4/30/97 IL chase by Gilbert Sebenste


Subject: Chase summary, 4/30/97: Beginner's luck!

Wx-chasers,

Been real busy (so when am I not anymore? ;-(   ), but I want to briefly
recap what we saw yesterday.

As you might guess, I was REALLY anticipating April 30th, even from the
day before. A powerful surface low pressure center was forecast to move
into northwest IL by 0Z. As it continued to deepen and develop, winds
ahead of the cold front and powerful upper low would continue to back.
Moisture wasn't great but adequate: mid-upper 50's would have to do.
However, couple that with surface temps near 80 and 500MB temps of -20C
advecting into the area on a negatively tilted trough, oh my!---the front
was going to bulge into west central IL enhancing low level convergence. I
set my sights on central IL for the day.

As the morning of the 30th dawned, it became obvious that the storm system
was moving slower than forecast, a by-product of it's rapid development.
Strong pressure falls across northern IL (over 5MB in 3 hours) indicated
the surface low was on target to move across there, bringing the warm
sector up to I-80. SPC had a moderate risk out and feared a significant
tornado outbreak IF enough heating would occur. And that was a very valid
and serious question/problem.

The area of synoptic scale lift was so broad and strong I feared that
clouds would envelope the warm sector and kill off our chances of
surface heating and the threat for severe thunderstorms. But by 11AM it
was obvious that much of the warm sector of the storm was at least partly
cloudy. Temperatures in south central and southern IL surged to 75 by
noon. Things were looking good!

At 12:30PM I left work. I was stalled by a viewer in the parking lot who
wanted to show baby pictures and photos of the Fermilab tornado
seminar in April briefly, so I took a moment to check 'em out.
She knew what was about to happen down south and knew I was a chaser, so
she kept it brief. And then, I headed for DeKalb, munched down a sandwich,
and hit the phone.

No one was answering the phone at the NIU weather office. Strange, we had
made arrangements for me to call them at 1:30PM! I grabbed my chase bag
and headed over there. I found out the ringer on the phone was turned off,
and found 9 enthusiastic chase partners with engines running waiting for
me! Ryan Williams, David Paul and a cast of characters (some seniors,
juniors and freshmen) were watching the radar. Tornadoes were already
detected on NWS doppler radar near Quincy. Let's go!!!

We left at 2PM from DeKalb and headed west on I-88, then south on I-39.
I called the NIU weather office to make sure their phone was OK and that 
our cell phone was working. As I was about to relay some information to
our second chase vehicle via two way radio, suddenly, I wasn't
transmitting anymore. And then I couldn't hear a thing either! We pulled
into a rest stop in southern Lee county and after an excruciating 15
minutes, we headed out again, not being able to communicate with each
other, but sure that my unit was the problem. Just before we crossed
into LaSalle county, however, I figured it out: the radio in our car was
overheating! At first I thought that maybe we had a bad antenna and were
getting too much feedback into the radio (a high standing wave ratio, for 
you techies out there). But even without transmitting, eventually the
radio became warm and then cut out. After another 15 minutes and after I
had put the unit on an air conditioning vent, the radio worked and I
relayed what was going on with the storms and the radio situation. We
worked it out so that either of us would flash our headlights whenever we
wanted to talk to each other, and then I'd plug my radio in. This turned
out to be a minor inconvenience rather than a serious hassle.

As we continued south on I-39, I was disappointed that one of my favorite
stations in LaSalle, WLPO, for some reason had poor severe weather
coverage. As we got to about I-80, we started picking up WMBD-AM in
Peoria, which once again had the best darn weather coverage of any station
I've ever heard in Illinois.

We heard about the tornado warning for Schuyler and Macoupin counties.
Good! Looks like we are right on track. Then Tazewell county, including
Peoria, went under the gun with a confirmed tornado on the ground! To
intercept, I decided to take SR 18 west. But after seeing reports of
storms farther south in better CAPE, I wanted to go further to the south
to get to those. And a line of weaker storms had formed out ahead of the
main one about 50 miles north and east, and I knew that once the storms in
the second, western line moved into the airmass spoiled by these, it would
be all she wrote for any chances of our supercells surviving. So, we
turned around, hopped back on I-39, destination: U.S. 24.

But there was a problem. As the tornado touchdown reports started coming
in, it was obvious the storm wasn't really moving northeast, but
perhaps almost due north! A call back to NIU confirmed that this was
true!

