Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 240732

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
232 AM CDT Mon Sep 24 2018

232 AM CDT

Through tonight...

Broad and subtle southern stream shortwave trough over the
southern Plains will ripple northeastward across IL and IN this
afternoon. Meanwhile, low pressure over the northern and central
Plains will deepen as it moves northeast into Canada this
afternoon. Low level flow will turn southerly in response to this
low tracking to north, which will advect richer low level moisture
and an associated stratus deck north into our CWA this afternoon.
Plume of mid-upper level moisture associated with the southern
stream should creep northward today as the subtle southern stream
shortwave trough tracks northeastward. Forcing from this shortwave
trough could be enough to produce some scattered showers,
particularly over our southern CWA where the plume of mid-upper
level moisture will be closer and more supportive of shower

In wake of this lead southern stream shortwave tonight, look for
mid-upper level moisture to be driven back south of our CWA.
However, low level warm/moist advection will continue tonight with
forecast soundings suggesting the depth of the stratus deck could
grow to between 6-9kft. The deepening warm cloud depth, some
modest low level ascent from the WAA, and collision and
coalescence within the deep warm (>0C) cloud looks to be a pretty
favorable set-up for drizzle tonight. Have added drizzle into the
grids tonight and with cloud depth in some models nearing 10kft,
it`s possible could see some light rain as well.

As the stratus deck encroaches from the south today, look for
temps to flat line or even drop off a couple degrees as skies go
overcast. This should result in a warmest temps today over
northwest CWA where mid 70s look attainable, while under the
stratus deck temps will probably fall to near 70 to perhaps the
upper 60s. The cloud cover tonight should serve as a blanket
keeping temps nearly steady much of the night and have gone above
guidance for low temps tonight given this expectation.

- Izzi


155 AM CDT

Tuesday through Sunday...

A cold front will sweep across the region on Tuesday bringing a
showers and thunderstorms to the area along with a conditional
threat for severe weather. Longwave trough is forecast to be in
place across the northern Great Plains on Tuesday while a
positively tilted shortwave lifts across the Upper Midwest
during the afternoon and evening hours. Local area will fall
within the right entrance region to a strong upper jet over the
Upper Midwest enhancing large scale ascent over the region. At
the surface, a cold front is expected to be near the Mississippi
River by mid afternoon and will move across the forecast area by
late in the evening. Steepening mid level lapse rates atop warm
and moist surface conditions could result in modest to moderate
instability developing ahead of the front across portions of
northern Illinois and northwest Indiana. The amount of
destabilization remains the main area of uncertainty, but if these
ingredients do come together then the combination of instability
and strong flow aloft could support a severe weather threat over
the area on Tuesday.

For Wednesday and Thursday, high pressure will build back across
the Great Lakes behind the departing cold front resulting a
couple days of fair weather locally. Temperatures will turn cooler
once again with highs mainly in the 60s both days, though some
spots may tag 70F south of I-80 on Thursday.

Still some modest timing differences in models, but expect another
cold front to move across the region Thursday evening or
overnight. The surface ridge axis, meanwhile, will stretch from
New England southwest into Texas and does not provide favorable
trajectories for moisture return into the local area. Given this
and the unfavorable diurnal timing of fropa, instability will be
missing to support any thunder threat for the late week front, and
many models even suggest dry frontal passage.

Over the weekend, surface ridge axis shifts farther east while
baroclinic zone sets up across portions of the midwest. Increasing
southwest flow across the baroclinic zone will support precip
chances over the weekend. In addition, there is the possibility
for a sharp temperature gradient over or very near the forecast
area with upper 70s/80s south and 50s north. Given the proximity
of this gradient, forecast details remain unclear at this



For the 06Z TAFs...

Primary aviation concern is timing of MVFR cigs arrival and how
low cigs will get. Have pushed back the timing a bit given dry low
level easterly flow in place, have trended toward the slowest
guidance for timing of MVFR cigs arriving. Models suggest that
cigs could drop to IFR at times later tomorrow afternoon and into
the evening. While this is certainly plausible, synoptically and
climo don`t really scream IFR, so opted to not introduce IFR into
the TAF at this time.

- Izzi





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