July 19th, 2018 Central Iowa Tornadofest

Tornadoes: 6

Highest Wind Gust: ~70 mph

Largest Hail: ~1.5"

Now midsummer, and the Midwest still being in a boring weather pattern, I had little expectation of another chase for a while. Typically, there would be sporadic sleeper opportunities in Illinois and adjacent portions of other states, but these were nonexistent. At least, until models started picking up on some potential for a setup across Iowa on July 19th. This one seemed as if it could be less like a sleeper, though, and more like a potentially solid tornado setup. An anomalous pattern for midsummer was forecast to occur, with a 500mb jet impulse diving south and eastward through the Midwest (and later into the South). It was just barely nosing into the area of interest in central/eastern Iowa by 21-00z:

Although jet positioning wasn't quite ideal (and a few runs of certain models were pretty awful

with it in terms of tornadic favorability), there was at least sufficient flow established aloft.

Surface details were much more interesting. A surface low was centered over southern Minnesota, with occluded/cold front extending in a bow shape southward, bisecting the state of Iowa roughly along I-35. A warm front extended southeastward from the occluded/cold front near the Des Moines area, creating a triple point that would serve as the highest potential area for an intense supercell to develop. The tornado threat likely wouldn't be confined to the triple point/warm front, though. Winds along and north of the warm front were very well backed, with favorable thermodynamic profiles for surface-based storms. South of the front, however, winds were expected to veer with time, reducing the tornado threat if anything overcame the cap there.

Below is visible satellite imagery with surface observations overlaid shortly before initation, along with my crude drawing in of surface features. Note the presence of building cumulus towers being supported by the enhanced lift along the boundary intersection at the triple point. Additionally, note that winds are very well backed north of the warm front:

One final and very useful tool for analyzing this setup was the surface vorticity/0-3 km CAPE plot. Our group often refers to it as the "god composite," as low level CAPE is a very good indicator of whether or not a supercell will be provided with enough low level stretching to support rotation to the ground. Additionally, an already-favorable environment laden with lots of ambient vorticity can have some enhanced tornadic potential. In this case, both were very high. Especially of note is that 0-3 km CAPE values were bullseyed at 200+, double the general threshold of 100 j/kg I tend to use for anticipating more significant tornadic events.

Matt Zuro and Isaac Polanski met up with me at my house, where I was having car trouble. There was a grinding noise when I turned my steering wheel, and we were expecting to take my vehicle for the chase. I'm not very automotively literate, but eventually I figured out my power steering fluid was low. A very stupid problem that my dad and I had somehow overlooked doing basic checks under the hood. Luckily, it was also a very stupid(ly easy) fix. Matt and Isaac gave me a ride to Casey's in Granville where I grabbed a bottle of fluid, then came back home to pour it in. We were Iowa-bound.

Much to our excitement, current observations and short-term models had largely remained consistent with the scenario they'd been painting the night before. I did start getting a bit nervous as midday HRRR runs were a bit lackluster on helicity. Regardless, I was confident in raging supercells at the very least, with tornado chances still being in question. I did think some potential was there for a more significant event if everything aligned as models had projected over the previous couple days. SPC went 2% on tornado chances:

We thought SPC was sleeping a bit on the tornado threat, but we continued on to our target area with some level of confidence. We targeted Grinnell, IA, where we could eventually decide to head north or south. The warm front was just to our south, but hi-res models also popped supercells well north of the front in the backed flow, and we wanted to make sure we kept both areas in play. As we made it to Grinnell, we found a Walmart parking lot to sit and look over data. We ended up going into the store, where we messed around for half an hour, as well as bought a tennis racket and balls, along with some Rain-X. Sam Bertellotti and his mom met up with us in the parking lot, where I applied the Rain-X to the windshield and then we generally goofed off some more. My vehicle's back hatch was open, and in there was my old, useless, despised PVC dashcam mount I'd just recently uninstalled from my dash. I'd already damaged it throwing it around at home, so Isaac, not meaning to do too much harm, slammed it onto the pavement where the entire PVC rig exploded into pieces. Everyone started dying of laughter. I thought we were going to get the cops called on us at this point, and I was scurrying to clean up the mess. Little did I know that Isaac had just destroyed the final Iowa chasing horcrux, and that the state's curse would cease to exist afterward.

