of the shortwave kept us in SE Wyoming.

So that cell died, outflowed and probably hit the outflow of the original severe storm that developed by Medicine Bow. Again, another strong rotating signature on doppler radar. Structure had to be better than the first storm.

Oh and Ethan - about that satellite tornado. I don't know and it doesn't add up. I gave Cheyenne the photos, how long it was on the ground (30 seconds if that) and was only over an open field. So they say it did damage to a barn? We didn't see that so maybe there was another - but they used the time I gave them for that satellite. Odd.

I guess I'll do a proper report with photos soon.