Joliet, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

619
FXUS63 KLOT 230556
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1256 AM CDT Mon Jun 23 2025

.KEY MESSAGES...

- Dangerously hot and humid conditions will continue through
  Monday evening. A Heat Advisory remains in effect through
  Monday evening for the entire area 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 near 100 through
  much of the week. Heat headline extensions are not currently
  planned.

- Shower and storm chances increase incrementally Monday
  evening through the end of the week. Some may be severe with
  damaging winds and flash flooding.

&&

.DISCUSSION...
Issued at 225 PM CDT Sun Jun 22 2025

Through Monday:

The main forecast uncertainty with the heat on Monday is the
extent to which dew points mix out in the afternoon, which will
affect top end temperature magnitudes and afternoon heat indices.

Southwesterly winds have ramped up early this afternoon due to
the mixed layer deepening, with gusts up to 30-35 mph as of this
writing. We`ve also seen a couple spots lose a degree or two
with their dew points. With that said, dew points have generally
remained above a majority of the guidance. In other words, it`s
oppressive out there. Before slowly cooling towards and after
sunset, late afternoon-early evening heat indices will be in the
100-110F range.

Another exceptionally warm and muggy night is on the way with
lows likely only getting down to 80F or even just above it in
and just outside of Chicago, and mid to upper 70s for the rest
of the area as dew points remain in the 70s. Breezy
southwesterly winds this evening will gradually ease through the
night, but at least sporadic gusts will likely continue for the
Chicago urban heat island northward to Lake County IL.

Monday afternoon will be similar, thermally speaking, at 850 mb
(low +20s C) and 925 mb (~27C) to today. Thus, as already
alluded to, the only question mark is whether dew points will
mix out as (primarily) the foreign/non-NCEP guidance has been
insisting, in particularly the ECMWF/EPS and Canadian suites.
Forecast soundings do suggest a bit more dry air aloft than
today, so did lean a bit but not fully in the direction of
"mixing out" with upper 60s-low 70s Td at peak heating (mid 60s
in/near Chicago). If this occurs, or even more aggressive dew
point drops occur, that will up the ante for how high
temperatures get, but also correspondingly lower the heat
indices at peak heating.

With highest forecast confidence in dew points mixing out in the
city and points north up to Lake County, that appears to be the
most likely corridor for highs to reach the upper 90s (97-98F
range) and can`t completely rule out some spots near 100F. On
the other hand, if dew points only mix out a bit more than
today, then temps will probably only add a degree or so from
today`s (Sunday`s) highs. Our somewhat middle ground gridded
forecast with temps and dew points has peak Monday afternoon
heat indices in the 100-105F range.

It will be another breezy day amidst a tight pressure gradient.
Winds should start to gust up to 30 mph fairly early in the
morning, especially near/north of I-80, as mixing occurs into
the departing low-level jet. In the mid to late afternoon,
there`s a good signal for low-level winds ramping up similar to
today, so gusts should peak in the 30-35 mph range. Finally, our
attention will turn to the potential for storms developing
and/or moving into areas just to the north/northwest of the CWA
after 4pm.

Castro


Monday evening through Sunday:

Monday evening through Friday, the upper-level ridge responsible for
the current hot and humid conditions will gradually flatten allowing
for a low-level frontal boundary (current draped across the northern
Plains) to slowly pivot and sag southward through the Great Lakes.
Weak capping (owing to a slacking low-level thermal ridge) along and
south of the frontal boundary will set the stage for episodic
clusters of showers and storms, all tied to both the diurnal cycle
and any incoming subtle shortwaves in the southwest to zonal flow
aloft.

While showers and storms will erupt along the front Monday afternoon
and evening from the southern Plains to northern Lower Michigan, a
residual pocket of dry mid-level air should generally choke any
activity that tries to work southeastward into northern
Illinois. With that said, there is a strong signal for cool outflow
to surge southeastward across northern Illinois Monday evening,
leading to a break in the heat. Tuesday still looks like the
first day where scattered afternoon thunderstorms will develop
across our area, perhaps focused along any residual outflow
boundary and/or a lake breeze. Additional waves of showers and
storms are then favored all the way through Friday as ripples
propagate aloft atop the low-level frontal boundary stalled
across the Lower Great Lakes. Nearly continuous mid-range (40
to 60%) chances for showers and storms offered by the NBM remain
appropriate for the Tuesday through Thursday timeframe, though
many hours should be dry.

