October 29 Illinois Chase By Gilbert Sebenste

Subject: Chase summary, 10/29/96...We had to try!

Author's note: This is quite lengthy...be warned!!!

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The day of Monday, October 28 brought keen interest to my weather eyes.
Climatologically speaking, snow chasing in these parts is more appropriate
than storm chasing, as the last week of October typically brings it's
first flurries around here. But this week was going to be quite a bit
different!

A deep trough had dug in the southern Rockies, forming a cutoff low.
A powerful, 300MB 150 knot jet streak was rounding the base of the trough
with nothing that potent upstream. With the strongest winds forecast to be
east of the cutoff low, that means it should "open up" and lift out
towards the northeast. And as that happened, it should spin up a surface
low in thee of the Rockies. Given the magnitude of the jet aloft, the
powerful dynamics associated with it, and, thanks to an unusually warm and
moist airmass across Oklahoma and Texas just waiting to come back
northward, I knew something big was about to happen!

Flash ahead to 1PM. The 12Z ETA and NGM are in, and paint an astonishingly
similar and very scary picture: A surface low was going to develop in
Colorado and be in western WI by 0Z on the 30th (6PM on the 29th, CT).

At that point, the cold front would be moving into Illinois, with the 
triple point---where the occluded front, warm front and cold front meet--
moving across northwest IL, with the cold front bulging eastward across
central IL. This means that winds would have to back eastward in northern
IL, if the progs were true. The 500MB cut off low would become a
trough, and would turn "negatively tilted"---that is, oriented northwest
to southeast---favorable for cold air advection to come in and destabilize
the atmosphere. The mid levels would be remarkable warm for that time of
year...-5 to -10C...and that would be sorely needed as temperatures at the
surface would only be right around 21C (70F).

Using THE STORM MACHINE, by 0Z the following parameters would be in place
across northern IL ahead of the cold front: Helicity: 300-400 J/KG; CAPE
1,000-1500 J/KG; 60 knot low level jet at 850MB, 90 knots at 500MB and 130
knots at 300. Speed shear was not the problem; directional shear was good;
instability was fair at best. That would be the only significant negative
factor, as far as I could tell, as a direct result of warm temps aloft and
lack of great surface heating. My target: LaSalle, IL, some 80 miles
southwest of Chicago.

Oh, yes, and one more slight problem: mean wind vector on these storms
would be roughly 290/60. That is, moving northeast at 70+ MPH.

Hmmmm. That means if we make a mistake on a road...it could be our last in
a tornadic situation. We would have to be extremely careful. And it also
meant that more than likely if we saw anything we couldn't chase it...or
keep up with it, more precisely.

SPC issues their 2nd day outlook at 18Z. Bingo. Moderate risk dead
centered on north-central IL. And they are just as excited about it as I
am. I hit the sack that night, wondering if I will calm down and get some
sleep. As I think that terrifying thought, I nod off.
Undoubtedly, with a grin on my face.

THE DAY BEGINS, 10/29/96

I get up at my usual 3:30AM and head for work. The progs so far continue
to be in astonishing agreement and are verifying very well. SPC doesn't
change the moderate risk significantly in their day 1 outlook. It's going
to be a wild day!!!

I wake Brian Fugiel out of bed at 9:30AM and let him know it's a chase
day. The 12Z models are just coming out and they are again virtually
identical and consistent with the last two runs...except slightly slower
and further to the north and west with the surface low. That's even
better! That puts the triple point right where I want it, along the
IA/IL/WI border. Energy-Helicity Index values bullseye southwest Wisconsin
down through northwest Illinois. I let him know that my target area is
shifted northwest with the models: Sterling, IL, 40 miles east of Moline
and the Quad Cities. Surface and upper air analysis show no reason why
the models are going to bust this one. Now, the question is surface
heating...and the dry slot.

By 11AM I have essentially completed my analysis. High-resolution water vapor
loops show the dry slot...and the strong upper low racing northeastward,
the former pointing towards northwestern IL. An area of clear skies has
broken through ahead of the cold front, and temps are now 28C at Columbia.
Already, my visible satellite loop shows TCU and small CB just out ahead
of the cold front in central Iowa southward to northern Missouri. "No,
no, don't go yet!!!" I muse, but I know there's no cap in this situation.
A tornado watch covering central Iowa to northern Missouri is quickly
issued. 

I deliver my noon forecast at the TV station, and at 12:30PM I am done.
I head south back to DeKalb and brief Brian that all is well. The storms
are very slow to develop, and should wait before they get into IL before
everything comes together. This is great! We head to the NIU weather
office for a last minute check on what's going on. 

I bring up Greg Thompson's 1.5 KM visible satellite for the midwest. Ahhh.
An area of clearing just ahead of the cold front, and south of the warm
front. temperatures are climbing into the lower 70's in western IL.
Moline, IL jumps from 54F to 68F in one hour. FROPA! Everything is coming
together. Mark Russo wants to come along; the more the merrier! We have
yours truly doing navigator/videographer and riding shotgun, Brian
driving, and Mark doing backup navigator duties when I am videotaping and
also calling for in on the cell phone for information and reporting
severe weather. We leave the office at 2:25PM, not waiting for the watch
to come out. At 2:30PM, the red box goes out for all areas west of I-39 in
Illinois through extreme eastern Iowa and northeast Missouri. We hit I-88
at 2:30, but didn't hear the watch until 2:45 when we pulled in "MIX-96"
in the Quad Cities as we pass Rochelle. We call the COD weather lab and
talk to Jeremy Hylka to get all the details. Hail: 1.25". Max tops 42,000
feet. Oh yeah...mean wind vector...240/60. Arrrrgh.

