965
FXUS63 KLOT 161130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Mon Jun 16 2025

.KEY MESSAGES...

- Warmer and gradually more humid away from the lake today, with
  a low (<20%) chance of a few isolated afternoon thunderstorms
  mainly well west and south of the Chicago metro.

- Thunderstorm chances increase late Tuesday, with multiple
  rounds of storms possible Tuesday night through Wednesday
  evening. Some storms may be severe and have torrential
  rainfall, particularly on Wednesday.

- Hot and humid conditions are on track to arrive this
  weekend.

&&

.DISCUSSION...
Issued at 331 AM CDT Mon Jun 16 2025

Through Tuesday evening:

Warmer and increasingly more humid weather is expected as we
start the new week. Isolated to widely scattered thunderstorm
chances return to western/southern parts of the forecast area
this afternoon, with a greater potential for scattered strong to
severe storms Tuesday afternoon and evening.

Early morning GOES vapor imagery depicts a rather disorganized,
weak upper level trough from AR/MO into IL. Farther north, a
short wave trough was evident across the upper Mississippi
Valley. Guidance indicates some phasing will occur between these
features today, with the resulting mid-level trough axis
drifting east across the region through tonight. At the surface,
an elongated west-east oriented high pressure ridge stretching
from the Great Lakes through New England is forecast to move off
to the east, allowing low level winds to turn more southerly
this afternoon. While flow through the column is relatively
light beneath the baggy upper trough, this will allow for modest
warm/moist advection which will result in warmer and
increasingly more humid weather today. Low-level model thermal
progs support highs in the mid-upper 80s this afternoon, except
for cooler 70s near the lake with modest east-southeast onshore
winds there. Higher surface dew points in the mid-upper 60s from
eastern IA to central IL are expected to spread into the
southern and western parts of the forecast area this afternoon,
resulting in conditional instability (MLCAPEs ~500-800 J/kg
during peak heating) which may allow for some convective
shower/storm development in these areas, though ephemeral low-
level convergence and weak mid-level lapse rates (5.5-6.0 C/km)
and wind profiles suggest only isolated to widely scattered
coverage. The 00Z HRRR depicted a significantly larger footprint
of afternoon convection over east/northeast IA and suggests an
outflow boundary may spread into northwest IL, which would
potentially allow for greater thunderstorm potential into our
western cwa. Successive runs of the HRRR have really backed away
from this scenario however.

Any isolated showers or thunderstorms which linger across the
western and southern parts of the forecast area should dissipate
by mid-late evening with the loss of diurnal instability. The
reminder of the evening and overnight hours currently are
anticipated to be fairly quiet, with convection mainly focused
well west and northwest of the area along a cold front
stretching from the upper Mississippi Valley, southwest into
IA/KS. Some CAM guidance hints at the potential for an outflow
boundary originating with this upstream convection to spread
into WI/northern IL during the predawn hours of early Tuesday
morning, which could potentially produce some scattered
convection into northern IL. Confidence is low in this level of
detail at this point, though have maintained some slight chance
(<24%) pops northwest of about a Sterling to Highland Park IL
line for this possibility as well as to maintain some continuity
from the previous forecast. Otherwise, warm and somewhat muggy
conditions are expected overnight with lows in the upper 60s in
most spots and a light southwest wind.

Have maintained similar slight chance pops along/north of the
I-88 and Eisenhower expressway corridor across northern IL into
Tuesday morning, again for the possible outflow boundary
scenario. Greater potential for scattered thunderstorms
continues to be focused from later Tuesday afternoon into
Tuesday evening however, as a low-amplitude short wave tracks
east atop a diurnally very warm and seasonably humid warm sector
air mass across the region. Model low-level thermal fields
support afternoon high temperatures around 90 degrees (mid-90s
peak heat indices), with forecast soundings indicating
potentially 1500-2000 J/kg MLCAPE values by late afternoon.
Deep-layer bulk shear appears somewhat limited at <25 kts,
though given the instability a risk of damaging wind gusts and
perhaps some marginally severe hail would likely be supported by
any more organized storms. While confidence in the mesoscale
details at this distance remains on the low side, both the HRRR
and RAP depict some degree of MCS organization with convection
reaching the area late Tuesday afternoon/evening. SPC`s latest
Day 2 outlook maintains a marginal (level 1 of 5) risk of severe
weather across the IL portion of our forecast area for this
potential.