Deciding we probably shouldn't keep loitering in the parking lot especially after what just happened, we found our way to a park on the north end of town where, again, we messed around, but this time in a place meant for it. The sun started shining as the overcast pushed eastward, and any tiny doubts we had for destabilization dissolved. The cold/occluded front was very obvious looking at satellite imagery, as bubbling cumulus was churning all along its extent. By around 2pm, a group of towers began to develop into storms right near the triple point on the east side of Des Moines. We decided to hold off for just a couple more scans to see how it'd organize, and in the meantime got gas. Now leaving Grinnell to hop west on I-80, the still very much developing supercell went tornado warned. There was virtually no velocity signature, so we were a bit confused, then realized that if it wasn't radar indicated, it was probably confirmed. Sure enough, the warning cited a funnel report, and at the next update a confirmed tornado near Bondurant. We were still half an hour from intercept, and panicking that we missed the show. And, of course, as we got caught behind a semi going 60 mph in the left lane, reports came in of a 2nd tornado with the first still in progress. As we neared the storm, we had to core punch from the east, where we encountered quarter to half dollar sized hail as we took the exit between Newton and Colfax. I must say, the Rain-X was working extremely well as I had yet to use my wipers through the deluge and large hail.

We so far had had no visual on any base through the rain and somewhat dense tree canopy along the interstate, and we headed southward to try and gain one. A couple cells were firing just south of the main storm and one-by-one merged with the primary cell along/just north of 80. As soon as we were able to make out a cloud base through the rain, we immediately saw a stovepipe tornado descending down:

We were just ecstatic we didn't miss the show completely. We were at a bit of a distance from the tornado, but had a clear view of both it and the mesocyclone above. Wide angle shots proved to be breathtaking:

The tornado kept tearing away at the landscape for several minutes as a local in a pickup truck pulled up to us, speaking a bit to us about what was going on. We pointed to the tornado in the distance, as he was driving away from it and we assumed he hadn't seen it. He sat behind us outside his truck, taking photos with his phone as we documented with our cameras. Eventually it morphed into a thin tube as it began to rope out and become a bit wrapped in rain:

As the tornado became further obscured, we headed into Colfax where we gunned it south towards a new circulation associated with a different cell. On radar, it was little more than a small shower, but a boundary extended to the updraft base (that was well displaced to the south of its little core). There was a lowering in the base as we were still a few miles north of it in completely clear air:

It had been several minutes since we'd lost visual on the previous tornado, but as we continued southward, we found that it was still barely on the ground as a thin, fragile rope funnel that proceeded to dissolve within seconds of us regaining visual. Meanwhile, the base to our south slowly increased in motion, eventually becoming a small, pronounced, rapidly rotating lowering that was almost surely a large tube-like funnel, and maybe even a tornado at this point:

We arrived in the small town of Prairie City where, after taking a wrong turn, we emerged on a back road on the east side of town to witness full condensation just as it began to take place:

This was one of the most photogenic, graceful tornadoes I have ever seen. It was a perfect shot. I rushed to pull my clamp camera mount out of the backseat of my car, attached it to my car's roof rack, and loaded my camcorder onto it for an elevated mounted shot of this beauty. The tornado began a long rope-out process (at least relative to the duration of its life):

