Chattanooga Chase

July 10, 2013 by
Filed under: Summary 

Chattanooga & Lawton OK
17 Apr 13

SHORT: Intercepted a few supercells in the FDR-FSI corridor, power flashes with one SW FSI and another near FSI, followed by late intercept of Red River storm.

LONG:

Weather and logistic setup
This day featured a somewhat classical cold-frontal/dryline scenario ahead of a positively tilted middle-upper level trough, and all the geometric funkiness that can impart to winds in the 2-4 km AGL layer. Capping looked strong enough to preclude much dryline development except right near the triple point, which was a little concerning given the direction of the flow aloft bearing a strong component parallel to either the front or a vector drawn downshear from the likely genesis zone. Nonetheless, 0-1 km shear, deep shear, moisture and CAPE all were expected to suffice for supercells in late April in good chase country of SW OK, and I was off duty. That means just one thing: out the door in search of more.

Bryan Smith and his family had a red-eye flight scheduled for the next morning, but that didn’t deter him from wanting to feast on the atmospheric smorgasbord within a couple of hours of home. We left Norman, driving through some driving rain along the way, before fueling in FSI. There we encountered Mike Foster, recently retired from his post as chief of the Norman NWS Forecast Office, who (along with us and seemingly thousands of others) was targeting a new cell that had erupted over the Red River area and slowed down almost directly atop FDR. At least Mike wasn’t chasing on his motorcycle!

Initial FDR-FSI supercell
We used Spotter Network to avoid the heaviest stream of storm-bound dots for the longest time possible, taking some back roads S and W out of Indiahoma to a point a few miles E of FDR (and well ahead of the accumulation of dots). As we waited, the storm slowly came into view sporting a fuzzy but broad base, then a flat and (at best) weakly rotating wall cloud commensurate with a decidedly pronounced d(dot)/dt around our location.

Hey, these days, with all the chasers, pseudo-chasers and wannabe locals congregating around Oklahoma storms, it’s a sad but necessary part of photographically-minded intercept strategy to anticipate storm maturity, movement changes and mesocyclonic cycles, then claim good viewing spots in those potential areas ahead of the hordes, whenever possible. Understanding and anticipating likely storm morphology, motion and behavior is helpful, when dealing with a vast majority who just whack-a-mole chase the spinny screen icon in reactionary mode. It’s a chess game, in a way; and such foresight does reap benefits in the field sometimes. When the storms and the roads don’t make other plans, it can prevent a some stress later. My most consistent trouble instead seems to be picking the “right” storm among several. 🙂

Sustaining rotation aloft was a struggle for this supercell through most of its lifespan, thanks to a combination of those wind weaknesses aloft and upshear cell mergers, but it did manage to wrap up a couple of nice visual low-level mesocyclones. The first was seen from 8 WNW Chattanooga, looking W, as a low-hanging, rotating wall cloud with strong feeder inflow on the NE side. Between that shot and the next one was about as close as a storm can come to producing a tornado, without actually doing so; such was the vigor of cloud-base rotation. It also was transient–precip from both the storm’s own hook and some annoying upshear convection soon wrapped around, undercut and weakened the circulation. Lo…gaze to the dirt-road horizon in the previous photo–there comes a train of dots! It was time to reposition downshear.

Unfortunately, it looked like the storm was going to track right over the W and N sides of Lawton. Warnings were in full effect, covering that scenario. Still, having a city in the way of a potentially tornadic supercell is highly undesirable, most importantly for its citizenry and secondarily for observational strategy. We had a short viewing window from just SW of town, whereupon the storm tried very hard once again to wrap an occlusion and produce a spinup. We’re not sure if it did in this stage, at about 1719 CDT, though a suspicious column (enhanced crop of last image) soon appeared that was either a brief dust tube beneath the center of circulation or a dense precip shaft right behind it. That was a view looking W from Coombs Rd and SW 97th St in rural SW Lawton. Poor contrast precluded visually ascertaining the rotation, if any, of that column.

