257
FXUS63 KLOT 090159
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
859 PM CDT Tue Jul 8 2025

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms, mainly near and
  south of I-80, continue to diminish late this
  evening/overnight.

- Dangerous swimming conditions possible at Lake Michigan
  beaches Wednesday afternoon into early Thursday.

- There is a low chance (20 to 30%) for isolated thunderstorms
  on Wednesday, primarily near the lake breeze.

- Shower and thunderstorm chances increase Friday afternoon and
  overnight. Timing differences remain, and uncertainty
  regarding the magnitude of any strong-severe threats exist.

&&

.UPDATE...
Issued at 859 PM CDT Tue Jul 8 2025

Showers and a few isolated thunderstorms continue at mid-
evening, mainly near and south of the I-80 corridor. A remnant
MCV, embedded within a weak synoptic scale short wave trough,
appears to be the primary forcing, along with a myriad of
lingering surface outflow boundaries. Convection intensity has
decreased fairly quickly over the last hour or so however, with
the loss of diurnal instability with sunset. While light rain
and showers will continue to track slowly east across the
south/southeast parts of the forecast area with the MCV
overnight, thunder and associated gusty wind/flooding potential
should continue to quickly diminish. Farther to the northwest,
isolated thunderstorms over southwest and central WI are also on
a quick diminishing trend. A few showers may reach the Rockford
are in the next couple of hours, but again thunder potential
appears low.

Otherwise, with Light winds, clearing skies and surface dew
points holding near 70F, we`ll have to keep an eye on the
potential for some patchy fog development prior to sunrise,
Areas of northern IL west of the Chicago area appear to be most
favored, where decreasing cloud cover is most favored.

Ratzer

&&

.DISCUSSION...
Issued at 304 PM CDT Tue Jul 8 2025

Through Wednesday Night:

The slow moving MCV from our earlier update is located roughly
west of Lacon, IL as of this writing. Widely scattered
thunderstorms have developed on the eastern flank of the MCV near
and south of I-80 in Illinois to near the lake breeze in Indiana.
To the west of the MCV, nearly stationary pulse-type convection
resides near the MS River.

Rain and/or cloud cooled/stabilized air north of I-80 will likely
continue to limit the threat for deep convection into the early
evening. The only exception to this is well north of the IL/WI
state line in the Milwaukee metro (tied to differential heating),
and this activity isn`t expected to approach far northern IL.

Otherwise, the better overlap of less "mucking up" from existing
clouds/light precip, deeper moisture sloshing back north, and the
MCV trajectory generally to the south of I-80 to favor higher
thunderstorm coverage this afternoon. In the gridded forecast,
we`re continuing to highlight this (I-80 and south) area for
scattered showers and thunderstorms, with isolated to widely
scattered showers and perhaps isolated thunderstorms north of
I-80.

Despite the moist adiabatic lapse rate and weak deep layer
shear environment, up to or more than 1500 J/kg of uncapped
MLCAPE, and perhaps slightly enhanced 0-3km shear from the MCV may
contribute to isolated strong to marginally severe storms capable
of localized downbursts (50-60 mph gusts). Swath of seasonably
high 1.5 to 1.7" PWATs also sloshing eastward may support
localized corridors where torrential rainfall rates could cause
some flooding. We`ve already seen this behavior with some of the
more robust cells (non-severe microburst and ~1.25" of rain in ~25
minutes near Braidwood IL during the 2-3pm hour).

The primary threat timing window for scattered gusty storms,
localized torrential rainfall, and occasional to frequent cloud
to ground lightning strikes is roughly through 9pm CDT timeframe,
again favoring areas near and south of I-80. To the north, with
ongoing weak convection and perhaps widely scattered showers this
afternoon (~30% coverage but only isolated/20% chance of storms),
the main threat will be occasional localized downpours and
isolated sporadic lightning, winding down in the ~6-8pm timeframe.
As weak forcing pinwheels overhead through the overnight at a
diurnally unfavorable time, can`t rule out spotty showers and
maybe an isolated thunderstorm or two, with the best chances of
this primarily east of I-57. Forecast low temps are in the mid-
upper 60s outside of Chicago and around 70F in the city. If
there`s enough clearing well west of Lake Michigan later tonight,
can`t rule out some patchy fog development.

On Wednesday, in addition to isolated to perhaps widely scattered
afternoon showers/storms primarily inland of Lake Michigan, our
other forecast concern is at Lake Michigan beaches. A wind shift
to lake enhanced northerly winds behind a backdoor front/strong
lake breeze should quickly build waves during the afternoon and
into the evening. This will be driven by ~1020 mb high pressure
building down from the northern Great Lakes with the cooler air
mass in northerly fetch also bringing slightly unstable conditions
given the now 70+F water temps in the nearshore waters.

