Some Lessons from the Sulphur Tornado Intercept

Sleeping after an overnight shift, I awakened to Elke’s informing me of a storm erupting a couple counties to the south. I had asked her in advance to do this conditionally–the condition being a storm forming W of I-35 that could cross the Interstate not far away. This was close to the Interstate, likely necessitating a more eastern approach, especially considering all the road work and likely delays around SW Highway 9 and I-35 in Norman.

When I looked through sleepy eyes at a surface map, satellite image and radar loop, those eyeballs damn-near exploded out of my head–this storm was moving off the dryline and into a tremendously moist and well-sheared environment and obviously would stay that way for another 1-2 hours, and I had to get out the door fast. More on the environment later…

As it turns out, and unbeknownst to me at the moment, the supercell already was becoming tornadic before I pulled my vehicle out of the house. I quickly fueled, picked up my intercept partner for the day (Mateusz Taszarek), and booked out of town as quickly and safely as traffic and lights would allow. Five more minutes delay at any point in the process and we might have missed everything, for we would have had to divert the long way around through Ada instead of plunging straight southward on US-177 toward Sulphur to get ahead of the main mesocyclone.

Anticyclonic forward-flank tornado–inside the core!

Every year we learn new things about the atmosphere. Out of hundreds of supercells I’ve intercepted, this is the only one with an anticyclonic tornado inside the forward-flank core! Scarier still, another few minutes and we might have driven into it unknowingly. Then again, another few minutes and I wouldn’t have attempted to go southward through an even meatier part of the core with the risk of huge hail. We made the plunge because I still could see through it to the other side, and because the obviously intense mesocyclone still would be a few miles W of the road when we passed. Below is an annotated screen capture from a RadarScope display including our location at about image time.

This is the first time I’ve even heard of a well-developed anticyclonic tornado buried inside the forward-flank core. In making the transect, we saw no evidence of it in the gray murk of heavy rain to our W, nor any suspicious wind shifts. We had NW winds W of Roff (as it turns out, on the clockwise side of the mesoanticyclone); but you’d also, often experience the same in that part of a “normal” forward-flank core. Winds then shifted to SE as we exited the forward flank. This is nothing surprising either; a violently tornadic circulation lay a few miles to our WSW.

What was surprising was to glance at velocity information a bit later and see that clockwise-spinning couplet racing away from us to the NE as we positioned temporarily in that great photographic location directly in the big tornado’s path.

Why did the anticyclonic tornado happen? We only can offer speculation and conjecture at this point, pending more detailed mobile-radar information (were they scanning northward at all?) and numerical simulations, so I’ll offer my best guess.

Conceptualize a localized anticyclonic vorticity field along the forward-flank gust front (FFGF), perhaps cast there or reinforced by negative BL vorticity from an absorbed split or left-moving updraft…then reinforced by anticyclonic shear (negative vorticity) ambient to that distance from such a big and intensely tornadic mesocyclone. I’ve seen small, brief anticyclonic tornadoes on FFGFs before. Still, even when reckoning that improbable (yet physically plausible) stack of dominoes, I’m at a loss for how that tornado survives for >12 miles while buried in the static stability implicit to 59–64 deg F (based on car thermometer) surface conditions inside that FF core!

Below are a couple 0.5-deg radar loops Steve Miller (TX) made of the process. He saved me the trouble (thanks Steve!).

Will this change intercept strategies in the future when north of a storm? Maybe. Playing probabilities, the odds are extraordinarily tiny of ever seeing another anticyclonic tornado entombed in the forward-flank core of even a violently tornadic supercell. Yet now we know it’s possible, and we must be vigilant of that. This means paying more attention to wind cues and, when available, radar velocity imagery in and near that part of the storm.

Wet wedge, debris-filled RFD

Okay, this isn’t a great surprise after the fact, in a mesoscale-diagnostic sense, given: rich boundary-layer moisture, low cloud bases (low LCL), favorable deep shear, and strong low-level shear and helicity. The supercell interacted with a vorticity-laden outflow boundary from morning storms for a long time by moving eastward at about the rate and direction the boundary itself eroded.

This is not something that can be forecast more than a few hours out, as analytic details become better-defined, because neither the synoptic operational guidance nor the high-resolution convection-allowing models (CAMs) tend to predict such features well. Some CAMs forecast a supercell in about that area (right for the wrong reasons?), but none finished before 12Z had updraft-helicity (UH) tracks characteristic of a violent-tornado-producing supercell. Regardless, the value of careful human diagnostics shines through here, and helped to motivate a well-reasoned pre-watch mesoscale discussion, then tornado watch and follow-up discussion from the SPC.

The tornado itself was a well-developed wedge when we first saw it to our SW (video capture from Mateusz), breaking out of the forward-flank region, and remained over a mile wide as it crossed US-177 north of Sulphur, based on the annotated NWS Norman path map linked here. The next two photos were shot from the first good vantage we could find outside the forward flank, a spot that also happened to be in the tornado’s path (see map linked above). This also was in a beautiful country setting with green grass, trees, a pond and wildflowers-and unfortunately, a beautiful ranch home that was hit (survey photo and damage-assessment tool image courtesy NWS Norman).

