Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 031736

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 PM 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 18Z TAFs...

1236 PM...Forecast concerns include...

Chance of showers through early evening.
Mvfr cigs through this evening.
Gusty north/northeast winds through mid/late evening.

Isolated showers will be possible this afternoon into this evening
but confidence is fairly low for coverage. Best timing for the
terminals may be mid/late afternoon. Opted to continue vicinity
mention into early this evening. There will be a continued low
chance for showers tonight into Tuesday morning...mainly for gyy
and northwest IN and some mention may be needed with later

Mvfr cigs in the 1-2kft range have been fairly prevailing this
morning and are expected to slowly lift into the 2-3kft range this
afternoon but bkn/ovc mvfr cigs look to remain prevailing into
this evening and may continue through the overnight hours...
especially for mdw/gyy into northwest IN. Some lifting and then
scattering is expected this evening for the rest of the area.

Winds are slowly turning northerly early this afternoon in the
10-12kt range and are expected to become north/northeast this
afternoon with gusts increasing into the lower 20kt range...with
the highest gusts for ord/mdw/gyy and near the lake. Speeds and
gusts are expected to slowly diminish during the evening and
remain northerly tonight. cms


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 until 10 PM Tuesday.

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



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