619
FXUS63 KLOT 290514
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1214 AM CDT Sun Jun 29 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 341 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:

The muggy air mass from Sunday will still be in place over the
region at the beginning of the day on Monday. Between a
deepening upper-level trough swinging into the Great Lakes and
an associated southeastward-tracking cold front, the more
coherent synoptic-scale forcing in the region should lead to a
greater coverage of diurnal convection in our area on Monday
compared to Sunday. Residual mesoscale features from Sunday`s
convective cycle, including remnant outflow boundaries or MCVs,
could also augment the synoptic-scale forcing and serve as foci
for convective development, though the extent to which those
might play a role on Monday likely won`t be known until Sunday`s
convection has shown its hand.

The stronger-than-usual synoptic-scale forcing for this time of
year overspreading a moisture-laden and unstable air mass has
been causing coarser-resolution models with parameterized
convection to be quite overzealous with their QPF output. This
has bled into the NBM, whose PoPs for Monday are likely too high
based on the few 12Z CAMs whose range currently extends into
Monday. That said, between whatever leftovers there are from
Sunday/Sunday night`s convection and any subsequent new diurnal
convective development, have to think that there will at least
scattered showers and storms around in our forecast area during
the daytime on Monday given the overall setup even though some
of the finer-scale details of the forecast remain uncertain at
this time.

If any morning convection/cloud cover doesn`t end up being too
much a hindrance to diurnal destabilization efforts, then a
similar to perhaps slightly better environment for deep
convection compared to our past several days of convection
should materialize during the afternoon. Slightly greater deep-
layer shear would tend to promote loosely-organized multicell
clusters with a pulse-like character. The most robust convection
would likely be capable of producing water-loaded downbursts
that could cause wind damage on an isolated/localized basis. The
potential for this appears to be greatest across southeastern
portions of our forecast area, where there may be more time for
destabilization to occur and for thunderstorms to mature, though
it`s not out of the question at this time that this threat could
end up materializing farther to the north, pending the mesoscale
details. The latest Day 3 Convective Outlook from the Storm
Prediction Center accordingly has our southeastern CWA in a
Marginal (level 1/5) Risk for severe thunderstorms, which
appears to look appropriate at this time.

After the aforementioned cold front passes through the region,
surface high pressure and a slightly cooler (though still
seasonably warm) and less humid air mass will briefly settle
into the region on Tuesday and Wednesday. However, as ridging
sets up over the central CONUS during the mid to late week time
frame, a gradual warming trend should ensue going into the
weekend. Richer moisture will also be advected back into the
region, supporting renewed chances for showers and thunderstorms
going into the upcoming holiday weekend.

Ogorek

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1214 AM CDT Sun Jun 29 2025

The key aviation weather messages are:

- Isolated thunderstorm development possible along a lake breeze
  this afternoon. Coverage/chances currently too low for VCTS
  mention near Chicago terminals attm.

- Additional low chances for showers/storms Sunday night-early
  Monday AM, but confidence once again too low to justify TS
  mentions.

Variable to light SE winds will prevail tonight with VFR
conditions. Winds will gradually become SW through Sunday
morning around 5-10 kt.

Airmass will become hot/humid and unstable during the late-
morning and afternoon. A sharpening lake breeze may serve as a
focus for isolated thunderstorm development, but confidence in
TS remains under 50% threshold for VCTS mention at this point.
Have continued with "dry" TAFs at the Chicago-area sites, but
consideration for TS mention may be necessary in future
issuances. If storms develop on the lake breeze, east/northeast
wind shifts would be in play in the afternoon.

Additional isolated-scattered showers and storms may develop
Monday evening and overnight. Signal for TS during this period
again appears to nebulous to justify a thunder mention however,
with chances currently around 20-30 percent.

Carlaw

&&

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

&&

$$

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