July 29, 1996 Maquoketa, IA Chase By Gilbert Sebenste


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: sebenste@geog.niu.edu    (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                  *
*******************************************************************************