8/23/97 MN Chase by Don Lloyd


From: Don Lloyd (dlloyd@TCCCOM.NET):
Subject:      Chase Notes for Aug 23rd

A most interesting chase day for many reasons including some excellent
storm structure with at least three classic supercells, a run-in with
an HP at dusk, and a near miss with a possible tornado after dark
on I90.

The ingredients for this severe outbreak were already becoming apparent
on the models Friday morning. A strong upper level jet streak would dive
southeast through MN Saturday afternoon initiating cyclogenesis in northwest
MN strengthening southerly low-level flow leading to strong warm air and
moisture advection into MN. The ETA showed southeastern MN to be under the
favored exit quadrant of the jet, with strong UVVs and helicities progged
over 400. Instability looked to be moderate with LIs of -4 to -7dC. The
biggest question mark was the amount of moisture available. Model updates
were consistent and began to show a tongue of moisture extenting from the
southwest into southern Minnesota. It was enough for me to start setting
up the van which had not fully prepped for a chase since July 1st.

This setup only improved and by the 15Z Day-1 discussion, the target area
in southeast MN had been ungraded to MDT Risk. Final analysis of 18Z satmaps
and surface data confirmed that the area between Rochester and Austin, MN
was the target area with the convection likely initiating around 5pm.

We left Fond du Lac at 1:30, driving through steady rain and a low
overcast. Drove out of the rain at Tomah and into a clearing sky as
we crossed the Mississippi at La Crosse at 4:15. Ragged bits of cloud
scudded quickly across the sky from the southwest, while robust
building altocumulus moved southeast in bands--some with sufficient
vertical development resembling towering cumulus. Dewpoints (Td) which
were AOA 60F around La Crosse were now at 68F as we approached Rochester.
Surface temp was 82F. Stopped for gas just after 5pm at the Stewartville
exit on I90 and watched as sheered towers exploded virtually overhead,
orientated to the southeast. These initially high-based storms quickly
assumed supercell characteristics. Drove south on US63 to MN16 and turned
east. By 5:45 we had a solid rainfree base (RFB) at the northwest corner
of a tower that was sheered to the southeast. Structure was crisp with a
cumuliform anvil. RFB was ovoid with definite signs of rotation and
curved inflow bands. Another tower which had gone up further west was
developing the same structure, though was not quite as crisp. At 6:30,
we were about 6 miles southwest of Preston on a county road. The RFB
just to our north had well defined wallcloud with marked vertical
motion and rotation. Updraft was still very crisp and sheered to the
southeast, the cumuliform anvil nicely knuckled. This cell looked
remarkably like the South Storm (Biddle convention) of the May 1st,
1997 Oklahoma chase and was quite beautiful at this point. With this
structure, a tornado seemed almost inevitable. This seemed the peak
of the chase, positioned as we were between two potentially tornadic
supercells. But by 7pm, the storms started to weaken. As they moved
southeast, we moved into more stable and lower theta-e air--or they
were starting to choke on their own outflow--we could not be certain
from where we were but Tds had dropped to 60F, temps to 70F. What
amazed us was even with the significant low-level rotation, no
warnings were ever issued for these storms. They attempted to push
up new updrafts several times but it was obvious their severe potential
was finished.

Meanwhile, directly north, we had also been watching the storm north
and northwest of Winona exploding in the late afternoon sky. It was
also beautifully structured with a stairstep flanking line. We drove
east on MN44 to intercept at Hokah but by then realized the storms were
backbuilding out of Winona County and were forced to backtrack on MN16
to avoid a nasty looking gust front and core. We were running out of
time, it was now near 8pm, the sun setting--though with clear sky to the
west, we felt it would buy us a bit more chase time. We turned north at
Houston on MN76, a road dominated by curves, hills, and trees. The
lightning was virtually continuous, with several tornado reports just
to our north. I thought we might still get a look at the RFB before
dark and then the chase got a bit hairy. A good stretch of MN76 cuts
east and we realized there would be no RFB--this was a big old nasty
HP supercell--but by that time, we were heading east and the wrapping
precip was cutting off the escape. To out northeast, a definite lowering
was visible in the flicker of lightning. We were in the notch of an
HP! The interstate was only a few miles north so we decided to press
on with a better chance of finding cover there if need be. As we
turned east on I90, we regained NOAA weather radio which we had mostly
lost in the hills just as they issued a tornado warning for our location
at around 8:20pm. I pulled under an overpass--well off the road--as we
got blasted by rain and hail from the northwest, winds very gusty AOA
40mph and rapidly increasing. I got out of the van and heard a roar that
seemed quite different from the wind and noticed the wind was veering
more and more to the north. With nothing visible but driving rain and
hail we decided it was time to bug out of the van and duck under the
overpass. The wind continued to veer to the northeast, the roar grew
louder and seemed to pass to our east. The wind quickly intensified
to an estimated 70-75mph until the rain and hail were virtually
horizontal. The hail, between dime and quarter-sized was really the
worst of it and quite painful on any exposed skin--Jennie had welts all
over her legs--the hail blowing right up into the girders where we were.

By 8:35, feeling the threat had passed, we drove east towards home.
About three or four miles east of MN76, we crossed a mile of roadway
where the hail had accumulated to about an inch. After that, it was
heavy rain until we crossed the Mississippi. We may never know if a
tornado passed just to our east--the meso did for sure--but it certainly
felt like it. I'm sure a few will question our judgement in choosing
the route we did but it also happened to be the way home. Sooner or
later, we had to cut through the storm. Good knowledge of storm
structure kept us out of harm I believe, though chasers with lesser
experience might've gotten into deep trouble. The people I really
wonder about were the few drivers who continued to drive in that
incredible maelstrom, oblivious to the potential danger.

Still wet, we managed to make Applebees in Fond du Lac by 11:30 for
dinner and a much needed refreshment.

Chase distance: 541 miles.

A few photos posted on our homepage.

(If anyone has LSR's for Winona County, I would appreciate it if they
forward them to me by email)  


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