The funnel thinned, wriggled, disappeared, then reappeared for one final show before dissipating completely. I got one of my dream video shots from this tornado, and it's certainly a memorable one for me despite its short duration and EF0 rating. As the cell that produced this tornado merged with a primary supercell getting its act together just north of us, we made our way eastward down the gravel road network. Realizing the storm was making a dive southeastward, and that Highway 163, a divided highway that ran parallel to storm motion, was only a mile or so south of us, we made our way to the highway, where we enjoyed the 65 mph speed limit and well-maintained pavement. We made up lots of ground on the storm as the RFD filled in with rain. We noticed some mischief under the partially rain-wrapped base as we drew closer to Monroe. Emerging from underneath an overpass, a cone funnel showed itself to us, likely on the ground at this point. We had some trouble viewing it with corn fields blocking it, but the road turned due east right before Monroe, giving us a clear view down the highway to look straight at it. It turned into a large, tall, stout stovepipe for a short time:

We were completely awed by this time. It appeared that this tornado was part of a satellite circulation, as the main mesocyclone was still churning to to the northeast. Although I might be completely wrong as we had a poor view of the whole storm at once due to rain/less favorable lighting. The tornado dissipated after making one more brief attempt at touchdown. We kept blasting southeast down the highway, making more progress on the storm. As we passed Otley we began to get out ahead. There was higher contrast now with our better positioning leading to a backlit view of storm structure. A massive, very low wall cloud was present under the ugly base:

A solid inflow band soon formed to the right feeding into the circulation as it ramped up. It was at this time that there was very visible rotation through the entire mesocyclone, even as we drove at 70 mph. It was apparent something big was probably about to happen, but we were battling trees and more corn on the north side of the highway. We eventually hit better clearing as we arrived in Pella, and we immediately saw it: a large stovepipe tornado well to our northwest, somewhat low contrast due to rain. We pulled over to document the tornado as it moved closer. Rain started to clear out a bit as RFD surges became drier. The structure was dramatic, with very classic RFD cut wrapping into the meso and large tornado underneath:

Also dramatic were the colors. A teal glow originating from higher up in the storm illuminated the circulation. The dark, beastly feel in front of and underneath the storm stood in stark contrast to the sunshine and brightness behind. It was truly breathtaking. Rain cleared out even more, and the tornado drew even closer:

It morphed back and forth between a large cone and large stovepipe at this time, with the structure show still ongoing above it. Sam had pulled up behind us after finding us driving down the highway. We were speculating that this could be a violent tornado. Thankfully, it did little more than skirt structures at this point in its life, doing minimal damage. Had it hit homes head-on, I fear this tornado may have been rated in the violent category.

Through our time sitting at our location documenting the tornado, I took notice that there had been almost no left to right motion of the tornado's position. It was still far away, but I wanted to make sure of our safety. I checked radar, and the tight couplet had made a noticeable right turn directly at us. I yelled to the others that it was moving our way. We weren't going to move yet, but I warned them that we only had a limited amount of time before our safety was in jeopardy.

Occlusion was beginning to affect the tornado as it still was moving right at our location, it becoming taller as RFD carved out the cloud base around it:

Motion at the base of the tornado was rapid. Soon after, it appeared to be in a dissipating stage, as the condensation funnel lifted. There was certainly ground circulation as small debris and dust was still being lofted. Multiple vortices began to condense and dance around, briefly and periodically connecting the funnel to the ground:

A noticeable increase in debris being lofted was noted and we realized that the tornado was perilously close to hitting Pella. We didn't realize it was mostly east of town at the time, so we began to become a bit frantic about the potential for disaster. The tornado was about a mile from us, still moving at us, when we decided to move. It was during our relocation that our fears came true. What looked like entire structures were lofted into the air. This turned out to be one or more of the buildings that made up the Vermeer plant in Pella. The base of the tornado went from a thin dust swirl to a massive debris cloud. Transformers exploded as the tornado took down power lines:

Within only about 30 seconds, the tornado began to broaden out a bit, rotation slowing:

And only seconds after this, a very weak ground circulation was all that was left. It was still in progress as rain once again began to wrap into the updraft region of the storm. The tornado began to reorganize into a modestly-sized cone as a satellite touched down under the inflow band to its north:

