June 11, 2004 Northwestern Iowa Classic Tornadoes

Meteorology:

Once in a while, everything comes together in one spot at one time to produce beautiful tornadoes. Such was the case today. Having had marvelous success in southwestern Nebraska the prior day, we didn't rest on our laurels because the same powerful upper-level system that produced the June 10 tornadoes was going to be progressing eastward into the Midwest the next day where the atmosphere was clearly going to be very moist and unstable. Moreover, most computer model forecast soundings suggested that the cap would not be particularly strong on June 11, resulting in early development of tornadic supercells. Therefore we drove to Lincoln, Nebraska on the night of June 10, arriving at 12:30 am and departing at 9:00 am. On the basis of early morning analyses and model forecasts, we had already determined that northern Iowa/southern Minnesota was the target area. However, it wasn't clear how quickly the surface wind shift/dryline would move eastward or how early convection would develop. The first key decision of the day had to be made at Omaha: do we go east to Des Moines and then north/northwest or do we head directly into the heart of northwest Iowa? An early surface map (Fig. 1) provided the answer: head northwest on Hwy 30 through Denison and then on to Storm Lake because strong low-level moist advection was underway in western Iowa, while the surface dryline was still in eastern Nebraska with several hours yet to go until vertical mixing pushed it eastward. We also expected the warm front to move steadily northward during the day due to diurnal downward mixing of strong southerly momentum from the top of the boundary layer. Thus, we anticipated that moisture would deepen steadily over NW Iowa beneath a band of strong southwesterly flow aloft (Fig. 2). Furthermore, the strong easterly component of the surface winds near the warm front would ensure that low-level shear was quite strong, favoring significant tornadoes in any isolated supercells that developed. Our early decision to take a direct route into northwestern Iowa (albeit off the interstate system) proved critical to our success on this day. While driving towards Storm Lake, a series of surface charts (not shown) displayed an impressive trend of rapid low-level heating/moistening over the target area. We could also see deep, vertically sheared towering cumulus clouds to our west and northwest as we drove through the gently rolling farmland southwest of Storm Lake. By the time we arrived at Storm Lake (noon local time) the sky was filled with crisp towering cumulus clouds, haze and stratus with a light southeasterly breeze. It also felt like a sauna outside. The 17Z surface map (Fig. 3) indicated a potentially explosive setup, with strong focused moisture convergence over northwest Iowa and very high dewpoints (mid to upper 70's F). Based on the 12Z Omaha sounding, such low-level moisture and surface temperatures in the middle to upper 80's F yielded MLCAPE greater than 3000 J/KG over the region. Furthermore, this high CAPE atmosphere was also strongly sheared along the warm front, producing a rare superposition of extreme shear and CAPE. At this point I got the "big-day butterflies" and all of my senses went on alert. We moved to a point just northeast of town and did another surface analysis, which merely reinforced what we already knew about the setup. Both our eyes and a visible satellite image (Fig. 4) showed a developing CB (cumulonimbus) near Cherokee, Iowa and our radar system quickly identified this thunderstorm cell once it developed a precipitation core. We pounced almost immediately, heading towards Aurelia where we picked up this menacing, growling storm (the growl was due to a pronounced hail roar). We followed it northeastward towards Linn Grove, watching it undergo several small RFD occlusions and even one pronounced weakening phase southwest of Sioux Rapids until the storm went crazy about 10 miles southwest of Webb. The rest of the story provided in chase summary below.

Route and Maps:

Fig. 1: 14Z surface map centered on Iowa showing the moist southeasterlies extending into far northwestern Iowa.	Fig. 2: Impressive negatively-tilted 500 mb trough (6 hour RUC forecast). Note the 40-45 kt winds from 225-230 degrees forecast to be in place over Iowa by late afternoon!	Fig. 3: 17Z surface map shows well-defined "triple point" over northwest Iowa. Note the 82/79 observation at Storm Lake (SLB)! The magenta dot shows the approximate location of the place where the storm of the day produced the long-lived tornado about 2 hours later.	Fig. 4: A 1754Z visible satellite image (downloaded in the van) showing the development of the first substantial convection (at the tip of the blue arrow).

Chase Summary:

