June 21 2009 chase



This looked to be one of the last hurrahs of the 2009 chase season, as a toasty subtropical ridge was in the process of retrograding toward the Plains states.  I had been

scheduled to work every substantial tornado day thus far during the Spring, with my weeks off being complete duds.  Since I had June 21 off, I was eager to take advantage of

this interesting-looking day over Iowa.  A mobile upper trough and weakly cyclonic 5H wind max, loose remnants of an anomalously deep low-latitude mid-level low that had

persisted off the southern CA coast for a couple weeks, would be the primary impetus for potential severe storms.  The upper trough would lift a synoptic warm front through

the state of Iowa during the day.  Despite warm upper tropospheric temperatures associated with an exiting subtropical feed (remnants of T.D. One-E over Baja a few days prior),

strong instability was expected in the wake of the warm front.  This would include extreme low-level CAPE in the warm sector due to evapotranspiratively-enhanced moisture

and seasonably modest capping above the boundary layer.  With weak low-level cyclogenesis, modestly-sized curving low-level hodographs (0-1 km SRH around 100-150 m2/s2)

would persist to the cool side of the warm front.  A threat for small tornadoes seemed likely here given strong boundary layer destabilization amidst low LCL heights.  This was

assuming that warm frontal convergence and lackluster background ascent was successful in initiating deep convection.  Storms did in fact form along the warm front, with a

cluster of primarily brief tornadoes fortuitously occurring in the area I chased along Highway 20 between Fort Dodge and Waterloo.  Interestingly, the parent cells were quite

small in nature, with mostly transient supercell structures--not helped by the numerous cell collisions which took place in that particular area.  This was in stark contrast to the

similar setup the day before over central/eastern KS, which produced no less than three rather long-lived and dry-side-of-classic supercells (with 2.25" precipitable water

sampled just north of the storms on the 00Z TOP RAOB that evening!).  It's possible that substantially more neutral QG forcing in the June 21 case was less supportive of

long-lived supercellular convection, but I haven't looked into this aspect of the event closely.






Initial struggling convection, and possible tornado near Williams IA


My chase partner and I initially targeted Storm Lake Iowa and got there by 1230PM.  Rapidly sorted through the latest observed/model data for about 20 minutes, and decided to re-target Fort Dodge given that low clouds were a bit more stubborn to break than previously forecast.  By the time I looked up from my laptop when finished with the data, the low stratus and fog had completely lifted and the bright sky was littered with bubbling stratocumulus.  We got Taco Johns (they really need to expand that restaurant into the southern Plains) and headed southeast.  Upon arriving in Fort Dodge, showery multicellular convection was developing on the surface warm front immediately to our west.  This convection struggled for quite some time, looking pretty "soft" as it rose through the troposphere.  The rain was extremely heavy though with such high moisture content in place.

The convection also exhibited some unusual motions at cloud base, such as pictured here looking SW.

Same cell with cascading motions on the back side, view looking E.

The convection finally began deepening and looking more vigorous, and began to produce lightning.  We followed this cell through the town of Stratford (hit by an F3 tornado on Nov 12 2005) and onward toward I-35.

A leading cell to our northeast--the origins of which we had likely been observing earlier--was then tornado-warned.  We zipped east and north in time to see heavy rain overtake and obscure its rainfree base and small lowerings near I-35.  We dropped south a bit to keep an eye on the storm we'd just left, which had developed a linear look.  Then, apparently, the northeastern storm came back out of the rain just to our north... with a strongly occluded mesocyclone visible.  This meso produced a funnel/possible tornado at 543 PM, which broke in half (as in the photo, just above the horizon) before dissipating.




Longest-lived supercell in Hardin county, with a couple tornadoes


As we continued on, a new storm cell developed ahead of us and became dominant.  I had some navigational issues around the same time due to the fact that Highway 20 in Hardin/Grundy counties had been moved south several miles, compared to what my out-of-date road atlas and Streets & Trips software were showing.  Anyway, this cell actually became rightward deviant and moved eastward just N of Highway 20 for the next 1-2 hours.  Viewed here is a healthy low-level mesocyclone from the intensifying storm, looking NNW in an area south of Iowa Falls.

An RFD clear slot began to advance from behind.  Meanwhile, several pieces of scud developed beneath the wall cloud and made contact with the ground.  I saw very little evidence of rotation and no violent motions to speak of, so am not sure this was tornadic per se.

Moving eastward on Highway 20, we watched what was a definite short-lived tornadic circulation... occurring shortly before this photo was snapped about 630 PM.