We hit Sterling at about 3:15PM. It's cloudy, it's foggy, it's cold.
We know we are just north of the warm front. We decide to head south on
former Illinois 88 (can't recall what it is now), into the warmer air.
Within a few minutes, we punch the fog, the clouds break, and it's
70/60 air!!! Ahhhhhh. We have strong southeast winds, broken clouds,
and low level scud ripping northward. We call the chief meteorologist
at my TV station. It's a squall line, BUT...there are a few cells starting
to pop out ahead of the main line. Excellent! We are 7 miles east of
Prophetstown, IL, and head there to make our next move.

After another call, this time to Paul Sirvatka, he thinks we have two
choices: Peoria/Galesburg, which is his best guess, or keep heading west
towards the mighty Mississippi. With only an hour and twenty minutes to go
until sunset, we go for the latter. It's now 3:30PM. We decide to head
north on SR 78 from Prophetstown back to I-88, and then west to intercept
the storms. We flip on WSDR-AM 1240 out of Sterling, which was an
excellent source of severe weather information during my infamous
May 9, 1995 Morrison, IL chase (see that report on the SCH reports page).

Suddenly, warnings start to fly. Cedar County, Iowa, just to the west of
Moline, is under a tornado warning for a radar indicated tube! But, the
county we are in, Whiteside (in Illinois), goes under a severe
thunderstorm warning. This is for a cell out ahead of the main line.
YESSSSSS!!!

At this point, a little panic sets in. The area we are in is in a map
boundary between one page and another, and continuity isn't great.
We get off I-88 on the next exit, Albany road. The sky is turning black to
our southwest. We need to head south. We head into Erie, IL and try to
find a road that goes east-northeast, the small one on our map. Instead,
we hit River Road, which heads southwest. And there's no place to turn
around, with the Rock River to our left, and a deep gully and cornfield
to our right!!! It's 4:05 PM...

As we head southwest, we can see the outflow boundary from the storm
tearing towards us. Finally, we hit a farmhouse and we quickly turn
around. But as we do, we are slammed by a 50MPH west-southwesterly gust
of wind with blinding rain!

The next few minutes are a lesson in colorful language, so I won't get
into details. We try to get northeast again on River Road but we are
forced to go 30 MPH or slower as we are hit with winds. Did I mention
there are power lines whipping around on the west side of the road,
seemingly ready to come off which would land in our car? Just asking,
Brian and Mark. :-(

In two minutes, however, the storm is history. We get northeast of Erie,
and get back on Albany road. As we do that, we notice the sky has turned
black again to our southwest. It's the main line of storms with the cold
front!

We get back on I-88 and head east. We can see low scud to our southwest,
but to our east and northeast with the broken line of showers (no
lightning yet) we can see no discernable features. At this point, it's
starting to get dark. This is all elevated-based crap (you won't find that
in the AMS Glossary of Meteorology, but to keep it simple, the showers
look pathetic and show no signs of becoming supercells, or thunderstorms,
for that matter). Still, a severe thunderstorm warning goes out for
Whiteside county until 5:00PM, for high winds associated with this
"storm". We have only seen one bolt of lightning, and that was a distant
one off to our east in the cold sector when we were by Sterling earlier
on.

As we head east, we punch into the shower again and are hit by 50 MPH 
winds. WSDR notes at their studios on the east side of town that they are
getting the high winds now. We pull underneath a bridge into the grassy
area next to the support in the median as the rain and the high winds have
made it unsafe for us to continue in my Escort hatchback. It's now 4:35PM.
20 minutes until complete darkness.

We get the word that we are in a 50 dbZ rain shower. No kidding, Dick
Tracy. NLDN lightning images show a flash here and there, but we can't see
them, nor hear them on our AM radio. At that point, with darkness
encroaching rapidly, we discuss the conditions and deem this situation
hopeless. We won't chase a mesocyclone at night in high winds with it moving
potentially at 70 MPH. We call it a day. Er, night.

But as we head home, at about 5:20PM, the rain and the wind picks up
again. Suddenly, a semi flipped over into a ditch comes into view! I hit
the cell phone and dial 911. The driver appears to be okay, and others are
standing around him. Still, I call the police and get them out there. 
The warnings keep flying as we drive to our east with storms producing
severe weather in the form of "primarily high winds". No doubt!!!

The rest of the trip home was interesting. For over an hour, as we drive
through rain squalls, we occasionally have to pull over as the downward
transfer of momentum from the very strong low level jet aloft gets pulled
down to the surface, creating heavy rains that are blowing sideways with
visibilities occasionally down to near zero. As we near DeKalb, we
finally see some in-cloud lightning. At last, we make it home, and
pop on The Weather Channel.

We did the right thing by staying with the broken line ahead of the main
squall line. It produced all the wind damage. Paul Sirvatka was right
though, as I later learned of a brief, weak tornado near Peoria according
to the Fulton County ESDA folks. Still, that would have pure luck to have
seen that one, even if we had seen it.

Lack of instability was the cause of the lack of widespread surface based
thunderstorms. The few that were (down by Peoria) didn't last long.
Everything else came together, but it was one parameter that fell short
which killed our chances for seeing a tornado that day. The radar
indicated rotations in Iowa never saw the ground. And we were left without
even getting pea-sized hail.

Oh well. Given what happened, I would have done it all again. Everything
pretty much verified. And who knows, maybe it will happen in November!
If nothing else, it's at least another lame excuse I can use for not
getting my end-of-the-year-compendium out yet...

Gilbert

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