Just a couple of minutes after we headed back E onto Coombs Rd toward LAW airport, and at about 1725 CDT, a power flash occurred to our NW, while: 1) we were in motion and 2) the mesocyclone took a NNE jog back into the core. I immediately pulled over and shot a few photos, including this one, of the suspicious area from which the flashes came. This was the same part of the storm into which Jared Leighton and Scott Blair filmed power flashes, at about the same time, but in their case, looking WSW from within the developed part of Lawton. Tornado or not, we were certain that damaging wind of some sort was underway inside and beneath the low-level mesocyclone and its ragged, diffuse cloud and precip features.

Soon thereafter, that mesocyclone also got undercut by its own merger-assisted RFD surge. The storm kept tripping over itself! A resulting interlude before the next potential wrap-up gave us a much-needed time span to trudge through the S side of Lawton along OK-7, amidst wailing tornado sirens, then find a short-lived, hilltop vantage about a mile E of I-44. We watched the storm get ill-defined as what was left of the main mesocyclone area moved to our N, decided it was too messy and going into a less favorable environment, and headed fo a newer supercell evident both visually and on radar to our SW, again near FDR. This maneuver took us back down I-44 and W on OK-36–an intersection through which we would pass thrice more!

Second and third FDR-FSI supercells
Visually, as seen from 6 N of Chattanooga, the second storm looked pretty in a ragged way–but also, rather undercut and surfing its own outflow to some extent. This predicament looked familiar, but we kept abeam of the storm in its inflow region, back through the I-44/OK-36 junction and right back to the high spot on OK-7 just E of Lawton.

This time, we watched from a church parking lot across the street, seeing a power flash in the northern fringes of Lawton (or near FSI) at about 1857 CDT. I took a few photos immediately afterward looking back into the mesocyclone area, and they don’t reveal much (even in ridiculous enhancement). Fortunately, Lawton was spared any substantial tornado impact for the second time in less than two hours; though some damaging hail hit the FSI area and the northern fringes of the city.

After watching this storm blow its way off to our N to join the freight train of heavy rain, we headed SW to another apparent supercell on its heels, running back through the I-44/OK-36 intersection again. Darkness was descending, and we only briefly watched this rather messy but still weakly rotating storm follow a very similar path and process to the others. Getting very hungry by now, we jumped a couple of miles back N behind this storm to a Taco Bell for a quick bite.

Grandfield supercell and return home
Pumping cheap quesadilla slices down one’s esophagus isn’t the ideal activity while seeing a raging velocity couplet bust out of the radar screen on another storm to the SW. I damn-near choked. This was a cell that was fixing to cross the Red River between Vernon TX and Grandfield, OK, a storm that apparently was getting very happy with the onset of the low-level jet (and resulting hodograph enlargement). This was easy to forecast; we just let hunger and fatigue get the best of us. In retrospect, we likely wouldn’t have gotten there in time to see either of its tornadoes anyway; but we still tried to see what was left.

Zigzagging S and W past the OK-36 exit and through backroads, some impressive structure was evident in fast-fading twilight. I wish I had stopped to photograph this. Both darkness and intervening low clouds conspired to obscure much of the storm within minutes, as we closed in, ENE of Grandfield. A navigational error of mine took us eastward one section road N of where we thought we were, and we found ourselves ahead of the mesocyclone, in the dark, on what disturbingly became a rough, dead-end pathway through grass. Fortunately, we still had several miles of leeway, and I had a high-clearance 4WD. As such, were able to double back down the section “road”, zip S promptly out of the hook’s projected path, and let the storm slide along to our N. Still, it was a valuable reminder about double-checking one’s positioning.

As for the storm, the mesocyclone area assumed a rather ragged, circular and toothy appearance to our W, NW and N, as seen via lightning flashes and assorted nearby town lights. Cold outflow, felt well SE of the main updraft base, didn’t portend a healthy future for this formerly grand supercell; and it slowly decayed to our E and SE as we cut behind it to head home via US-81 and OK-9. Bryan slept most of the way back—and needed it. We arrived in Norman just before midnight, with Bryan still having to drive up to his place in Edmond and get a few more hours of sleep prior to heading to the airport for that red-eye. I didn’t envy his predicament; but he indeed survives to this day.

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