395
FXUS63 KLOT 202328
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
628 PM CDT Fri Jun 20 2025

.KEY MESSAGES...

- First heat wave of the summer expected this weekend into early
  next week with multiple days of highs well into the 90s and
  peak afternoon heat indices of around 105F likely.

- Extreme Heat Warning remains in effect for the city of Chicago
  and Cook Co. where special 3-day Extreme Heat Warning
  criteria of peak heat indices of 100F+ are expected to be
  reached.

- Heat Advisory remains in effect for the rest of the area
  Saturday-Monday. Some areas may need to be upgraded to an
  Excessive Heat Warning if Tuesday again has ~105F heat indices.

- Saturday-early Saturday evening will be unusually windy given
  the heat, with southwesterly gusts up to 35-45 mph I-80 and
  north and up to 30-35 mph south of I-80.

&&

.DISCUSSION...
Issued at 320 PM CDT Fri Jun 20 2025

Through Tonight:

Skies have largely cleared out across the area this afternoon
in the wake of the earlier rain, allowing for temperatures to
warm into the 80s across the area. The core of a low-level jet
and an associated plume of rich low-level theta-e remains
positioned just to our northwest at press time and is expected
to expand northeastward this evening. The nose of this low-level
jet is where additional shower and thunderstorm development is
expected to be focused later tonight, particularly as the jet
starts to strengthen this evening.

Model guidance is largely in agreement that widespread
convection likely won`t develop until the mid-late evening, when
the low-level jet`s nose is pointed into central/northern
Wisconsin. The resultant arc of convection would remain
northeast of our forecast area if this were indeed to be the
case, but models, in general, tend to struggle with where and
when to initiate convection in these summertime low- level
jet/isentropic ascent setups on the peripheries of EML plumes.
Haven`t seen any upstream agitated cumulus growth of note yet
that would be of cause for concern, but opted to err on the side
of caution and maintain some low-end slight chance PoPs for our
northeastern CWA late this afternoon/early this evening in the
odd event that something were to get going along the nose of the
low-level jet earlier and/or farther south than expected.

Otherwise, southerly winds will persist through the night with
occasional gusts near and in excess of 20 mph. These winds will
help keep temperatures propped up overnight while also advecting
higher dew points into the area. Low temperatures tonight will
thus likely remain above 70F across our forecast area, setting
the stage for the beginning of our upcoming heat wave.

Ogorek


Saturday through Friday:

Overview of the heat wave through Tuesday:

Saturday: Blast of heat and oppressive humidity driven by strong
southwest winds gusting to ~40 mph, strongest north of I-80. Heat
indices around 105F.

Sunday-Monday: Both breezy but not as windy, temps a degree or so
warmer each day and dew points/humidity not quite as oppressively
high, especially on Monday. Temps may near 100F in spots on Monday.

Tuesday: Potential for another day of near to a bit above 105F
heat indices but highest confidence south of I-80 due to higher
storm chances and winds possibly turning onshore near the lake.

Meteorological Summary:

As the powerhouse 500 mb ridge amplifies northward from the Mid
South on Saturday, strong for June mid 990s surface low pressure
over the northern Plains and high pressure to our southeast will
set up a tight pressure gradient (and strong southwesterly winds
aloft for this time of year). Conditions will be akin to a "blast
furnace" or perhaps a blast of steam is the better verbiage. Model
forecast backwards trajectories of the air mass overhead
tomorrow afternoon suggests our source region will be Arkansas
and far southern Missouri, where it`s been exceptionally wet
recently, and dew points are in the mid to locally upper 70s.

Analogous (but in opposite direction) to a direct Arctic
discharge in the winter from a PV lobe, the unusually strong
southwesterly flow up to 850 mb will result in close to direct
transport of the air mass well to the south today. With this in
mind, and aided by moist ground from recent rain locally,
expecting dew points to mix out less than they typically would
on a windy day. Temps may not quite max out vs. climo of progged
925 mb temps (mid-upper 20s C) because of the very high dew
points, but low-mid 90s will be enough for heat indices around
105F and perhaps locally near 110F. Dew points will probably mix
out slightly more in and just outside Chicago and points north,
resulting in temps up in the 96-97F range there.

The strong southwest winds are a key forecast message given
increased confidence in gusts into the 35-45 mph range I-80 and
north and up to 30-35 mph south of I-80. Lightweight structures at
outdoor events (for shading or otherwise) may be compromised by
winds this strong. Expect southwest winds to only slowly subside
Saturday night, pairing with the high (low-mid 70s) dew point
temperatures for an exceptionally warm night, especially in
Chicago (around 80F). The very warm and muggy overnight
conditions, particularly in urban areas, will make it tougher to
cool off and recover from the day`s heat, especially for
vulnerable individuals and/or those who spent prolonged time
outdoors during the most oppressive conditions.