Shoot. Well, now we were east-northeast of our Peoria storm. We turned
around on SR 17 and headed back north. We had to be quick; these storms
were now moving at almost 45 knots (over 50 MPH)! We headed back to SR 18 
and headed west for about 2 miles. Then, we stopped to watch and wait.

We got out of the car to survey the scene. Strong south to southeast winds
were blowing into the storm itself; however, *storm relative* inflow
would not be that strong with the storm moving at over 50 MPH (in fact,
I was wondering if the updraft would be on it's north side). And as we
waited, the sky to our southwest quickly grew dark. We turned up WMBD
to listen for the latest warnings. The warnings off to the south were
being cancelled quickly except for a tornadic storm southwest of 
Champaign. I wondered if my friend Scott Olthoff and his colleagues were
on that one. As I was pondering that thought for a moment, a terrifying
sound brought me back to reality...

Suddenly, a low but distinctable loud buzz came across the radio...and
stayed. My eyes grew wide and I yelled "Everyone in the car! NOW!!!!"
In chaos everyone hit the cars. Seconds later, a bright flash followed by
a tremendous BOOM shook both cars! Ryan (our driver in my car) and I
looked at each other and let go of our breaths. That lightning bolt was
TOO close, hitting the field a short distance away. Some of our other
chase partners didn't realize up until then that we were in or
near the ionization path of a powerful lightning strike!

After a few moments of allowing everyone time to clean out their pants, 
I saw that we were still too far east. I told our hesitant driver to go
forward into the blackness, assuring him I wasn't going to get him into
the core. I had to be careful since visibility was rather poor---under 5 
miles. at that point. We got into brief heavy rain on the far east side of
the cell. Approaching the city of Magnolia, I thought I had caught a
glimpse of a rain free base. I also noticed spiral bands ahead, above the
trees and hills. I wondered what we would see on the other side of this
valley. We passed through Magnolia, and went through a valley (for those
of you with the "Gazetter" atlas of IL, you can see the elevation contours
are pretty packed just west of Magnolia). Then, we crested a hill two
miles west of town. No more than a mile and a half away, to our west
southwest, was an unmistakable rotating wall cloud with beaver tails
coming in from the north AND south sides! And then, a narrow funnel
briefly developed in the center of it! Green off to our north and a nasty
looking core as well made this a classic supercell to an "S"!

We quickly pulled off and got out to watch. I saw the beginning of a clear
slot on the south side of the meso. "Oh, my God, it's going to go!" I
thought to myself! Suddenly, a plume of dust shot up from the ground
just north of the wall cloud. Several started yelling "Tornado!" but I
wasn't sure what that was. It was too distant and not impressive to really
tell. It might have been, but whatever it was, only lasted about 30
seconds and was a "dust whirl" at best.

After a few minutes, the wall cloud passed across 18 and headed north. I
told everyone we had to get in our car, turn around and get north quick,
or else it would outrun us. My navigator found SR 89 which headed due
north. Perfect! We headed east back to 89, and turned north. We followed
it north into the small community of McNabb (pop. 350). But the meso was
now rain-wrapped, and the storm was starting to get northeast of us. So,
we headed east on McNabb Blacktop Road (which was actually made of
concrete  ), and watched as another meso formed to our north
(doppler radar would later confirm for us the presence of two mesos, this
second one having "blue next to red" (strong shear/rotation implied) on
the base velocity tilt 1 scan from Davenport. Inflow bands quickly formed
southeast and northwest of the meso. We headed east to US 251 and headed
north. We watched it produce a brief funnel on the north side of the meso
as it continued to move quickly north-northeast. Unfortunately, we were
now almost in the LaSalle/Peru area, and theres only a few bridges at that
point to go across the mighty Illinois river. We got back on I-39, but by
that point, our storm was way out ahead of us, and it looked anemic at
best.

We called COD as the NIU phone was acting up again. The call confirmed my
fears: the line was dying, our storm sucked in rain-cooled/spoiled air and
committed convective suicide very, very quickly. I got on the radio and
relayed that the day was over. We headed back to NIU to watch the radar
loops to see what we had seen.

Given the restraints we had (time wise), information wise and
communication wise, this chase was a big success. Everyone went home
happier than a cat locked in a mouse cloning factory. And we all saw a
great tornadic supercell! Thanks to Dr. Silberberg, Mark, and the
others at NIU and Walter over at COD for their help in getting the info 
we needed to make our trip a success. And tomorrow (Friday, May 2) is
another day...in a risk for more tubes! Stay tuned, everyone...

Gilbert

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Gilbert Sebenste                                                     ********
Internet: sebenste@geog.niu.edu    (My opinions only!)                 *****
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