Regarding the somewhat low end convective threat, residual low-
level moisture and the warming boundary layer may allow for an
isolated shower or storm to develop by early afternoon,
particularly near the lake breeze/backdoor front. However,
coverage may be quite limited owing to increasing mid-level
subsidence and very dry air as well. Hence capped PoPs in the
20-30% range, with the focus shifting well inland of the lake by
the mid to late afternoon. Highs will range from the 83 to 87F
inland, but only the mid to upper 70s lakeside. While the cooling
will not be nearly as pronounced as with springtime lake enhanced
("pneumonia") fronts, a more noticeable lake cooling footprint
(for early July) will spread inland during the afternoon and early
evening. Wednesday night will be fairly pleasant with lows in the
low-mid 60s outside of Chicago and upper 60s in/near the city.

Castro

Thursday through Tuesday:

The influence of an area of high pressure will begin to wane across
the region on Thursday as it starts to shift to the east. That being
the case, instability largely looks to remain displaced to the west
and south of the region, with some degree of lingering subsidence
evident in area forecast soundings. The northern fringes of a
returning EML plume will begin to advect into our far south and west
locales through the morning and afternoon. Suppose it`s not entirely
out of the question that a few elevated showers/storms attempt to
fire along this axis, although any coverage at this point looks like
it`d stay quite limited. Guidance does also suggest that one or
several thunderstorm complexes may develop across central Iowa at
the nose of a modest 35-40 kt LLJ as it intersects a returning plume
of steeper mid-level lapse rates. Steering flow/upper-level winds
should safely guide any complex south and west of the CWA, however.
Blended guidance re-introduced some slight chance PoPs south of I-
80, and based on the latest guidance, didn`t see a big reason to
remove these, but planning on mentioning a largely dry day. An
afternoon lake breeze will hold temperatures in the mid-upper 70s
lakeside.

A more robust shortwave trough is forecast to shift across parts of
the Upper Midwest Friday afternoon and into Saturday. Timing,
intensity, and location differences continue across the guidance
suite today, however, with the GFS still somewhat of a south and
stronger outlier. This has implications on any strong-severe weather
threat in the region, along with timing of storm chances. At this
point, there`s still too much variability in the guidance suite to
to say one way or another if an organized severe weather (and
localized flash flood) threat will materialize, mainly across our
northwest, late on Friday into the evening, but this something we`ll
be keeping a close eye on. Regardless, gradually increasing large
scale forcing and returning moisture and instability should result
in increasing shower and storm chances, particularly late on Friday
afternoon and into the overnight hours, with at least some
attendant threat for strong storms and locally torrential rainfall.

Flow will turn northwesterly across the area on Saturday, but it
doesn`t seem like we`ll totally scour out boundary layer moisture.
With a sharp shortwave in the vicinity, additional scattered
convection may develop through the late morning and afternoon as
things destabilize with heating. By Sunday, current indications
point to moisture finally getting shunted south of the region, with
things looking largely dry. Semi-active quasi-zonal flow will
continue into next week, however, with additional shower/storm
chances returning during the Monday-Wednesday timeframe.

Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 643 PM CDT Tue Jul 8 2025

Key messages for the 00Z TAFs:

- Isolated SHRA possible early this evening for Chicago
  terminals. Have included VCSH (VCTS at GYY in closer proximity
  to higher TS threat) for this.

- Low potential for shallow fog development prior to sunrise
  mainly outside of the metro terminals. Will monitor trends
  later tonight especially for RFD.

- Isolated SHRA/TSRA possible midday/early afternoon Wed with
  cold front/lake breeze moving through Chicago terminals.

- Light S to W winds into Wed AM, shifting NE around 10 kts with
  front/lake breeze passage early afternoon for CHI terminals.

Area of SHRA/TSRA currently working east along/south of the I-80
corridor early this evening. Expectation is that most of this
will pass south of the terminals, though spotty SHRA remains
possible for the Chicago metro sites. GYY, being farthest
south has a higher TS potential, and have indicated VCTS there.
Activity exits the area late this evening (isolated convection
over parts of NE IA/WI is expected to dissipate this evening
before reaching the terminals). Lingering low-level moisture,
decreasing cloud cover and light winds overnight may allow for
shallow fog development and MVFR/IFR visibilities prior to
sunrise Wednesday. Greatest threat appears to be west of the
Chicago area and south of RFD, though can`t rule out some
visibility impacts at RFD and perhaps DPA. Confidence is low
enough to leave out of TAFs for now, but will monitor trends
later tonight and make tactical amendments if needed.

On Wednesday, a cold front is forecast to push south-southwest
and combine with the lake breeze across the area during the
midday/afternoon hours. This will result in a wind shift to the
northeast and potentially initiate isolated to widely-scattered
SHRA/TSRA. Timing of the wind shift looks to be earliest at
ORD/MDW (~17Z) and shortly after that at DPA-GYY. Farther
inland, the front is expected to move more slowly, not reaching
RFD until around 00Z. Have included a brief prob30 SHRA for the
Chicago terminals where convergence along the front/lake
boundary will be stronger. Can`t rule out isolated TS, though
coverage is expected to be low enough and of short duration so
have not mentioned explicitly in TAFs.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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