Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 031119

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
619 AM CDT Mon Aug 3 2020

300 AM CDT

Through Tuesday...

The main weather focus today remains on building wave heights
across the lake behind a fast-approaching cold front, and the
potential for waterspouts mainly this morning and perhaps
lingering into the early afternoon hours across the far southern
tip of the lake.

Our latest much-advertised cold front is tough to discern in the
surface fields early this morning, with a gradual theta-e gradient
noted into portions of north-central and northwest Illinois.
Eventually, this boundary will move through our entire region (by
later this afternoon), injecting a much drier and cooler airmass
across the CWA. Until then, near-surface moisture remains a bit
elevated, especially across the southeastern half of the forecast
area where dewpoint depressions have fallen to about a degree or
less in spots. Clearing skies in the wake of yesterday afternoon`s
convection and also immediately ahead of the encroaching low-
stratus deck north of I-80 should allow pockets of fog to develop
through sunrise this morning. Have added some patchy fog wording
to the gridded forecast roughly south of I-88 as a result with
some locally dense pockets possible. This set up doesn`t look like
a Dense Fog Advisory situation, but we`ll continue to keep an eye
on trends this morning.

Watching our initial wind surge dropping down the lake, with buoy
observations upstream indicating gusts increasing to 20 to 25 kts.
Observations indicate what appears to be a mesovortex-like
feature at the southwestern bowl of the lake--and this signal has
been in hi-res guidance for the past few nights. Enhanced
convergence associated with this feature, combined with an
upstream approaching shortwave should continue to force convection
near the land-lake interface the rest of this morning. While the
coldest 850 mb temperatures are still north of our region,
somewhat steep mid-level lapse rates combined with lake surface
temperatures in the lower 70s are currently supporting lake-
induced CAPE values approaching 1500 J/kg with equilibrium levels
to almost 35,000 feet. In addition, area VWPs indicate about 30
kts of effective deep layer shear in place over the lake, which
seems to be supporting transient supercellular structures with two
recent impressive-looking cores tracking southeast across the lake
(on the right-moving supercell vector). Conditions will remain
favorable for the development of some stronger cells as well as an
attendant waterspout threat this morning, especially for any
showers/storms that are able to develop near the convergent axis
associated with the aforementioned front-mesovortex intersection.
Have greatly pared back PoPs inland today with crashing inversion
heights and incoming subsidence likely to limit precipitation
potential. In addition, have tried to limit the thunder wording to
the immediate lake and lakeside locales today with instability
really lacking elsewhere--save perhaps for our far southeast later
this afternoon ahead of the cold front.

Hazardous swimming conditions and a threat for lakeshore flooding
remain, and expected to ramp up today and late this afternoon and
evening as the strongest winds surge down the lake. See the
MARINE section below for further information.

Precipitation chances drop off tonight but low chances linger
downwind of the lake into northwest Indiana. Lake parameters start
to get a little less supportive by daybreak Tuesday, with things
probably trending more towards just a broken-overcast lake-effect
cloud deck during the day.



210 AM CDT

Tuesday night through Sunday...

With the upper-level jet stream well to our north and the trough
responsible for our recent stretch of fall-like weather pivoting
eastward by mid-week, the end of the workweek looks quiet and dry.
Daily highs and lows will increase by a few degrees each day with
ample sunshine and daily lake breezes. Longer-range forecasts
call for a return to a more summer-like thunderstorm pattern from
the weekend onward, with ensemble model guidance perhaps
highlighting the Sunday through Tuesday timeframe for episodes of



For the 12Z TAFs...

A stratus deck rooted between 800 and 1200 feet remains locked in
place at all terminals with (lo and behold) prevailing IFR at
DPA/MDW/ORD. As the daylight hours progress, would think cloud
bases should respond to even modest low-level heating (via solar
irradiance) and accordingly have offered a return to MVFR
everywhere by 15Z. Should the IFR overstay its welcome, a
secondary cold front this afternoon will afford a stronger push of
wind that will mix cloud bases upward. Winds should also adopt a
subtle easterly component behind the secondary front, as well.

A convergent band of showers over Lake Michigan may meander west
and east at times this morning leading to showers impinging on
the 10 mile vicinity radius at ORD/MDW/GYY. Otherwise, current
thinking is most if not all hours will be dry at the terminals.

After sunset, sufficient lake instability and low-level
northeasterly flow will enable lake effect stratus to stream over
all but RFD.



IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104 until 7 AM

     Lakeshore Flood Advisory...ILZ006-ILZ103-ILZ104 until 4 PM

IN...Beach Hazards Statement...INZ001-INZ002 until 7 AM Wednesday.

     Lakeshore Flood Advisory...INZ001-INZ002...1 PM Monday to 10 PM

LM...Small Craft Advisory...IL nearshore waters until 4 AM Wednesday.

     Small Craft Advisory...IN nearshore waters until 4 AM Wednesday.



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