This satellite was getting pulled into the main tornado. It didn't dawn on us until seconds before that the two would probably merge. They eventually did so, the two funnels doing a bit of Fujiwhara as they ripped each other apart:

The single circulation created by the two tornadoes became disorganized for a short period of time as turbulence from the merge sorted itself out. Fleeing down the highway, a single cone dropped once again, larger that the previous one. This was still the Pella tornado as debris was lofted from a barn being ripped apart:

The tornado finally lifted for good after a few more minutes. However, one final satellite funnel descended:

This was never officially confirmed as a tornado by the NWS, and I didn't officially count it as one until another chaser who was much closer, Devin Pitts, confirmed there was a ground circulation. Our 6th and final tornado of the day.

We kept up with the storm for a little while longer, all the way to Oskaloosa, where we decided that the storm was very much outflow dominant and we could leave it without missing anything substantial. And even if we did miss something, we were exhausted. We allowed the center of circulation to cross the highway to our north to ensure we didn't drive into a rain-wrapped tornado while getting back north to begin heading home. We didn't let the storm pass completely, though, as we ended up in the RFD. Winds gusted to 70 mph or so, and I was having some trouble keeping the vehicle in our lane. A medium-sized tree snapped up the road from us, luckily falling the direction opposite the road. As we got into Oskaloosa, the hail began. We were in a 60+ dbz core on radar, and hail to half dollar sized was pelting the car. We found a bank drive-thru where we sheltered from the hail with 2 other vehicles.

Sam said that he was back in Pella to assess some of the damage that had been done. We decided to join him and headed back toward town. What was eerie to me was that only an hour after this massive tornado event went down, nothing but blue skies could be seen where it occurred:

As we got into Pella, we discovered that the bulk of the town had been spared. We learned that the building(s) we watched get lofted belonged to the Vermeer plant on the east end of town. I felt bad becoming part of the gawker traffic jam to view the damage, but we decided to make a run by before our trip home:

Several vehicles completely splattered with insulation were pulling out from the plant, many with their windows blown out. We deduced that these must've been workers' vehicles. Water pipes were spewing over the mangled remains of the buildings.

We'd also learned about what had happened in Marshalltown, well to our north. Unlike Pella, Marshalltown was not mostly spared. The tornado went right through the business district as a large wedge. Thankfully, the tornado wasn't violent, but it still did considerable damage through town.

Despite only a 2% tornado outlook from SPC, local NWS offices, NWS Des Moines in particular, did an exceptional job in warnings. They tornado warned the initial Bondurant storm before the first tornado of the event touched down, even as velocity signatures were nonexistent. It appeared that they had a great handle on the environment and the types of storms they were dealing with. Marshalltown had an entire 43 minutes (!!!) of lead time before the tornado struck town, and a Tornado Emergency was issued as it became apparent a significant tornado was about to impact. There is no doubt in my mind that NWS DMX saved lives in both Pella and Marshalltown due to their swift and accurate warnings. It's a marvel of the science of meteorology that we can say there were zero fatalities with this event.

A total of 13 tornadoes were confirmed across central Iowa, with both the Pella and Marshalltown tornadoes being rated EF3. This graphic was put together by NWS Des Moines of all the tornadoes produced from the Pella storm (and its mergers):

NWS Des Moines also has a very good summary page of event: here.

It was a localized tornado outbreak. Not exactly what I was expecting in the morning. However, thoughts of a potentially bigger day that had crossed my mind ended up coming true.

My video documentation of tornadoes we witnessed:

SPC Storm Reports:

GPS Location History:

Human impact aside, this chase is at least tied with Carpenter, WY 6/12/17 for my #1 chase. I did much better with documentation this time, though, so it's been my biggest success so far. But with all the destruction, it made for a much less fun and gratifying experience.

Iowa and 2018 finally gave it up. Thanks to Isaac destroying my cursed PVC dashcam mount, the jinx of Iowa has been reversed forever. Or, well...at least until the next setup.