Stair Stepping Several Supercells

August 7, 2008 by
Filed under: Summary 

LeMars to Defiance, IA
11 Jun 8

SHORT: Difficult stair-step chase. At least 5 tornado-warned, line-embedded supercells intercepted in NW/W-central IA. Several tight HP mesocyclonic wrap-ups observed including rain wrapped tornado near Pierson.

LONG: We chose FSD for lodging the night before given our good history with motels there and its likely proximity to the next day’s tightly would surface low. While munching lunch, it became apparent that the “bent back” area in close proximity to the low wouldn’t destabilize enough to give us an early show of tornadic bliss. In caravan with the “Three Dudes and a Dog” chase team, we then headed SE toward SUX to get into a narrow notch of solar heating — purifying warm-sector air just ahead of the bulging blend of cold front and dryline — where the morning’s persistent low clouds were breaking up. On the way down I-29, and through breaks in the stratocumuli, we saw towers bubbling along the boundary to our W in Nebraska – a good sign.

After a short stop in SUX for data and refreshment, we cruised NE to LeMars to stay just E of the slowly progressive boundary and its line of discrete, gradually deepening towers, some of whose convective plumes got very steeply tilted in their early stages by the strong vertical shear. Small Cb’s began to pop all along the line from our NW southward, but one especially vigorous and expanding storm very nearby to our WSW captured our attention. Right after it went up, and while it still was in this rather disorganized, multicellular stage, out came the TOR warning! I was surprised. FSD was wasting no time jumping on new development in that regard, even if little or no shear yet was evident in the low levels (via onboard GR interrogation) and the radar and visual structures weren’t yet supercellular. I called to inform them that I had a great view of this storm, and that had a rather rainy and featureless base with no rotation visible. They seemed glad for the information. Unfortunately, cell phone access got surprisingly spotty the rest of the chase, which surprised me also, since Iowa has a rather dense road and town network.

As we watched this storm grow and move nearer, a tall, lanky and very old farmer pulled up and chatted for a spell. I guess he picked me because I wore a Cowboys jersey, and he immediately professed his disdain for the Packers. 🙂 He was a Korean War vet, very fit and nimble for his advanced age, clad in overalls, his bald pate dappled profusely with splotches of variously colored melanin and a few stray gray hairs, his face leathery and deeply weather-worn from decades in the field. His wit was keen as a good knife, his tongue even sharper. He was happy to entertain me with a story of a tornado he saw “pretty near ten years ago.

Here is what he said, nearly verbatim:

    “Yes sirree Bob, that son of a b__ch came right past that hill, that rise where the road goes now. Stretched out so long the top of it was right over my head and the bottom was spinning out in the field a quarter mile away like a g____mn washing machine. That motherf___er tore the haystacks all to hell, blew up the shed and threw my tools all over the g____mn place! But I gotta admit the damn thing was fun to watch, I tell you…scared the s__t out of me when I first saw it over yonder because it was a lot bigger at first, and looked like it was coming straight at me…”

I would have liked to hear more of those passionately told, profanity strewn tornado tales from years of yore, but the forward-flank precip was spreading onto us, and it was time to bail E through LeMars and beyond. It is insightful to converse with these old folks when there’s time to do so. Hearing storm stories from the nonmeteorological perspective helps to keep a scientist well-grounded in the understanding of why we do what we do for a vocation — forecast severe weather — and as an avocation, observe and document them both for pleasure and for learning. As we parted, I advised the old soldier-farmer not to drive west. [Oh, don’t worry, I’m not about to drive into that g___mn mess!”] I also did take the time to thank him for serving this country. That’s important. It’s the least we can do for those still left from the dwindling numbers of what Tom Brokaw aptly termed the Greatest Generation.

That storm moved off to the NE along the expanding line and got better organized as we headed E to a vantage near Oyens. For a short time, it wrapped a mesocyclonic occlusion with a broad, conical, slowly rotating lowering, but soon entrained too much precip from another storm to its immediate S. That storm was a little more ragged and disorganized (looking W), but did show cyclonic shear in its short-lived updraft base.