Sunday and Monday will both be similar thermally at the 925 mb
level, so the key drivers to temps and heat indices will be the
extent to which dew points mix out each afternoon at peak heating.
The pressure gradient will remain tight as the deep surface low
mentioned earlier tracks north of the Great Lakes, though it won`t
be quite as windy, with gusts instead peaking in the 30-35 mph
range in the afternoon. Given strong evaporation from the moist
ground each day and some air mass recycling, Sunday appears likely
to see afternoon dew points in the lower to locally mid 70s, which
should enable temps a degree or so warmer. As such, peak heat
indices should be very similar to those of tomorrow (Saturday).

Monday is the day where guidance agreement remains solid in dew
points mixing out into the 60s, especially I-80 and north. With
925 mb temps near +27C, assuming moisture mix out does occur, this
could set the stage for temps nearing 100F in/near Chicago and
Rockford (similar setup to June 21, 2022 which had a high of 100F
at RFD and MDW). Heat indices will likely be more so in the
100-105F range, but accounting for cumulative effects easily
supportive of maintaining the going Heat Advisory (and Extreme
Heat Warning for Chicago/Cook County).

Looking ahead to Monday night and beyond, variance increases
enough in the guidance to lower confidence. The big question is
the timing of the ridge flattening out enough for convective
activity far to our north through the weekend to draw closer.
Convective influence on the surface pattern conceptually should
result in a weak surface boundary sagging south and the weaker
pressure gradient opening up the possibility for lake influenced
onshore winds by Tuesday. In the most recent 12z cycle, the
ECMWF/EPS suite points strongly toward Tuesday being one more day
of extreme heat. Even if convection and cooler temps, especially
near the lake, do come into play on Tuesday, capping may hold
farther south. This is important because then this would be a
fourth consecutive day of near/around 105F heat indices, which may
necessitate issuance of an Extreme Heat Warning for parts of our
area (due to cumulative effects of the heat).

By Tuesday night into Wednesday through Friday, the ridge should
have flattened enough by then to increase confidence in occasional
rounds of convection and also lake influence on temps near the
lake. The air mass will remain very warm to hot and humid aside
from lake cooling and rain bringing brief relief from the heat. It
will be a pseudo "ring of fire" type pattern, though at this
juncture, mid-level flow may be just marginal enough to prevent
a higher end severe threat, though not isolated strong/severe
storms. In addition, the high PWATs and large instability
reservoir should also be favorable for potential episodes of
flash flooding mid to late next week.

Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 628 PM CDT Fri Jun 20 2025

Only forecast concern this period are the winds.

Southerly winds will gust between 15-20kt tonight, though some
areas may lose gusts this evening. Wind directions may also
shift more to the south/southeast this evening then turn back to
the south/southwest overnight. Low level winds will steadily
increase this evening with low level wind shear expected through
daybreak Saturday morning. Surface winds/gusts will quickly
increase through mid morning with gusts into the 30kt range and
these gusts will increase into the mid 30kt range for the
mid/late afternoon on Saturday. Gusts will diminish into the
lower 20kt range with sunset Saturday evening with low level
wind shear again expected Saturday night. cms

&&

.MARINE...
Issued at 403 PM CDT Fri Jun 20 2025

A Gale Warning has been issued for the Illinois nearshore waters
and the Indiana nearshore waters from Calumet Harbor to Burns
Harbor from 9 AM to 10 PM Saturday.

We`re expecting strong south-southwest winds to develop by the
late morning due to a tight pressure gradient across the region.
Hot temperatures and deep mixing on land but the stabilizing
influence of the still cool lake waters will result in the
strongest winds likely being confined to the first couple miles
or so of the nearshore.

Confidence is highest in gale force speeds/frequent gale force
gusts to around 35 kt in the Gale Warning area, with a bit lower
confidence from Burns Harbor to Michigan City. We may need to
consider expanding the warning if confidence in frequent gales
increases for this nearshore zone.

If hazardous winds for small boats don`t persist through
Saturday night, they`re expected to redevelop on Sunday, with a
similar process repeated into Monday.

Castro

&&

.CLIMATE...
Issued at 114 PM CDT Thu Jun 19 2025

Temperatures this weekend into early next week could potentially
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 from 10 AM Saturday to 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 from 10 AM Saturday to midnight CDT
     Monday night for ILZ103-ILZ104-ILZ105.

IN...Heat Advisory from 10 AM CDT /11 AM EDT/ Saturday to midnight
     CDT /1 AM EDT/ Monday night for INZ001-INZ002-INZ010-
     INZ011-INZ019.

LM...Gale Warning from 9 AM to 10 PM CDT Saturday for Winthrop
     Harbor IL to Burns Harbor IN.

     Small Craft Advisory from 9 AM to 10 PM CDT Saturday for Burns
     Harbor to Michigan City IN.

&&

$$

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