544
FXUS63 KLOT 301926
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
226 PM CDT Wed Jul 30 2025

.KEY MESSAGES...

- A Flood Watch remains in effect for the entire area through 1
  PM tomorrow afternoon. A portion or all of it may be able to
  be cancelled early.

- Periods of rain are expected this evening and overnight as a
  slow-moving weather system moves overhead.

- High waves and rip currents will lead to a high swim risks at
  all Lake Michigan beaches through at least Friday afternoon.

- Cooler temperatures and more comfortable humidity levels will
  arrive tomorrow and last through the weekend.

&&

.DISCUSSION...
Issued at 226 PM CDT Wed Jul 30 2025

Through Thursday:

Recent radar imagery depicts showers and storms moving eastward
across the area at press time ahead of an MCV/shortwave moving
eastward across the Mississippi River. Meanwhile, a second area
of showers and embedded thunderstorms stretches from far
northern Iowa eastward through much of southern Wisconsin along
the northern flank of the MCV and along a broad frontal zone
drifting southward through the Great Lakes. Between both
features, relatively tranquil conditions reside with broken
cloud cover and temperatures in the mid 70s.

Over the coming hours, the expectation is for the main axis of
showers and storms to shift eastward out of our area only to be
replaced by the secondary area of showers and embedded storms
dropping southward out of Wisconsin. A broad area of upper-
level diffluence in the right entrance region of a passing jet
streak in tandem with PWATs near 2" should afford surprisingly
efficient rain rates this evening particularly where pockets of
instability still reside. With that said, the signal for
widespread, torrential rainfall tonight is much lower than it
had looked 12 hours ago (likely as a result to a different
evolution of convection today than perhaps expected). As a
result, will go ahead and trim the Flood Watch where rain has
already passed and let the evening shift remove the rest later
today. Note there may still be a very low-end threat on a very
localized basis (call it a 5% chance for flooding at any given
location). Meanwhile, increasing north to northeasterly flow off
the lake will lead to building waves on Lake Michigan,
resulting in a High Swim Risk this evening onward at all Lake
Michigan beaches.

Tomorrow, the back edge of the shortwave will shift eastward
causing showers to taper from northeast to southeast. Showers or
sprinkles may last the longest at lakeshore locations where
onshore flow and tumbling 850mb temperatures may afford modest
lake instability before dry air rushes into the region. Outside
tapering rain chances, tomorrow will feel quite different
compared to the past week with highs only in the low to mid 70s
and much lower humidity levels. Continued north to northeasterly
onshore winds will maintain a High Swim Risk at all Lake
Michigan beaches. Finally, have introduced near-surface smoke to
the forecast owing to upstream observations of 2-5 mile
visibilities across the Upper Great Lakes (that airmass will
come our way behind the front tomorrow).

Borchardt


Thursday Night through Wednesday:

Surface high pressure will remain stationed over the Great
Lakes Friday through the weekend. Abundant sunshine can thus be
expected across the region, though it`s possible that lingering
wildfire smoke aloft could continue to give the sky a milky look
at times. Anticyclonic flow around the surface high will
reinforce the cooler and drier air mass over the region, leading
to continued below to near-normal temperatures (highs in the
70s to low-mid 80s and lows in 50s to mid 60s) and comfortable
humidity levels. Synoptic conditions favor little to no
precipitation in our area through the weekend. However, the
unseasonably cool low-level air (850 mb temperatures in the
bottom 10% of regional sounding climatology) spreading over a
mild Lake Michigan with water temperatures still in the 70s
could support a conditional potential for isolated lake-induced
showers or drizzle Thursday night into Friday, pending the
presence of sufficient low-level moisture and the development of
any low-level convergence zones. Boosted PoPs a bit for this
time frame to account for this possibility, but have maintained
a dry forecast for now given the lingering uncertainties and
only marginal lake effect parameters in play.

Medium range guidance continues to indicate that the Great
Lakes surface high will depart going into early next week,
opening the door for warmer and more humid conditions to return,
along with our next chances for precipitation as upper-level
shortwave disturbances start to get funneled back into the
region. However, there is still a fair amount of spread amongst
the latest ensemble output regarding how quickly this will
occur and just how warm it might get next week, along with the
timing of any potential showers or storms next week. This
spread appears to be tied to the strength and trajectory of a
reinforcing upper-level trough diving into the Great Lakes
and/or northeastern CONUS from Canada, which will influence the
extent to which an upper-level ridge over the southwestern
CONUS is impeded as it attempts to slide eastward. Regardless,
the overall trend should still be towards warmer and more humid
conditions next week, and this trend is reflected in the latest
NBM output. The NBM`s PoPs, however, are likely too broad-
brushed as a result of the ensemble spread, and will likely need
refinement as trends emerge in ensemble guidance.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1235 PM CDT Wed Jul 30 2025

Concerns:

-Showers and thunderstorms through early-mid afternoon

-IFR or even brief LIFR visibility in heavy downpours

-Potential for IFR ceilings later today and tonight

A complex of thunderstorms will continue to slide by just to the
south of the Chicagoland terminals. KMDW will get grazed by
lightning, with about a 40-50% percent chance of direct terminal
impacts, For KORD VCTS more likely tells the story. For both
terminals, heavy showers may still reduce visibility to IFR (1
1/2-2SM), though LIFR may briefly occur. There will be a break
in the rain later this afternoon into the early evening, before
another batch of rain/showers will shift into the area. Ceilings
will likely drop back to MVFR, with a 50% chance of IFR.
Confidence on how long IFR will last is low. Rain will taper off
north to south overnight. Winds will increase out of the
northeast, especially tonight. Gusts to 25 KT appear likely at
times tonight into Thursday.

KMD

&&

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

     Flood Watch until 7 PM CDT this evening for ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-ILZ108.

IN...Beach Hazards Statement through Friday afternoon for INZ001-
     INZ002.

     Flood Watch until 7 PM CDT /8 PM EDT/ this evening for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 9 AM CDT Friday for the IL and IN
     nearshore waters.

&&

$$

Visit us at weather.gov/chicago