From: "Gilbert L. Sebenste"
Subject: Chase Summary, 7/29/96: Gotta love the Iowa mesonet! ----------------------------------------------------------------------- It didn't look all that impressive early that morning. A dreaded MCS had scoured out the instability from northeast Kansas, northwestern Missouri, and southern Iowa. That should have helped push a slow moving cold front well to the south, significantly reducing the threat of severe weather. However, unseasonably cold temperatures aloft and dewpoints in the low 60's along with temperatures around 80 would ensure popcorn instability thunderstorms that afternoon. However, things began to change rather quickly very early in the afternoon. As I got off work and into the NIU weather office, I pulled up a 19Z (2 PM CDT) plot of the Iowa mesonet. It revealed some very interesting things... A surface low had developed in central Iowa, to the southwest of Fort Dodge. This was causing winds to back across central Iowa, and winds were already south at Des Moines! The low, I surmised, would develop further as PVA aloft ahead of a vort max with a 500MB trough overhead would help spin up the low. That would back the winds more, causing stronger point convergence along the front, which was now moving back north across Iowa...along with 70 dewpoints as well! Unfortunately, I was running out of time. It was already 2:40PM! I called Mike Falout, who had the day off, and he agreed to the last minute chase. We hit I-88 west out of DeKalb at 3PM, and headed for Davenport, Iowa on I-88. When we reached Sterling, we heard SPC had issued a tornado watch. We listened to the NOAA weather radio station in Davenport for the coordinates. The watch was HUGE! 160 miles east and west of a line that covered the entire state of Iowa and northwest Illinois. What was going on, Armageddon? Inquiring minds were extremely curious to know!!! Well, we saw a storm off to our northeast that was putting down heavy rain, a "pulse" storm whose top kept rising, then collapsed over and over again. Deciding that this was moving into more stable air in Illinois, we chose to ignore it. We saw some cirrus off to our west and northwest. But Davenport was saying nothing was going on. OK. We'll stop by the Davenport NWS office in eastern Iowa, but only for a quick radar look if they would let us, and then we'll be off! We arrived at 4:45 PM and were greeted warmly by Mike Boehmke, Tom Phillip and a host of others who were watching the fun. They showed us a plot of the Iowa Mesonet and the PUP so that we could see what was going on, both at the DVN site and at Des Moines. We were dismayed to learn that over the past two hours, the surface low and the associated convergence had re-developed into SOUTHWEST Iowa, winds had backed in the Des Moines area and supercells and one tornado were already under way! We were three hours away from the action, and we knew we couldn't make it in time. Now what? Well, winds were still west where we were, and in northwest flow in July, ANYTHING can happen, as I saw the night before with a large supercell that formed to my west and moved into LaSalle county, IL producing numerous funnels. The office was in a chatty mood and we talked about the situation, and we agreed that their assessment that the best of the fun stuff would stay to the southwest until after dark was reasonable. Was there any hope? Of course there was, with 3,000 CAPEs lying around in northwest flow with strong dynamics aloft! Several storms started forming east of Waterloo. Then, about 5:30PM, there it was. A thunderstorm, east of Waterloo near Manchester, intensified rapidly. VILs went up to 35, a V-notch formed, and an inflow notch quickly appeared. Gate to gate shear of 35 knots at 52 miles out made for a minimal mesocyclone; and it was moving at roughly 320/15. At 5:20PM, after saying thanks to everyone for the information, we cruised north on US 61 for an intercept that I thought would take it into Maquoketa, IA at about 6:15PM. We drove north as the sky off to our northwest grew darker. I noticed a band of low to mid-level clouds pointing into the dark mass on the northwestern horizon. At 5:50PM, the rain free base came into view. Suddenly, traffic came to a screeching halt! A construction worker standing in the middle of the road with a stop sign was holding traffic up for over a mile! "You IDIOT!!! What in the @#$% are you doing? Don't you know the meaning of 'RUSH HOUR'?!?" We watched as the rain free base emerged from the northwest and moved east, with a developing wall cloud embedded in the rain on the west side of the precipitation area! It was amazing. We sat there for 10 minutes as traffic backed up for goodness knows how far. Finally, after waiting what seemed to be an eternity, as Mike and I blamed this on Roger Edwards' jinx and goodness knows how many other myths, the worker threw his sign in the back of an oncoming truck and drove away! Confused drivers moved on, and we finally started making good progress as traffic quickly loosened up. We approached Maquoketa shortly after 6 PM. We could not believe what we were seeing. A gorgeous, rotating classic/HP hybrid supercell with a low wall cloud off to our northwest! Just before we pulled into town, I told Mike that since the storm has or would likely turn right, and since the mean flow was at 310 degrees, we needed to be south-southeast of the storm, and pray the meso doesn't get completely enshrouded in rain. We turned around, went about 3-4 miles south of town, and we pulled of US 61 onto a gravel road and headed west a half mile, just north of Iowa SR 136. What a view! On top of a hill surrounded by trees and power lines except to our north, northwest and west, it was a dynamite location to watch the events unfold. The supercell organized and quickly eveloved into an HP, but we could still make out the enshrouded mesocyclone from time to time in the center of the storm, more toward the northeast side of it. Lightning was infrequent, occasional at best and mostly in and around the core that we could see; smooth striations appeared as the storm slowly approached us; there were lots of great video and photo opportunities, and none of them were wasted. A beaver tail marked the low level inflow from the east, mid level inflow bands also came in from the southeast and west from what we could see at our location. Even better was that there weren't any storms off to the south and southwest, so rain-cooled air from other storms would not get ingested into our storm. Unfortunately, winds at the surface were calm, and it was so quiet you could hear a weather radio drop. But, at 6:20PM, it was time to move as the storm creeped southeastward. So far, no warnings from Davenport; even though we could clearly see a large hail shaft to our north, it was passing over no-mans land. I suspect had it passed over a populated area, we would have gotten severe hail reported. As it was, that's fine by us; it's not really affecting anyone and giving two chasers quite a spectacle! We headed back to U.S. 61 at 6:40PM and proceeded to head south to a position just north of Welton. The storm was looking meaner by the second, and then an attempt at an occlusion was made by the mesocyclone. We couldn't believe it, and we thought a tornado was imminent! But it was not to be. In the next 15 minutes, the storm died. I mean it weakened VERY rapidly...after a downburst in the storm core, inflow at all levels stopped, and a half hour later, the storm was completely gone along with the heating of the day. A weaker thunderstorm, generated by the outflow from the storm, fired a short time later, but it looked like it would produce only a downpour at best. Nonetheless, during it's life span, it was a supercell with very good structure, and with an unusual, haze-free view, we were able to see nature at her finest. As we begun the 90 minute drive home, "Celebration" by Kool And The Gang came on the radio, and we sung (well, we attempted to eminate sounds that remotely resembled those of sharps and flats of the musical scale without shattering the windows) a song of celebration of our success as we cranked the stereo on the long drive home. Gilbert ******************************************************************************* Gilbert Sebenste ******** Internet: firstname.lastname@example.org (My opinions only!) ***** Owner of the Storm Chaser Homepage/SCH Canada *** URL: http://www.stormchaser.niu.edu/chaser/chaser.html ** SCH Canada URL: http://www.stormchaser.niu.edu/chaser/canada * *******************************************************************************