294
FXUS63 KLOT 220539
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1239 AM CDT Sun Jun 22 2025

.KEY MESSAGES...

- Dangerously hot and humid conditions will continue through at
  least Monday evening. A Heat Advisory remains in effect through
  Monday evening for the entire CWA except Cook County IL where
  an Extreme Heat Warning is in effect.

- The magnitude of heat should lessen by Tuesday, though heat
  indices will still be in the upper 90s to lower 100s through
  much of the next workweek. (No planned extensions of the
  ongoing heat products).

- Chances for episodic thunderstorm clusters increase
  incrementally Tuesday onward. Some may be severe with
  damaging winds and flash flooding.

&&

.DISCUSSION...
Issued at 152 PM CDT Sat Jun 21 2025

Through Monday:

No meaningful changes needed to the forecast through Monday
evening with hot and humid conditions likely to continue. Sunday
should be a near carbon copy of today with highs in the low-mid
90s, warmest in the urban corridor of NE IL. Dewpoints Sunday
will also likely be very similar to today with low-mid 70s
leading to peak afternoon heat indices mostly in the 100-105
degree range. Guidance does suggest somewhat deeper mixing
Monday, which could allow dewpoints to mix out some in the
afternoon. Confidence is low as to if that deep mixing will
occur, and if so, to what extent dewpoints will drop in the
afternoon. Deeper mixing and lower dewpoints would likely allow
temps to get a couple/few degrees warmer on Monday than today
and Sunday. If dewpoints really tank Monday afternoon, it is
possible that heat indices could fail to reach 100 degrees, but
given low confidence in this scenario, opted to stick with the
NBM guidance which has dewpoints mixing into the 60s with
another day of 100F+ heat indices.

Strong and gusty southwest winds will continue the rest of this
afternoon and only gradually ease this evening. As low level
jet develops this evening, its possible that we could see a few
hours of stronger winds/gusts around the urban heat island of
Chicago where higher temps should keep the boundary layer
somewhat mixed late into the evening. Wouldn`t even rule out a
few gusts to around 40 mph in Chicago until late evening. Sunday
should be another windy day, though likely averaging about 5kt
less so than today.

The relatively high dewpoints and southwesterly winds should
result temps being pretty slow to fall this evening and again
Sunday evening. In fact, in the urban heat island corridor of
the Chicago area wouldn`t be surprising to see low temps only
bottom out in the lower 80s tonight and again Sunday night with
heat indies unlikely to fall below 90 until midnight or a little
after.

Skies are expected to remain mostly clear through Monday. By
Monday afternoon the mid-upper level ridge is forecast to begin
weakening, mainly especially to our northwest. This should allow
the "ring of fire" of convection should inch closer to our CWA,
though through the afternoon hours Monday it still looks like
convection should remain northwest of our CWA.

- Izzi

Monday night through Friday:

Toward the middle of next week, the upper-level ridge
responsible for our heat wave is expected to flatten somewhat
resulting in quasi-southwest to zonal flow across the Upper
Mississippi River Valley. As a result, the low-level baroclinic
zone acting as a highway for episodic showers and storms (the
"ring of fire") should drift southward toward the Lower Great
Lakes by the middle of the week. At the same time, the strength
of the low-level thermal ridge is expected to weaken, resulting
in gradually less capping. When taken together, confidence is
increasing in a pattern change in the Tuesday to Wednesday
timeframe toward stormier conditions likely tied to both the
diurnal cycle and any incoming subtle shortwaves in the
southwest to zonal flow aloft, albeit with continued warm
temperatures.

At this point, it appears that Tuesday will be the first day to
feature low-end chances (30-40%) scattered afternoon
thunderstorms, perhaps focused along any residual outflow from
prior convection to our north and/or a lake breeze. Coverage of
afternoon thunderstorms may then incrementally increase
Wednesday through Friday owing to progressively weaker capping
and the southward drift of the baroclinic zone. Blended NBM
guidance offers nearly continuous mid-range chance PoPs (40 to
60%) Wednesday through Friday, which seems reasonable given the
pattern. Of course, many hours will be dry.

With the upper-level jet and associated availability of shear
currently expected to remain focused north of our area, the
airmass within and south of the baroclinic zone will be somewhat
stagnant and prone to moisture pooling. As a result, the
overall kinematic and thermodynamic regime next week will be
characterized by high PWATs (potentially nosing north of 2" at
times), weak mid-level lapse rates, daily afternoon MLCAPE >2000
J/kg, and meager deep-layer shear. Such a regime should limit
the threat for widespread/high-impact severe weather (e.g., no
derechoes), but can nevertheless support episodic clusters of
thunderstorms with a threat for torrential downpours, prolific
lightning, and strong to damaging winds. Both NCAR and CSU ML
probabilities for severe weather hang in the 5 to 15% range
Tuesday through Friday, consistent with a Level 1 to 2 out of 5
threat for severe weather each day. Also, could also see a
threat for localized flooding where thunderstorms impact the
same area.

Outside of thunderstorms Tuesday through Friday, it will remain
hot and humid. Daily highs in the low to mid 90s with dew
points in the low to mid 70s will support daily afternoon peak
heat indices in the upper 90s to lower 100s each day (not quite
105 on a widespread basis). With the baroclinic zone so close by
or directly overhead, perhaps at times modulated by convective
outflow, there will finally be an opportunity for Lake Michigan
to provide local cooling to shoreline locations. So, at this
point, the end time of the Extreme Heat Warning for Cook County
at midnight Tuesday morning appears to be in good shape.
Elsewhere, the Heat Advisory is currently slated to also end
early Tuesday morning, which appears reasonable. (If heat
indices were expected to remain at or above 105 on a widespread
basis through Friday, we`d have to consider alterations to the
ongoing heat products).

All in all, the pattern next week should be quintessential of
mid to late June.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1238 AM CDT Sun Jun 22 2025

VFR and dry conditions are forecast through the period. South
southwest winds have eased across the area early this morning
around 10kt with only sporadic gustiness lingering. Plan to
hold onto low-level wind shear (LLWS) through 12Z this morning
with the low-level jet (LLJ) expected to gradually weaken
through the remainder of the overnight hours. While not as
strong as yesterday, today is expected to be breezy again, with
gusts in the upper 20kt range developing by the afternoon. Winds
ease again toward sunset with another round of LLWS possible,
mainly toward RFD, with the LLJ expected to be a bit weaker
further east over the Chicago metro.

Petr

&&

.CLIMATE...
Issued at 152 PM CDT Sat Jun 21 2025

Temperatures through early next week will threaten record highs
and record warm lows for Chicago and Rockford. Here are the
current records:

Chicago              High       Warm Low
Saturday 6/21     101 (1988)    74 (1923)
Sunday 6/22        97 (1988)    76 (1923)
Monday 6/23        97 (1930)    79 (1923)

Rockford             High       Warm Low
Saturday 6/21     100 (2022)    71 (1995)
Sunday 6/22        97 (1923)    73 (1908)
Monday 6/23        97 (1923)    74 (1908)

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until midnight CDT Monday night for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-
     ILZ107-ILZ108.

     Extreme Heat Warning until midnight CDT Monday night for
     ILZ103-ILZ104-ILZ105.

IN...Heat Advisory until midnight CDT /1 AM EDT/ Monday night for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory from noon today to 7 PM CDT this evening
     for the IL and IN nearshore waters.

     Small Craft Advisory until 3 AM CDT early this morning for
     Wilmette Harbor to Calumet Harbor IL.

&&

$$

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