The next four images follow the tornado from our second vantage (green dot on the above map), after we got out of the tornado’s way and let it pass to our immediate NW and N.

Violent tornado(es) and disconnect between winds and damage

The storm already had produced one violent tornado before I got there, and another was underway. For the record, I refer to the first (“Katie”) as “violent” given its EF4 damage rating to one house, and the second (“Sulphur” wedge) also as “violent” due to EF5-level mobile-radar winds ~50 feet above ground, even though the highest damage level found was EF3. Official damage ratings are based on the EF scale, which is a damage scale used to estimate winds. It does contain damage indicators that extend 50 ft or more above ground, which also is where the mobile Doppler radar did its sampling in this case. The EF scale, however, does not contain accommodation for actual wind measurements at or near ground at the level of varius DIs, such as by wind instruments or mobile radar.

We can argue about the problems with this until our faces turn blue, but regardless of science or what you and I think, official policy since mid-2013 forces ratings to be based strictly off damage only, regardless of any and all other evidence. This event piled more fuel onto the flames of disconnect between tornado intensity and tornado damage that began with 2013’s El Reno OK and Bennington KS radar-EF5 tornadoes (the latter of which I also observed and photographed) that “only” produced EF3-level structural damage due to their most intense winds missing structures capable of yielding EF5 results. That happened with Sulphur.

The American Society of Civil Engineers has commissioned a group of meteorologists, engineers and a forestry expert to revisit and standardize the EF scale, which ultimately will refine and add damage indicators. It’s a years-long process. A lot of cooks are stirring that stew, including some I respect hugely. [I’m not part of that process due to lots of night-shift work and associated inability to be coherently present for many meetings.] However, the official policy of rating tornadoes based only on damage is a separate matter, and imparts a strong bias toward populated and densely constructed areas. As such, the official tornado rating is essentially a pathetic joke from a scientific perspective.

The EF Scale as it now stands also is about as useful as a $3 bill in rural areas where violent tornadoes occur, but damage indicators capable of EF5 results either aren’t present or aren’t spaced fortuitously with respect to the tiny area of a tornado that has those top-end winds. That’s a good thing from the human-impact perspective! However, it leads to unrepresentatively low ratings for many high-end Great Plains tornadoes.

A well-developed tornado between supercells!

We had a little trouble finding Highway 7 east out of Sulphur, but once we did, we noticed a new supercell well to the southeast (but conceivably reachable). This would become the Wapanucka/Atoka supercell, its first tornado causing a fatality SE of Connerville. Meanwhile the Sulphur storm charged eastward past Hickory, appearing to lose organization as a rampart of convection erupted between it and the Wapanucka storm to its SE.

We had a decision to make: go after what was left of the Sulphur storm, go home, or try to intercept the Wapanucka storm from a typically unsafe NW approach, as that supercell headed toward the steep terrain, road voids and thick forests immediately east of Atoka. Imagine our surprise, then, when this materialized from a small, ragged, otherwise unimpressive updraft several miles to our ESE, NW of Connerville and in between the supercells!

The atmosphere made our decision for us: “None of the above!” Even though the updraft was rotating, it’s a stretch to call the feature a supercell, since its radar presentation was hardly recognizable, and since the parent cloud form was so disorganized and shredded in appearance. The cloud base is uneven and scuddy, what little updraft there is very tilted and small. Imagine this “storm” with no tornado—would you then expect a tornado to form any second? Below is a zoomed-in perspective.

Coincidentally, we viewed this remarkable tube from 7 miles away at a spot along the same road and very near where I observed another tornado (in a different direction), five years minus two weeks before this. The Connerville tornado offered a spectacular, protracted rope-out.

Thus delayed by observing this tornado, and concerned by the highly unfavorable logistics of intercepting the southern supercell around or east of Atoka from the NW, we crept E on the nearest county road behind the old Sulphur storm in case it revealed any tubes from the back side. That not materializing, we headed home in time for late dinners.

The lessons of this intercept are many, but they include:

  1. Beware the possibility of an anticyclonic tornado buried inside the forward-flank core of an intense, cyclonically tornadic supercell.
  2. Exercise due diligence and great caution with any approach of a potentially or actually tornadic storm from the north (its left flank).
  3. In high-vorticity (high-low-level-shear) environments, such as old outflow boundaries. watch for spinups from any persistent updraft, even if it appears disorganized visually and/or on radar.
  4. When sufficiently interesting storm-observing potential occurs amidst a set of night shifts, have someone alert/awake you to nearby development in a potentially ripe situation (such also was the case for me on 3 May 1999).
  5. Official (EF) ratings are not scientific and don’t necessarily represent tornado intensity. I’ve known this for many years. Chuck Doswell has too, and reinforced the point in a recent entry on his BLOG. One can have violent, EF5 tornado winds without a violent EF rating.

To quote the late, great Paul Harvey: “Good day!”

[EDIT: 19 May 16 for added image links]


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