306
FXUS63 KLOT 171751
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

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

- Hot and humid conditions expected to arrive this weekend.

&&

.UPDATE...
Issued at 1125 AM CDT Tue Jun 17 2025

Forecast looks to be on track with no big changes made this
morning. Surface analysis shows a very moist air mass in place
with dewpoints in the lower 70s across most of our CWA late this
morning. Some slightly drier air is seeping into our far western
and northwestern CWA where dewpoints are in the mid-upper 60s.
Plenty of sunshine should allow boundary layer to become
moderately unstable this afternoon with MLCAPE of 1500-2000
J/kg, highest in the axis of better moisture.

Forcing this afternoon and evening looks somewhat nebulous, so
that does lower confidence in convective forecast some. However,
guidance is keying in on a subtle shortwave tracking eastward
across IA leading to a ramp up in coverage of storms very late
this afternoon and into this evening. While high cloudiness is
making it tough to see this shortwave on any of the water vapor
channels, there has been mid-level ACCAS development noted on
visible satellite imagery over southeast IA/northeast MO, which
could be a sign of some increased ascent ahead of this model
depicted shortwave in IA.

Vertical shear is and should remain quite weak this afternoon
and early evening, so threat of any organized severe weather
looks quite low. Storm mode should be pulse to weakly organized
multicellular with perhaps an isolated wet microburst or two
leading to some locally stronger wind gusts.

- Izzi

&&

.DISCUSSION...
Issued at 350 AM CDT Tue Jun 17 2025

Through Wednesday Night:

Early morning surface analysis depicts low pressure over
western Lake Superior, with a cold front trailing through
northwest WI into KS. Several significant outflow boundaries
associated with ongoing MCS clusters across NE/KS have muddled
the location of the front in those areas. Closer to home, a few
spotty showers have occasionally bubbled south of a PNT-IKK-VPZ
line from an apparent weak MCV and outflow from scattered late
Monday storms. This little disturbance should continue to drift
southeast of the area this morning. To our north, isolated
showers/storms have been dissipating across east-central WI, also
along a weakening outflow boundary. This remnant outflow may
settle into far northern IL this morning, though subsidence in the
wake of an eastward-drifting upper trough axis is expected to
help maintain a capping inversion through at least midday which
should preclude any significant potential for shower/storm
development during that time. Another remnant MCV was also noted
across southeast IA, associated with an MCS which moved through
the Dam area last evening. This feature is expected to drift
east- southeast into central IL later today, and may work to erode
capping somewhat by early afternoon across our southwestern cwa.

While effects of some of these more subtle features are
uncertain for our cwa through midday/early afternoon,
thunderstorm potential does look to increase by late afternoon
as a less subtle (and likely convectively-assisted by ongoing
MCSs) mid-level short wave moves slowly east from the Plains.
CAM guidance is in fairly good agreement in developing
convection into our western/northern cwa by late this afternoon
and spreading it eastward across much of the remainder of the
area this evening. Model forecast Mlcape are generally in the
1500-2000 J/kg range initially, though with marginal deep shear of
20-25 kts. This should allow strongest cells to have a
wind/marginal hail threat into the evening hours, before
instability gradually diminishes with the loss of diurnal heating.


Confidence then diminishes with respect to timing/details of
additional rounds of thunderstorms overnight and into Wednesday.
Synoptically however, the more amplified Plains short wave
continues to track east toward the mid-upper Mississippi Valley
tonight, with general guidance consensus in a deepening surface
low developing into eastern IA by 12Z Wednesday. This induces a
strengthening 30-35 kt southwesterly low-level jet late tonight
into Wednesday, which is likely to support additional
thunderstorm development. Overnight storms may be somewhat
elevated, particularly if evening convection leaves a more
substantial surface cold pool. Increasing moisture
transport/theta-E advection (p-wats approaching 2.00" or 175% of
normal) will present a threat of locally heavy rainfall amounts
and flooding potential with these clusters.