Beyond midnight Tuesday night, confidence really decreases in
convective trends through the overnight hours. Guidance has
trended a little slower with the more amplified upstream short
wave over the Plains, which may make for a lull in shower and
thunderstorm threat until after sunrise Wednesday. New 06Z 4km NAM
however depicts redevelopment of elevated convection overnight.
With such model spread have lowered NBM pops slightly, but
confidence remains low in trends after midnight.

Ratzer

Wednesday through Sunday:

An upper level trough over the northern Plains will deepen
Tuesday night and move eastward over the forecast area into
Wednesday. It is likely that there will be additional showers
and storms on Wednesday, but there is still so much uncertainty
surrounding the details of the event, which was still apparent
in the 00Z model suite. The EPS still has a fairly wide spread
on the track of the surface low with the bulk of the members
passing directly over Chicago, the GEFS is trying to hone in on
southern Wisconsin. Meanwhile the deterministic solutions have
the Euro`s surface low along the Wisconsin stateline, the GFS
over Chicago, and the NAM has it in Central Wisconsin. There is
also disagreement on shear profiles on model hodographs through
the day in both strength of low level shear as well location of
the best shear. Nevertheless, models are still suggesting CAPE
values greater than 2000 J/kg in the afternoon, a strengthening
low level jet, and deep layer shear around 40 to 50 knots. This
keeps a signal for a convective event which could be severe (the
SPC keeps the forecast area in a level 2 out of 5 "Slight" risk
of severe weather) on Wednesday. Lastly, model p-wats remain
over 1.75 inches during the day Wednesday with the potential for
up to 2.00 inches. The threat of downpours and localized
flooding will also be possible, especially if training of cells
occurs.

Toward the end of the week and into the weekend, ensemble model
guidance exhibits a strong signal for a deep trough over the
western United States and a pronounced ridge to the east. This
set up will allow for a very warm and humid air mass to push
eastward and encompass the Lower Great Lakes. Ensemble mean high
temperatures nose into the low to even mid 90s by this weekend.
With dew points climbing above 70F, this will result in
afternoon heat indices exceeding 100 degrees. It should be
mentioned that such a pattern may attract additional clusters
of showers and thunderstorms to develop, which may delay the
onset of heat. At any rate, this weekend is trending to be the
first instance of summer heat and humidity.

DK

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 630 AM CDT Mon Jun 16 2025

Key Aviation Messages for the current TAF period:

- Less than 30 percent chance for a shower/storm at KRFD today

VFR conditions are expected through the TAF period. Winds are
expected to be light and out of the southeast, gradually
becoming south around 00Z and southwest overnight. However, wind
speeds should be around 10 knots this afternoon, and therefore
no impacts expected to terminals.

There are two opportunities for shower chances near KRFD today.
First, there is a weak disturbance over SW Wisconsin. Perhaps a
brief shower develops near/at the KRFD terminal this morning,
but confidence is low. Additionally, instability and moisture
will increase through the afternoon away from the Chicago Metro
area. This could allow for isolated showers and storms late
afternoon/early evening. However, with both the morning and
evening scenarios, the chance for precip is less than 30 percent
and thus was kept out of the TAF presently.

There is no precip expected at Chicago terminals currently.
There is a weak signal for isolated showers and storms around
12Z tomorrow. However, there is not great agreement in models in
it which lowers confidence. For now, with the probability less
than 30 percent, the TAFs were kept dry with only SCT mention
for the slight uptick in clouds between 2000 to 3000 ft.

DK

&&

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

&&

$$

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