Leaving Lincoln at 9 am, we headed to Storm Lake where surface analyses clearly showed a region of intense sustained moisture convergence ahead of an apparent mesoscale circulation centered near SUX. Driving up towards the target region, the sky was amazing – filled with rock hard towering CU ALQDS with warm front haze and stratus just to the north. When we arrived in Storm Lake at noon, it felt like a true sauna outside with dewpoints in the mid-upper 70’s. We maneuvered to the north side of town and scanned the sky. Soon, a small CB appeared to our WSW south-southwest of Cherokee and we targeted it, driving WNW to just northeast of Aurelia. We watched the storm slowly grow, the base widening and several small RFD occlusions happening. The storm underwent an apparent brief weakening stage south of Linn Grove as the leading edge of the FF core crossed the warm front. However, as we watched the updraft base about 7 miles SSW of us, a strong RFD cleft appeared in the middle of the base. Shortly thereafter, scud began RACING cyclonically around the outer periphery of the entire circular updraft base from the dryslot around the northeast side! This motion was very rapid, fluid and dynamic – like smoke. I don’t recall seeing such strong fluid motion around the entire storm updraft base – usually only within the tornado cyclone/wall cloud itself. We repositioned to the NE of Sioux Rapids on Hwy 4 and the show began. A classic tapered cone tornado materialized beneath the circular updraft base and a debris fan sprang to life. The funnel quickly condensed to earth and a classic, perfectly backlit tornado was born – dark bluish-gray-black against blue sky. We stood on the bumper of the van and watched the tornado march over a rise in the land into the farmlands to our SW. We sat in one spot for about 5-10 minutes watching the tornado steadily approach, occasionally hitting a farmstead and causing the hapless buildings to explode as if bombs were dropped on them. At some points, large chunks of debris could be seen spraying outward from the tornado for several hundred meters. As the tornado got closer, we saw Tim Samaras race south down a farm road to deploy. We repositioned east a couple more times, eventually stopping about 2.5 miles west of Webb, Iowa. We watched this amazing classic tornado cross Hwy 4 at an oblique angle (from 230 degrees) about 1.1 miles west of us and then into the fields to our WNW. Lighting was perfect the entire time as this classic stovepipe rampaged across farmland, occasionally obliterating farm buildings and trees. We got many, many close up tripoded shots of the debris fan on HD film – with all sorts of debris flying around and whole trees flying around in the air. Since the ground was wet, we had an unusually clear view of the condensation cone “penetration” into the core of the debris fan – this was a dramatic change of pace from the usual situation where the tornado moves over dusty fields that haven’t received rain in a while so you quickly get a big dust-dobber. We could hear the roar so clearly – it was one of the most audible and distinct tornado roars I’ve heard in quite some time. I get tired of hearing people post about how they saw the most incredible tornado ever but this was certainly in my personal top 5-10. We went north on farm roads in front of it to watch the rope out stage and then quickly repositioned east to watch the new mesocyclones and tornadoes. All in all, we saw 6 tornadoes, including the tapered cone tornado that occurred SSE of Ft. Dodge on tail end Charlie later that evening. We flank-jumped a while and then called it a day. Iowa was very good to us once again and we’ll certainly be back for more!

Movie Clip (c) 2004 Jim Lea. Note: Jim is a professional videographer who accompanied us during Tour 4.

Video Captures: Note; I just decided to do a "core dump" of video stills to give the viewer a good idea of how the tornado looked at various stages.

Tornado #1: Approximate time on ground - from 2:20 pm CDT to 2:55 pm CDT; Tornado #2: Approximate time on ground - from 2:40 pm CDT to 2:45 pm CDT

The route taken during the tornado intercept (light blue highlighted line); the video stopping points are shown by red dots. The tornado tracks and F-ratings are adapted from the Sioux Falls, South Dakota NWS survey. However, I believe their survey contains an error; the northeastern most tornado track occurred north of Hwy 18, not south of the highway as indicated on their survey map. This is our tornado #4.	The following video captures are of the first (and most impressive) tornado of the day. All video was taken at the first two stops indicated on the route map west of Webb along SR 374 in southern Clay County.	A second tornado forms in open farmland north of SR 374 as the big one (shown in the other images) crossed the highway (e.g., in images webb15-webb17.jpg below). This vigorous dust whirl occurred beneath the rear portion of the forward flank updraft base and is our tornado #2. This tornado passed just in front of Bill Gargan, who was chasing with us during this period and was even closer to the initial tornado as it crossed SR374.	Note the trees in the air around the debris cloud. These are mainly whole trees being lofted over one half-mile into the air!
			At the time of the last image shown to the left, we quickly repositioned to the east in anticipation of additional tornadoes. We actually tried to establish a position to the southeast of tornado #1 during its mature occlusion/rope-out stage just north of SR374 on CR N14 but a new tornado was forming in the field to our immediate north-northeast - tornado #3 - so we blasted south in an effort to outflank the new development.

Tornado #4: Precise location unknown - must've been a separate updraft west-northwest of the occluded one that produced the first 2 tornadoes. Note: Tornado #3 was not captured on video due to its brevity.

Tornado #5: about 3 miles south of Graettinger, Iowa. It took a pretty long time for the storm to produce another photogenic tornado after #4. Eventually, it did so - culminating in a white elephant trunk tornado south of Graettinger (about 8 miles northwest of Emmetsburg, Iowa. After this tornado, we continued stair-stepping north and east on the excellent county road network of northern Iowa, following the storm into Minnesota even after it became hopelessly high-precipitation. Eventually, we ended up dropping south into Iowa to intercept the tail-end charlie supercell and saw a distant truncated tapered cone tornado southeast of Ft. Dodge, Iowa (Tornado #6).

Other pictures:

The following wonderful set of images were taken by customer John S. from Ireland and all copyrights belong exclusively to him.

	All images in this set are (c) 2004 John S

Bill Gargan's pictures. Bill rendezvoused with the tour on June 9 in northeastern Colorado and remained with us until the end.