CASE REVIEW: 17-18 July 1996
THE ENVIRONMENT THAT SET UP A "TRAIN ECHO"
RECORD RAINFALL AND FLASH FLOOD EVENT IN THE
MIDWEST.
PART I
Learning Objectives:
(1) To reinforce the concepts of flash flood forecasting by using this rather
"classic" example.
(2) To consider the ingredients leading to excessive local rainfall in a frontal
or mesohigh type event.
Materials Provided:
(1) Hand-analyzed maps of surface mean sea level pressure (every 2 mb)
and dewpoint (every 2oF ) from 0000 UTC 18 July.
(2) Plots of 850, 500, and 250 mb observations from 0000 UTC 18 July with
hand analysis of 850 dewpt and jet, 500 mb height, and 250 mb jet.
(3) Map with county background of northern Illinois and vicinity with
Chicago (KLOT) and Davenport (KDVN) radar sites.
Assignment: After a weather briefing for the situation:
(1) Review any material through 0100 UTC 18 July 1996. Where does the
low level support (jet/dewpoint/theta-e…) currently suggest a potential
regeneration area? Mark this on the map with an “X”.
(2) Do the conditions suggest enhanced intensities, or is enhanced duration
the main concern?
(3) What storm cell movement is likely?
(4) Do upper level features suggest any jet dynamics supporting upward
motion?
(5) So, with your thoughts in numbers 1-4, where do you expect the
“regeneration and training” to maximize your QPF between now and
1200 UTC? Draw isohyets every two inches beginning with 1.0-inch.
Hand that map in for compilation.
CASE REVIEW: 17-18 July 1996
THE SHORT-FUSED PORTION OF A "TRAIN ECHO"
RECORD RAINFALL AND FLASH FLOOD EVENT IN THE
MIDWEST.
PART II
Learning Objectives:
(1) To get a feel for the lead time you may be able to provide by interpreting
the complex mesoscale interactions of this flash flood event.
(2) To consider the optimal use of the many high tech tools (radar, satellite,
and gridded models) as well as a good old fashion hand analysis.
(3) To reinforce the ingredients-based forecasting knowledge to go along
with flash flood composites.
Materials:
(1) Blank map of counties in northern Illinois with Chicago (KLOT) and
Davenport (KDVN) radars.
(2) Surface plots of area for 0600 through 0900 UTC 18 July 1996.
Assignment: After a weather briefing for the event as of 0600 UTC:
(1) One person in each team should analyze the 0600 UTC surface map
(isobars at least every 2 mb, and the 72 and 74oF dewpoint). The other
can begin reviewing any data through 0600 UTC. Discuss with each
other what the current data on the workstation and the hand analysis
show. Is there a quasi-stationary frontal boundary? Is there indication of
a mesohigh?
(2) Indicate on the blank map where you are at least 50% confident that the
axis of strongest echoes will be between 0700 and 0900 UTC. Hand
this in for compilation.
(3) Step ahead to 0800 UTC. One person should look at data on the
workstation while the other analyzes the 0800 UTC surface for isobars at
least every 2 mb. Frontal boundaries? Mesohigh?
(4) Now step through 1000 UTC. Any difference between KLOT and
KDVN radars?
CASE REVIEW: 17-18 July 1996
A SUMMARY OF THE NORTHEASTERN ILLINOIS
RECORD RAINFALL AND FLASH FLOOD EVENT.
PART III
Excessive rainfall struck the western and northwestern suburbs of Chicago
between 0000 and 1000 UTC 18 July 1996. Flash flooding in these fairly
urbanized areas resulted in 8 fatalities, flooded more than 35,000 homes,
and caused more than $600 Million in private, public, and agricultural
damages.
An Illinois 24-h record 425 mm (17 inches) fell in the suburban Aurora and
resulted in flash flooding. The peak rainfall occurred between 0800 and
1000 UTC. Several rivers and streams were severely impacted, including
the DuPage, Fox, and Illinois Rivers, and Blackberry Creek.
The set-up was rather classic for a “frontal” type event although with time it
evolved into more of a “mesohigh” type event. Regardless of what "type"
event it was, the important consideration was that strong low level support
into the quasi-stationary boundary allowed for the regeneration of echoes to
balance the movement of cells to the east-southeast. Furthermore, jet
dynamics suggested that perhaps some enhanced upward motion was
present in this area of the left rear quad. Hand analyses of surface features
from 0600 and 0900 UTC show that the mesohigh had become more welldefined just prior to the peak of the storm. However, signs of the
developing mesohigh in the rain-cooled air existed even at 0000 UTC.
The Chicago radar (KLOT) was experiencing very heavy precipitation on
the radome during the latter part of the storm. It was at this time that the
difference in radar-derived rainfall between KLOT and KDVN was greatest,
most likely due to attenuation at KLOT. Other possible issues: somewhat
enhanced precipitation efficiency due to warm rain processes that could
result in radar underestimation when using the default WSR-88D settings.
Nevertheless, the radar provided excellent spatial detail for this event.
Rainfall Duration: enhanced during this event due to the regeneration of
echoes in a common area and subsequent movement across the same area
(training).
Rainfall Intensity: was also enhanced somewhat in this very moist lowlevel atmosphere due to increased precipitation efficiency. However, this
was not an extreme example of efficient precipitation production (warm rain
process) as evident in the dry mid levels, hail, and very cold cloud top
temperatures.
Hydrologic Response: was almost certainly enhanced in the urbanized
areas of the Chicago suburbs. Restricted floodways (culverts and bridges)
and impermeable surfaces would have been unable to handle the excessive
accumulations.