Gilbert's Iowa Chase Summary, 7/5/95

Chase Summary, 7/5

After working a 12 hour day, I decided to take a quick nap Wednesday afternoon. I knew that the atmosphere would once again become unstable, with LI's at -5. A good amount of moisture, cold air aloft, but the wind shear was near zero. However, that didn't stop tornadoes from forming the day before, so I was still guardedly optomistic. I thought if this was going to happen, it would be a late afternoon starting event, so I rested for an evening chase.

BEEEEEEEP! AUUUUUGH! Well, at 2:47 PM, my wx radio alarm, set on maximum loudness blew me out of bed. After hitting the volume control, I heard that most of eastern IA and northwest IL was under a tornado watch until 9 PM! Everyone I knew around DeKalb was gone for the early and mid afternoon, so I called the College of DuPage. They relayed the details and discussion of the watch, and I asked if anyone wanted to go. Lee Fitzgerald volunteered.

He met me at 3:30 and we went to the NIU weather office for a quick analysis: A developing squall line had already produced one tornado and more could follow as it approached an axis of maximum instability near the Mississippi river near Moline. I was definitely not thrilled by the wind shear, which was virtually zip, but I did notice that the winds were backing just ahead of the line. This would enhance the potential for low level storm rotation, and since we would not have to travel far if we busted, we made the final decision to go.

As we headed off on I-88 westbound towards Moline, we noticed that the skies were virtually clear. Surface winds were strong, but southwesterly; heating would be good for the rest of the afternoon. Then, as we approached Moline and got into weather radio range, they indicated in their short-term forecast that the line had intensified in the past hour, and that it should continue to do so as it approached the Quad Cities. An Iowa chase! I asked Lee where his road maps were. Uh oh...no Iowa roadmap...and I had forgot my atlas.

Fortunately, Lee was on his toes, and remembered the "Iowa Welcome Centers" gave those maps away as freebies right on the interstate, along with a rest stop with phones. Sure enough, we got on I-80 west and came to the first rest stop about 6 PM. While Lee got the map, I telephoned COD to see what was going on. No answer. I tried NIU; NIU chaser Walter Swiston, who had to work late, was monitoring there. After pulling up the NOWRAD image from eastern IA, the echoes were up to almost 60 dBz... just to our northwest. Sure enough, we could see the tower approach, coming at us at 40 MPH.

I decided to go to U.S. 61 and head north. We would be north of the incoming bow echo, now evident to us; and the strongest storm was north of the bow. Since we couldn't see any isolated cells of worthiness, I was hoping the storm north of the bow would spawn a tornado.

We watched the gust front move in, followed by a 40-50 MPH gust...a minute of heavy rain, and it was over.

THAT'S IT?!?!? Yep, that was about it. Lee and I couldn't believe how narrow that line was. Couldn't have been more than several miles wide. We followed that line back eastward on I-80, then I-88 over the next two hours, until it faded away completely, encountering heavy rain, some 40 MPH wind gusts, and that's that.

What went wrong? Lack of shear didn't help, of course, but a trough had passed through earlier, and that took some of the moisture away, as well as nearly all of the shear. I was hoping that the still mid-60's dewpoints and temps in the lower 80's would cut it. They didn't. And since the upper level trough was creeping along, advection of cold air and PVA at mid/upper levels was nil, since there were no discernible vorts or disturbances on the visible and water vapor satellite images early that afternoon. It was marginal, but since it was close, I decided to take a gamble.

I was hoping to exit this chase season by batting a thousand (up until then I had seen a tornado on my chases this year), but on this day it was not to be. But now that the NW mid level flow pattern has become entrenched for the moment, a southwest wind at the surface is now 45 degrees of low to mid level shear. And my hopes are still up for a successful late-season chase.

Gilbert

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Gilbert Sebenste ********
Internet: sebenste@geog.niu.edu (My opinions only!) *****
Owner of the Storm Chaser Homepage ***
URL: http://www.stormchaser.niu.edu/chaser/chaser.html **
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