By now the whole boundary had lit up, way off to our SW into SErn Nebraska, and was moving E as a string of at least loosely connected supercells. Our strategy by now was to slowly work SSE just ahead of the line, intercepting embedded supercells one by one until something looked interesting enough to stay with. The next one in line exhibited a broad, ragged wall cloud on the W end of a large inflow band (looking NW from near Remsen), so we kept ahead of it a short while to 1 N Marcus. The wall cloud tightened up into a weakly rotating lowering with strong rising motion, until it, too, was engulfed by rain. Onward…

As we drove SE toward Quimby, the sky to the SW grew noticeably darker, and Keith’s descriptions of a much stronger supercell, gleaned from the radar images they were seeing in their vehicle, sounded very promising. We headed W out of Washta toward an excellent vantage point located 2 N Pierson, with an intense HP circulation and suspicious, partly rain-wrapped lowering coming into view to our WSW. In wide angle views, this one actually had some structure, and the lowest part of the cloud mass began rotating very fast — tornado fast.

Even though storms were moving quickly up the line, our advance position allowed some time for Thunder the dog, who was rather unperturbed by the thundering mass of whirling clouds and rain to our WSW, to get out and get some exercise. While he was doing so, intermittent fingers of condensation — some on the ground — appeared under the area of intense cloud-base rotation (super enhanced zoom of previous image). Other spotters in the area also reported the tornado. We were unsure how long it had been going on, and how long it lasted later, since the tornadic circulation got ever more deeply buried in rain (enhanced crop-n-zoom) with time, the tornadic portion retrograding farther into the precip while the storm as a whole kept moving N (enhanced crop-n-zoom). [In fact, though confident we had seen a tornado, it took a good look a the LCD screen of my camera later that night to be positive.] The storm’s front flank rain area surged out NE of the mesocyclone and across the road to our W, and all visibility of the circulation was lost.

We skipped down the line into Ida Grove, fueling up as sirens began going off, and right before the staff closed the Casey’s gas station. At the time, no warning was in effect for that county, and it was between the paths of two storms in the line. Our best guess was that the local authorities pressed the button because of the scary dark (and harmless) clouds all about. Beneath the pall of gloom, Thunder the dog casually greeted passersby while Scott Weberpal drove up and showed us some of his tornado video from the day. Then, the chase resumed…

The next storm in the line was tornado-warned somewhere through the murk to our SW, over NWrn Harrison County. The meso looked tight and deeply wrapped, at least as intense on the .5 deg SRM presentation from Omaha as anything we had seen elsewhere, all day. We didn’t know it, but at that moment, that storm was spawning a tornado along a 14-mile path of rampage through the Loess Hills, killing four boy scouts at the Little Sioux camp. We intercepted it just S of Denison.

This was the most electrically active storm of the day, CG strikes slamming with fury all around the mesocyclone region. We saw all manner of “hangy downy thingies” — some of which appeared under the broad area of rotation, but nothing certifiably tornadic. While I was trying (with only modest success) to shot more daytime lightning shots, the Three Dudes and a Dog headed to a nearby higher vantage. Alas, neither the dudes nor the dog could see any debris or power flashes under the circulation, which (like its predecessors) quickly enshrouded itself in rain and raced off to the NE.

We briefly considered intercepting yet another wrapping circulation to our SSE, but road vectors and impinging darkness caused us to reconsider, and wisely so. We rode out the NW fringe of the storm’s core (nothing certifiably severe) n Defiance, then headed through Harlan and several heavy rain cores in the backside of the MCS, toward dinner with the crews at the Council Bluffs Applebee’s. It was there we heard about the Little Sioux tragedy. TV stations in OMA were showing an ugly mess there, with injured scouts and adults from there still are arriving at various regional hospitals. It had been a long day, a hard chase, and a sad ending — but still memorable in some good ways too. It was a fascinating event meteorologically, shared with friends, generating stories to carry for a lifetime. By the time we found a room and went to bed, we it was after midnight, and we were mentally drained and ready for a good night’s rest.

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