Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 091723

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1223 PM CDT Tue Aug 9 2022

Issued at 252 AM CDT Tue Aug 9 2022

Through Wednesday...

Cooler, much less humid, and generally dry conditions expected the
next couple days. GOES-16 nighttime microphysics RGB shows
shallow convective cloud band over southern Lake Michigan. Thus
far, not seeing any evidence of sprinkles or showers with this
band and forecast bufkit soundings keep lake induced inversion
heights generally at or below 5kft. While a lake effect sprinkle
cannot be rules out this morning, chances look low and would be
pretty non-impactful if it were to occur.

Only other forecast concern in the short term is fog potential
tonight. NAM boundary layer looks unrealistically moist as it
often does, with remainder of guidance suggesting considerable low
level drying today with their forecast soundings tonight not
looking particularly worrisome for fog. Despite dry soundings in
the GFS, the MAV guidance is hitting dense fog potential
surprising hard tonight outside of Chicago urban heat island.
Certainly many areas in the CWA have very moist soils now and with
surface high overhead providing nearly ideal radiational cooling
conditions, saw no reason to make any changes to the patchy fog we
have going for late tonight. Suspect any fog tonight would be
generally shallow and confined to the typical low spots/cold air
drainage areas in rural locations.

Otherwise no significant wx in over the next couple days.

- Izzi


Issued at 134 AM CDT Tue Aug 9 2022

Wednesday night through Monday...

A backdoor cold front well-removed from its parent low over
Quebec will sag southwestward across the area late Wednesday night
into Thursday morning before stalling or washing out just south
and west of the CWA Thursday night. A narrow ribbon of moisture
may be sufficient for isolated showers and possibly a thunderstorm
along the front, but a nocturnal to pre-noon passage for most of
the area will prove unfavorable for higher precip coverage and
better thunderstorm chances.

A reinforcing shot of relatively cooler conditions with 850 hPa
temps around 12C will support dangerous swimming conditions on
southern Lake Michigan Thursday into Friday amid persistent NNE
winds in an unstable marine regime.

Northwest flow will dominate the pattern across the western Great
Lakes late week through the weekend in a stalled pattern owing to
pronounced ridging across the western CONUS and a weakening
trough along the east coast. Multiple shortwaves embedded within
the NW flow will traverse the Upper Mississippi River Valley
Saturday through Monday. Low-level (specifically surface) moisture
advection will be lacking prior to the onset of a LLJ Saturday
night ahead of the first wave. Will maintain a dry forecast for
Saturday, but chances for showers and storms will ramp up Saturday
night into Sunday. Early signals point to the potential for a
narrow swath of very heavy rain and a localized severe threat
Saturday night into Sunday morning where the veered LLJ orients
perpendicular to the mid- level flow. Consensus guidance is in
quite good agreement with this setup, with even deterministic
global guidance advertising pockets of 1-2"+ rainfall amounts.
Location remains a bit less clear, but we will need to continue to
monitor this period as the week progresses.



For the 18Z TAFs...

Aviation weather concerns for 18Z TAFs:

* Areas of fog may develop early Wednesday morning at terminals away
  from the Chicago metro.

Surface high pressure to our north is creating north to
northeasterly winds at 10 to 15 kt. The MVFR cloud deck from this
morning has lifted and will continue to scatter out. High pressure
will move overhead tonight, bringing light northeasterly to variable
winds throughout the night. With residual moisture from Monday`s
heavy precipitation in place, there is a chance for areas of fog to
develop early Wednesday morning at locations to the west (RFD and
DPA), which should burn off shortly after sunrise. Winds will remain
light and northerly Wednesday during the day with mostly clear



IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104 until 4 PM

IN...Beach Hazards Statement...INZ001-INZ002 until 4 PM Tuesday.



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