849
FXUS63 KLOT 180543
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1243 AM CDT Mon Aug 18 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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1243 AM CDT Mon Aug 18 2025

A cluster of showers and thunderstorms just east of the
Mississippi River should continue weakening over the next few
hours as it encounters decreasing instability. Given proximity
to RFD, will convert the inherited PROB30 group to a TEMPO group
for -SHRA (note a flash of lightning cannot be ruled out). A
brief westerly wind shift may occur as well. The cluster is
expected to fully decay before reaching DPA/ORD/MDW/GYY.

Attention then turns to the threat for another round of showers
and storms as an upper-level shortwave currently moving across
the central Plains arrives this afternoon. At this point, the
expectation is for either the reinvigoration of thunderstorms
currently ongoing along the NE/KS border or for new storms to
develop along residual outflow near the Mississippi River by
early afternoon. Such an evolution would set the stage for the
resulting line of storms to sweep across the terminals through
early evening vanquishing the instability pool. Note that some
guidance offers a separate scenario where coverage of afternoon
convection is minimized, in which case a prolonged period of
thunder would materialize after dark into early Tuesday morning
as shortwaves parade over the undisturbed unstable airmass. At
this point, we believe the former (line of storms) scenario is
more likely, as the CAMs that support the latter scenario
(prolonged overnight thunder) appear to be too low on dew points
during the afternoon preventing thunderstorms from
redeveloping on a widespread basis. So, will go ahead and offer
targeted TEMPO groups for thunder at all terminals tied to the
late afternoon to early evening hours. A strong westerly wind
shift with gusts to 40kt (locally higher) is probable as the
line of storms sweeps across the airspace.

Easterly winds at the start of the TAF period should gradually
veer southeasterly throughout the night and through Monday
morning. At ORD/MDW/DPA/GYY, lake influence may keep the wind
direction east of south even through early afternoon, while the
wind direction should veer south to southwesterly at RFD. Wind
directions should "go around the clock" behind the line of
storms from northeast to southeast before settling on
southwesterly by the very end of the TAF period.

Finally, there may be periodic periods of MVFR cigs (aoa 2kft) throughout
the TAF period.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement until 10 AM CDT this morning for
     ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement until 10 AM CDT this morning for
     INZ001-INZ002.

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

&&

$$

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