164
FXUS63 KLOT 061739
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 PM CDT Sun Jul 6 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected today (50 to
  60% chance), ending from north to south this evening. The main
  concern will be torrential rain with localized flooding
  possible. Some areas may stay completely dry.

- Dangerous swimming conditions at Lake Michigan beaches this
  afternoon through midday Monday.

- A period of dry and slightly cooler weather is on track to
  start the upcoming week, though isolated showers and storms
  cannot be ruled out in the Tuesday night through Thursday
  timeframe (20-40% chance or so each day).

&&

.UPDATE...
Issued at 1059 AM CDT Sun Jul 6 2025

The overall forecast remains on track late this morning. A
remnant outflow boundary and collocated localized vorticity
maximum associated with an earlier MCV over southern Wisconsin
has provided the initial focus for showers as of 10am. This
axis, roughly from Ogle county to Waukegan, will continue to
drift southeast through mid-afternoon as surface-based
destabilization increases. An cold front arcing from the Quad
Cities to just south of Madison to the northern Milwaukee metro
will overtake the initial boundary and allow for a renewed focus
for higher coverage of showers and some thunderstorms. 12Z
DVN/ILX RAOBs show favorable conditions (PWAT near 2", low-
shear, poor mid-level lapse rates) for low-centroid showers and
some storms with localized torrential rain rates. A very
localized flash flood threat cannot be ruled out, but in the
absence of deep-layer forcing, confidence in higher coverage of
flooding potential remains too low for issuance of a Flood
Watch.

Kluber

&&

.DISCUSSION...
Issued at 218 AM CDT Sun Jul 6 2025

Through Monday:

A cold front will be moving south across the area later today
into this evening with the models attempting to develop a
secondary, weaker area of low pressure along the front near the
southern end of Lake Michigan this afternoon. This should allow
for scattered thunderstorm development from late morning through
late afternoon, perhaps with the best coverage across northeast
IL and northwest IN. Overall coverage remains uncertain but
given the pattern, maintained likely pops across the south, but
increased pops to likely for much of the Chicago metro area and
its possible pops may need to be increased further as trends
emerge later today. Main threat from these storms will be very
heavy rain with precipitable water values approaching 2 inches.
While much of the area is in drought conditions, should this
heavy rain materialize over urban areas, there will a localized
flooding potential. Likewise, some areas may remain completely
dry today as well. Though the marginal/level 1 severe risk is
just east of the local area, a few stronger storms are
possible, with damaging wind gusts the main threat. Model trends
are looking good for this activity steadily dissipating and
shifting south through the evening hours. Maintained low chance
pops in the far southeast overnight, but even during this time
period, precip may be over. Monday is also looking dry for the
area and removed pops from the southern cwa.

Northerly winds will increase behind the cold front this
afternoon, possibly gusting into the 30 mph range over southern
Lake Michigan, especially if another surface low develops. This
will allow waves to quickly build into the 4-6 foot range,
creating dangerous swimming conditions at Lake Michigan beaches.
These northerly winds will continue into Monday afternoon when
they finally begin to diminish and depending on how fast that
happens, the beach hazard statement may need to be extended.

High temps today are somewhat dependent on cloud cover/precip
timing, but with much of the area likely being dry until late
morning and then the expected scattered nature to the precip,
highs in the mid 80s are still possible. Then as the cold front
moves south, temps will likely fall into the 70s across
northeast IL by late afternoon and possibly even into the upper
60s near the lake by evening. Cloud cover may persist south of
Lake Michigan into Monday morning, but with clearing skies west
of the lake, low temps may drop into the lower 60s, even a few
upper 50s Monday morning, across the western cwa. Highs look to
return into the 80s for most areas Monday, cooler near Lake
Michigan. cms


Monday Night through Saturday:

Monday night through Wednesday, the overarching upper-level
pattern from the northern Plains to the Great Lakes is expected
to be characterized by broad/disorganized northwesterly flow.
Ensemble model guidance suggests a shortwave moving along the
US/Canadian border, perhaps augmented by upstream convection in
the Plains, will move into the Great Lakes region sometime in
the Tuesday night to early Wednesday timeframe. If such timing
were to hold (definitely no guarantees), any convection that
develops Tuesday afternoon across Iowa would be prone to
weakening by the time it reaches our area Tuesday night. Tuesday
otherwise looks like a nice day with highs in the mid to upper
80s

Wednesday and Thursday, upper-level ridging will build
northward toward Hudson Bay atop weak troughing across the Gulf
Coast. Such a regime will lead to mid-level heights "spreading
out" across the Mississippi River Valley, altogether leading to
somewhat benign weather. Healthy cumulus fields each day may
sprout an isolated storm or two, particularly near daily lake
breezes where localized convergence will be maximized. With
little to no flow through the meager convective layer, any storm
that manages to develop would be fairly short-lived with
threats for occasional lightning strikes and downpours. Will
maintain the NBM PoPs of 20 to 40% through the period, keeping
in mind they are likely overdone for many areas. Highs both days
should be in the mid to upper 80s, with cooler readings in the
upper 70s in the wake of the aforementioned lake breezes.

Friday into the weekend, the upper-level low currently stalled
in the Gulf of Alaska is expected to swing southeastward and
track along the US/Canadian border. At the same time, the upper-
level low meandering off the California coast is expected to
swing east-northeastward atop a strengthening ridge over the
southwestern US, placing it in line to interact with the
approaching trough from the northwest. If and how both interact
will be something to watch as we get closer. Regardless, the
general idea is that chances for showers and storms should
return to the general region sometime next weekend.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1239 PM CDT Sun Jul 6 2025

Key messages:

- Scattered showers and thunderstorms ahead of a cold front this
  afternoon, mainly prior to 22z. Expect a short period of
  torrential downpours and possible IFR conditions.

- Gusty northeasterly wind shift with frontal passage at the
  Chicago area terminals (2030 to 2100Z at ORD and DPA and 21 to
  22z at MDW and GYY).

- Low MVFR CIGs return in the wake of the front, and may persist
  near the lake tonight.

Showers and thunderstorms continue to increase in areal coverage
across southern WI and far northern IL in advance of a cold
front early this afternoon. These showers and storms are
expected to impact all the terminals through 21-22Z this
afternoon. Very heavy downpours will accompany these showers and
storms, and will likely result in at least temporary IFR VSBYs
down around 1SM and locally lower CIGs.

The surface cold front has become lake enhanced over
southeastern WI, which is allowing it to race southward down
the western shores of Lake Michigan. It recently moved through
KMKE and KRAC in WI, and in its wake winds rapidly turned to the
northeast, with some gusts in the 20 to 25 kt range. Similar
conditions are expected at the Chicago terminals after 20Z. A
simple extrapolation of the front puts it in the KORD area
around 2030z and at KMDW ~2115Z. Once this front passes, expect
the threat of storms to gradually come to an end from north to
south across the area.

Northeast winds off the lake will likely drive some lower end
MVFR CIGs (~1,000 AGL) back in across the Chicago area
terminals late this afternoon into this evening. It is possible
that these lower CIGs scatter out for a period later this
evening, but may then return for a period late tonight into
early Monday morning before diurnally improving. Otherwise,
expect northeast winds through the day Monday.

KJB

&&

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

IN...Beach Hazards Statement from 6 PM CDT this evening through
     Monday afternoon for INZ001-INZ002.

LM...Small Craft Advisory until 9 AM CDT Monday for the IL
     nearshore waters.

     Small Craft Advisory from 6 PM this evening to 9 AM CDT Monday
     for the IN nearshore waters.

&&

$$

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