Very pretty and pronounced clear slot.

Despite the RFD having lost definition and filled with precip, another tornado developed at 636PM.  This was probably about 4 miles northeast of Owasa, IA.

The tornado lasted a few minutes and had an interesting look, resembling smoke coming up from a fire.

A fat cone funnel persisted aloft after lower-level condensation had dissipated... perhaps an evolution to a truncated cone tornado.  Another cell coming up from the southwest collided with the supercell at this time, with rain completely overtaking the mesocyclone.




Explosive young updraft produces damaging tornado just west of Dike, IA in Grundy county (video stills)


The consolidated storm was pretty messy.  Then around  715PM radar showed a new updraft exploding on its southwest flank and displaying considerable rotation.  Staying on Highway 20, this was probably too far away from us to see.  We considered moving south at Wellsburg (given the data we had, we probably should have!)... where we would have been in position to see another similar-looking tornado between Wellsburg and Holland, associated with the new updraft.  Instead we wandered eastward toward a new cell developing to our SE and heading toward Waterloo, while keeping an eye on the storm(s) behind us.  Meanwhile, cloud base rotation suddenly became apparent just to our south (740PM, as in photo).  My inexperienced chase partner was pretty excited by this feature, but it seemed rather benign to me--not associated with any particular updraft.  However, minutes later, I noticed a rock hard tower climbing overhead above the rotation.

The base produced a laminar funnel, persisting 1-2 minutes before dissipating.  Then a wildly spinning and perfectly circular cloud appendage evolved (as in picture top), and produced another laminar funnel with a ground circulation shortly thereafter (746PM).

Tornadic condensation reaching a bit lower, while the debris swirl passed behind a stand of trees.



The lower through left-hand half of this photo shows a convective inflow band that wrapped around and then up into the small updraft... quite an impressive feature considering the small size of the updraft itself.  The condensation of the tornado (at video capture top) was temporarily dissipating.

Heavy rain began falling in our location at this time.  The tornado moved quickly across both lanes of Highway 20 while gathering some dust, as in the video capture.  On the side of the road in the westbound lane (see car headlights in the video capture), a man standing outside of his car watched the tornado whiz right in front of him... in fact, I wouldn't be surprised if he was struck by the outer edge of the circulation.  I'm not particularly sure what he was thinking at the time, but he is lucky he wasn't picked up and thrown.  Hereafter, the tornado moved into the rows of trees, mowing a few of them down.

The tornado then condensed fully and developed a vigorous debris fan.  Unfortunately, the colors in the remainder of my video captures are washed out a lot... was quite a bit more vivid in person, even with the heavy rain.

Suddenly the tornado struck a hog barn.  Thick tendrils of pig manure wrapped cyclonically upward, while stark white pieces of the roof of the barn were ejected outward from the tornado's base.  All 200 of the hogs survived.  WFO Des Moines rated the damage EF-1.



The tornado and its processes were strongly reminiscent of the Shelton NE tornado of May 7 2005.

With heavy precip enveloping the tornado, a froth of wet debris continued wrapping/climbing to about halfway up to cloud base.  Surprisingly, the tornado became completely stationary at this time, and a fairly thick white rope could be seen for another 3 minutes or so before being completely blotted out by the dark rain curtains.

A review of radar confirms that this storm cell was only 15-20 minutes old at the time it produced the tornado, and only about 5 miles across.  With substantial 0-3 km CAPE in place, vigorous low-level stretching probably played a large role in tornadogenesis processes with the Dike tornado event.




Maps, weather data


00Z subjective surface analysis


00Z Davenport IA RAOB (should be strongly representative of Hardin/Grundy co. tornadoes)

Sfc T/Td: 83/74 F

MLCAPE: 2274 J/kg

MLCINH: 0 J/kg

0-3 km MLCAPE: 116 J/kg

MLLCL: 724 m

MLLFC: 766 m


20z Slater IA profiler hodograph using Hardin co. supercell motion

storm motion: 270 deg @ 21 kts

0-1 km SRH: 130 m2/s2

0-3 km SRH: 296 m2/s2

0-1 km bulk shear: 16 kts

0-6 km bulk shear: 48 kts

7-10 km SR flow: 24 kts


00z Davenport IA RAOB using Hardin co. supercell motion

storm motion: 270 deg @ 21 kts

0-1 km SRH: 129 m2/s2

0-3 km SRH: 182 m2/s2

0-1 km bulk shear: 18 kts

0-6 km bulk shear: 41 kts

7-10 km SR flow: 17 kts