With the upper-level jet and associated availability of shear
remaining focused north of our area, the airmass within and south of
the frontal boundary will be somewhat stagnant and supportive of
pooling moisture. As a result, the overall kinematic and
thermodynamic regime next week will be characterized by high PWATs
(rising above 2" at times), weak mid-level lapse rates, daily
afternoon MLCAPE >2000 J/kg, and meager deep-layer shear. Such a
regime should tend to limit the overall organization of each episode
of thunderstorms. However, upscale clustering and/or any locally
augmented shear via MCVs will nevertheless support threats for
torrential downpours, prolific lightning, and strong to damaging
winds on an isolated to scattered basis. Both NCAR and CSU ML models
continue to advertise 5 to 15% chances for severe weather range
Tuesday through Friday, consistent with a Level 1 to 2 out of 5
threat for severe weather each day. Conceptually, would have to
think a threat for flash flooding could materialize where
thunderstorms are most prevalent given the aforementioned hefty
PWATs, as well.

Tuesday through Friday still look hot and humid, especially along
and south of the frontal boundary draped across our area. Daily
highs in the low to mid 90s with dew points in the low to mid 70s
will support afternoon peak heat indices in the upper 90s to lower
100s each day. With the baroclinic zone directly overhead, perhaps
at times modulated by convective outflow, daily lake breezes appear
to be a good bet allowing for cooling along the Lake Michigan
shoreline. For now, don`t plan to extend the ongoing Heat Advisories
given heat indices in bulk should remain below 105F and the daily
bouts of thunderstorms should lead to cloud cover and outflow.
Nevertheless, can see the need for local extensions Tuesday and
perhaps Wednesday if temperatures and dew points overperform
current expectations, particularly south of I-80 where
convective coverage may end up somewhat isolated until the
frontal boundary arrives later in the week.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1255 AM CDT Mon Jun 23 2025

- Gusty SW winds to develop again during the day today, highest
  mid-late afternoon near 30kt.

- Outflow from thunderstorms in Wisconsin late afternoon could
  result in a gusty northwest wind shift this evening.

Winds have mostly eased early this morning across the area, but
sporadic gustiness will continue through daybreak. Winds then
steadily increase after daybreak through the morning, similar
to yesterday, with peak gusts mid to late afternoon around 30kt
again, remaining south southwesterly.

Showers and thunderstorms are expected to erupt along a cold
front across Iowa, far northwest Illinois, and southern
Wisconsin this afternoon. Some of this activity may attempt to drift
in and near RFD in the evening. Closer to the Chicago metro
terminals, hi-res model guidance remains consistent in these
storms pushing out an associated outflow boundary that
propagates across the area (focused near/north of I-80) which
would turn winds NW to N. Spotty showers/storms initiating along
this boundary also can`t be fully ruled out (15% chance),
though confidence remains too low to include in the TAFs with
this update. Thus, opted to handle this with TEMPO groups 3-6Z
for a gusty NNW wind shift at DPA/ORD/MDW and no precip mention.
Winds then turn somewhat variable under 10kt in the wake of the
outflow boundary overnight into early Tuesday morning.

Petr

&&

.CLIMATE...
Issued at 225 PM CDT Sun Jun 22 2025

Temperatures will continue to threaten record highs and record warm
lows for Chicago and Rockford. Here are the current records:

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

Rockford             High       Warm Low
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 tonight 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 tonight for ILZ103-
     ILZ104-ILZ105.

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

LM...Small Craft Advisory from 8 AM this morning to 10 PM CDT this
     evening for the IL nearshore waters.

     Small Craft Advisory from 1 PM this afternoon to 10 PM CDT
     this evening for the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago