405
FXUS63 KLOT 172336
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Sun Aug 17 2025

.KEY MESSAGES...

- Warm and humid conditions return Monday and may extend
  into Tuesday along with chances for thunderstorms.

- Strongest storms on Monday will be capable of gusty winds
  and heavy rainfall which may result in flash flooding.

- A pattern change toward drier, cooler, and less humid
  conditions will arrive by midweek.

&&

.DISCUSSION...
Issued at 351 PM CDT Sun Aug 17 2025

Through Tuesday:

The thunderstorm outflow from last night`s storms is currently
draped across the Mississippi River and is currently serving as
the focus for thunderstorm development this afternoon. While the
expectation is for storms this afternoon to remain along and
west of this boundary, there is a subtle convergence zone
stretching from the northern Quad Cities Area to southern
Iroquois County. If storms this afternoon push outflow back
towards this convergence zone then a period of isolated
thunderstorms may materialize given the weakly capped and humid
air mass still in place. Due to the low confidence in this
scenario have opted to maintain the slight chance (15-20%) POPs
for areas along and south of a Dixon to Watseka line as a
precaution through 9 PM this evening.

Outside of this small rain chance, dry conditions are generally
expected for tonight as subtle height rises develop overhead
and cap off the atmosphere to storms. Therefore, expect some
partly to mostly clear skies with temperatures dipping into the
upper 60s and lower 70s tonight. That said, there is a chance
for patchy fog to develop west of the Fox Valley but given that
winds may stay somewhat elevated tonight (speeds of 5-7 mph)
confidence was too low to warrant a formal mention in the
forecast.

Heading into Monday, the shortwave trough moving across the
Dakotas and northern NE will begin to move into northern IL
Monday morning bringing with it a cold front. While initially it
looks as if the atmosphere will be capped to any convection,
sufficient instability is expected to develop by early afternoon
which should allow scattered thunderstorms to develop from west
to east ahead of the shortwave and associated front. Wind shear
Monday afternoon is on the weaker side (bulk shear around 20-30
kts) but still sufficient to support some robust updrafts and a
low-end severe threat mainly in the form of gusty to locally
damaging winds. Furthermore, the continued humid air mass will
make any thunderstorms heavy rain producers. With the wind shear
vectors forecast to be perpendicular to the frontal boundary
storms should remain progressive enough to minimize the flood
threat, but if repeated rounds of storms move over the already
saturated portions of our CWA a flash flood threat may quickly
develop.

Aside from the storms on Monday, temperatures will once again
warm into the upper 80s and lower 90s with dew points in the
70s. These conditions will result in peak heat indices Monday
afternoon around 100F. While these are below formal heat
advisory criteria they are still high enough to result in heat
related illness so be sure to use caution if planning to be
outside Monday. Additionally, breezy southeast winds are
expected to persist on Lake Michigan which will support large
waves and dangerous rip currents especially at the northeast IL
beaches. A Beach Hazard Statement is currently in effect through
Monday morning but may need to be extend for the IL beaches if
winds remain elevated and waves are slower to subside.

The aforementioned cold front will continue to move through the
area Monday night through the day on Tuesday resulting in off
and on periods of showers and thunderstorms. However, the wind
shear is expected to move east of the area Monday night which
should lower the severe potential for the day on Tuesday. That
said, localized gusty winds and heavy downpours will remain
possible with any storms Tuesday which may result in a continued
threat for flooding.


Tuesday night through Sunday:

By Tuesday night the cold front should be mostly out of our
area resulting in mostly dry conditions for Tuesday night into
Wednesday. However, if the front is slower to exit than there is
a chance some isolated showers/storms could persist into
Wednesday morning. Regardless, a Canadian surface high will move
into the area on Wednesday and persist through the end of the
week and into next weekend. As a result, a period of notably
cooler and less humid conditions is expected from Wednesday
onward with highs in the upper 70s to mid-80s and dew points in
the 60s. Though, another period of gusty winds and high waves
does look to develop on Lake Michigan behind the front on
Wednesday which will likely result in rip currents and dangerous
swimming conditions at area beaches.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 636 PM CDT Sun Aug 17 2025

Forecast concerns include...

Chance of thunderstorms Monday afternoon/evening.
Wind directions Monday.

Current line of scattered thunderstorms southwest of the
terminals is expected to remain there through this evening,
along a boundary and better moisture axis. There is a low chance
for the thunderstorms over northeast IA to move into northwest
IL tonight but confidence is low and no mention at RFD for now.
There is also a chance of showers around daybreak for all of
the terminals. Confidence is low for this potential.

Scattered thunderstorms will be possible Monday afternoon into
Monday evening across all of the terminals but there remains
uncertainty for coverage and timing. There is also the potential
that the storms on Monday form a line as they move across the
area. Continued prob thunder mention, but did expand the time
window, which should be able to be narrowed down and possibly to
tempo mention as trends emerge.

Northeast winds this evening will become southeast overnight and
then southerly on Monday. Confidence is low for winds remaining
south/southwest as most recent trends suggest prevailing winds
may be south/southeast. Speeds into the 10-15kt will be possible
Monday.

There remains patchy mvfr cigs and these are expected to slowly
scatter out this evening. There is some potential for fog
overnight into early Monday morning outside the Chicago
terminals and trends will need to be monitored for possible
mention with later forecasts. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement through Monday morning for ILZ006-
     ILZ103-ILZ104.

IN...Beach Hazards Statement through Monday morning for INZ001-
     INZ002.

LM...Small Craft Advisory until 10 AM CDT Monday for the IL
     nearshore waters.

&&

$$

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