343
FXUS63 KLOT 281901
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
201 PM CDT Sat Jun 28 2025

.KEY MESSAGES...

- Heat and humidity return Sunday with peak heat indices around
  100 degrees.

- Isolated-widely scattered thunderstorms possible Sunday
  afternoon and night with greater coverage of showers and
  thunderstorms expected Monday.

&&

.DISCUSSION...
Issued at 155 PM CDT Sat Jun 28 2025

Through Sunday Night:

Main belt of westerlies remains well north of the area, close to
the U.S. Canadian border. To the south of the jet stream there
is smattering of MCVs across the Mississippi Valley region. One
such MCV is moving into northwest IL early this afternoon,
encountering a progressively drier and more stable air mass as
it moves eastward. Despite the lack of convection, there remain
some high based showers/sprinkles and a compact area of mid-
level cloudiness. Can`t rule out a few sprinkles making it into
our northwestern CWA this afternoon before precip likely dries
up with this feature as it continues eastward.

Air mass is quickly recovering across eastern South Dakota into
Minnesota in the wake of a pair of MCVs (one near MSP and the
other over northern WI/western upper Michigan). Fairly strong
signal in most available guidance that convection will develop
over central/southern MN later this afternoon into this evening,
likely organizing into an MCS and probably spawning another MCV
or two tonight. MCS is likely to track eastward across WI
tonight into early Sunday morning with precip expected to remain
to our north.

Of greater interest to our area is the potential for southward
moving outflow emanating from this MCS potentially impacting our
CWA Sunday. Sfc flow is expected to become southerly Sunday as
sfc high scoots off to the east, but gradient looks to be fairly
baggy, so there doesn`t look to be too much momentum to stop
southward progression of the outflow boundary, particularly if
the MCS were to remain most robust into eastern WI early Sunday
morning. Most CAM guidance keeps outflow to our north with
little impact on our area, while the 12z HRRR was most
aggressive with southward movement of this outflow and would be
most impactful to our area.

A hot, humid, and moderately to strongly unstable air mass
should encompass our CWA Sunday with minimal capping during the
afternoon. There appears to be a couple of plausible
scenarios for Sunday:

Scenario 1: WI MCS outflow remains to the north of the CWA.
Unstable and uncapped air mass could allow for some isolated
afternoon "air mass" thunderstorms to pop just about anywhere,
maybe more focused with any remnant MCVs meandering into the
region from southern MO. Any such MCV is really hard to
forecast this far out. In this scenario, a weak lake breeze may
attempt to move inland along the IL north shore with convection
likely to remain mostly just offshore, at least initially.

Scenario 2: WI MCS sends outflow boundary farther south. In this
scenario, where ever the outflow boundary ends up, it could be
a focus for more concentrated afternoon convection. In addition,
this scenario would likely feature a more robust lake breeze,
enhanced by the outflow, which would also likely be another
focus for convection.

CAMs and most guidance in general is pretty reserved in
convective development across our area tomorrow, likely due to
the weak forcing. Given the low confidence have maintain slight
chance pops across the entire CWA, with a corridor of higher
pops along the western shore of Lake Michigan where lake breeze
convergence could be enough to trigger convection given the
expected weak capping.

Temperature, dewpoint, and resultant heat index forecast for
Sunday is also pretty tricky. Most guidance suggests dewpoints
could mix out a bit Sunday, particularly in the urban corridor
of the Chicago metro. Upstream dewpoints are solidly in the
70s and guidance was too aggressive in mixing out dewpoints
earlier this week, so leaning toward the higher guidance with
respect to dewpoints and heat indices. Unless a 12z HRRR
scenario with a stronger lake breeze pans out, then lake cooling
tomorrow afternoon looks to be pretty limited and confined to
the IL north shore area.

More organized convection Sunday afternoon is expected to be
closer to the synoptic front to our northwest across WI into IA.
Given the weak shear and lack of a strong low level jet, it
seems likely that this convection probably won`t survive long
into the night Sunday night. Just carrying chance pops for some
of these storms surviving the night and making it into the area.

- Izzi


Monday through Saturday:

Forthcoming...

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1238 PM CDT Sat Jun 28 2025

VFR conditions are expected through the period. An area of mid-
level clouds with the remnants of convection could produce
sprinkles at RFD through mid-afternoon.

A lake breeze currently nearing ORD/MDW will result in ESE
winds up to 10 knots through the day, with light winds in the SE
quadrant continuing through tonight. Winds will then shift SSW
around 10 knots shortly after sunrise on Sunday.

While the TAF for ORD/MDW is currently dry for Sunday
afternoon, a developing lake breeze as well as any stalling
outflow boundaries (from expected convection well to the
northwest tonight) could provide an impetus for isolated TS over
northeast Illinois as early as 20Z Sunday. Future TAF
iterations may require of at least PROB30 TS if confidence in
occurrence and/or coverage continue to increase.

Kluber

&&

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

&&

$$

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