Surface low is progged to continue to deepen to around 1000 mb
Wednesday as it moves across southern WI. Despite the potential
for a stable cold pool across the area from overnight storms,
breezy southwest low-level winds would appear to favor at least
some recovery and destabilization of the airmass by early
afternoon. Uncertainty does remain with respect to remnant
outflow boundary position and cloud cover, though moderate
instability coupled with increasing 40-50 kt mid-level flow
suggests an increasing severe thunderstorm threat by Wednesday
afternoon. This environment would be supportive of organized
clusters and supercells, capable of all hazards. SPC has upgraded
their Day 2 outlook to include an enhanced risk across our
southeastern cwa for these threats. The mid-level trough axis and
surface cold front move across the forecast area from west to east
Wednesday evening, ending the thunderstorm and severe weather
threat.

Dependent on cloud cover today, very warm and humid conditions
are expected with afternoon highs expected from the upper 80s to
around 90 (peak heat indices in the low-mid 90s) in most areas.
Cloud cover and precipitation should limit temperatures to the
upper 70s to mid- 80s Wednesday, with warmest readings in the
southeast.

Ratzer

Thursday through Monday:

As the surface front exits to the east, an upper level ridge
over Texas will grow through Thursday and drift east on Friday.
As skies clear Thursday afternoon, temperatures should warm into
the low to mid 80s. Westerly winds on Thursday will slowly turn
to the southwest overnight into Friday, increasing warm air
advection into the region. Even with more mid level moisture
moving in to increase cloud cover on Friday, temperatures are
expected to continue to warm solidly into the mid to upper 80s.
Long range models are hinting at a few weak perturbations over
Wisconsin with associated vort maxes that may drift southward
over the stateline that could be just enough forcing to trigger
an afternoon shower/storm on either day. Confidence remains low
at this range, but felt no need to remove the slight (less than
25 percent) chance the NBM provided in northeastern Illinois.

That upper level high will continue to grow on Saturday and
Sunday with a surface high developing over the eastern CONUS.
With strong southerly flow into the western Great Lakes, 850 mb
temperatures will climb to 20 to 23C. That same flow will also
increase Gulf moisture into the region increasing dew points
this weekend into the 70s. Hot and humid conditions are expected
as surface temperatures expected to climb into the 90s and heat
indices into the 100s. Little relief can be expected overnight
as low temperatures Sunday and Monday morning are expected to
only get down into the 70s. There is still some concern that
with the amount moisture available, scattered diurnal
thunderstorms or even overnight clusters could prevent sunshine
from creating the heating necessary for the aforementioned heat
indices. But with strong subsidence over the region, the going
forecast remains dry. For now, Saturday and Sunday look to be
the hottest days. However, ensemble guidance does not show much
movement to the high early next week which could keep warmer
temperatures lingering and only gradually step down into the
middle of next week. While there is a signal for an upper level
trough out west to swing over the Plains and approach the area
around Tuesday/Wednesday next week, there is lower confidence in
details on its influence and any precipitation chances.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1251 PM CDT Tue Jun 17 2025

Key Aviation Messages for the current TAF period:

- Chances for thunderstorms this evening. Low confidence in
  timing and coverage.

- Additional chances for thunderstorms midday through mid
  afternoon Wednesday.

No major forecast changes from the previous forecast. Breezy
southwest winds will continue across the terminals through the
afternoon. Have maintained the PROB30 mention for thunderstorms
early this evening as we continue to monitor a disturbance
approaching from southeast Iowa and northeast Missouri.
Instability is in place so do expect at least scattered
thunderstorm development later this afternoon as this
disturbance arrives which would work eastward towards the
terminals early this evening. Debated adjusting to a TEMPO for
TS but confidence is not all that high, especially when it comes
to coverage. May need to adjust the timing slightly later and
the later portion of the PROB30 period may yield the best chance
for storms to occur. A lull in activity is expected late
tonight and early Wednesday morning before a cold front
approaches closer to mid day bringing another window for
thunderstorm activity into mid Wednesday afternoon. South winds
Wednesday will shift west-southwest with the frontal passage
later in the afternoon and MVFR ceilings look to be in place
behind the front as well.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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