549
FXUS63 KLOT 010652
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
152 AM CDT Tue Sep 1 2026

.KEY MESSAGES...

- Today and tomorrow will be hot and humid with peak heat
indices of 100 to 110F.

- There is a low (10 to 20%) chance for severe thunderstorms
today, mainly from mid morning to early afternoon.

- Chances for thunderstorms increase Thursday and Friday as the
hot and humid conditions are pushed south of I-80.

- A break in the humidity may materialize this weekend.

&&

.DISCUSSION...
Issued at 152 AM CDT Tue Sep 1 2026

Though Tonight...

Early morning GOES-19 water vapor imagery depicts an expansive
summer-like heat dome centered over the middle Mississippi River
Valley with RAP-analyzed 500mb heights near 5960m over Paducah,
Kentucky. Meanwhile, an upper-level low was centered near the
Manitoba and Ontario borders in southern Canada. Quasi-zonal
flow stretches between the upper-level low to the north and heat
dome to the south from the Black Hills region eastward through
the Lower Great Lakes. Closer to the surface, an expansive
region of unseasonably moist low-level air characterized by dew
points in the low to mid 70s encompasses much of the Great
Lakes, Upper Mississippi River, and middle Mississippi River
valleys. The large-scale pattern is more typical of the middle
of July than the first day of September.

The primary concern today are dangerous heat. Owing to the very
wet summer (90 day observed precipitation departures were 200
to 300% of normal across parts of Illinois and Indiana),
agricultural evapotranspiration is still in full swing. Dew
points yesterday climbed into the mid to upper 70s across much
of the forecast area, and frankly there are no reasons why the
same won`t happen today. With 850mb temperatures peaking in the
+22 to +23 range today and tomorrow, dry bulb high temperatures
should have no problem reaching the low to mid 90s. When
combined with dew points in the mid to upper 70s, widespread
heat index values of 105 to locally 110F (wet bulb globe
temperatures of 87 to 90F) appear to be a good bet today.

The only fly in the ointment is the low probability threat for
thunderstorms and associated free air conditioning (whether by
cloud cover or precipitation). Early this morning, have been
keeping a close eye on an elongated west-to-east band of ACCAS
extending from near Omaha, Nebraska to Davenport, Iowa. Per the
00Z RAOBs from OAX and DVN, the ACCAS field is rooted at the
base of an EML plume in which lapse rates range from 7.5 to 8
K/km from 850 to 500 mb. Over the past hour or two, the ACCAS
field has gotten increasingly agitated east of Omaha, presumably
owing to increasing low-level confluence on the nose of a
modest but nevertheless intensifying 850mb low-level jet and at
least a glancing blow of upper-level diffluence in the right
entrance region of a jet streak propagating into the Great
Lakes.

In the absence of more pronounced forcing for ascent, it seems
like the most likely scenario is for thunderstorms to struggle
to develop and maintain themselves across Iowa early this
morning. However, should they develop and become established,
they will have access to a highway of moisture-laden instability
(MUCAPE >3000 J/kg) that points directly to Chicago. When also
considering access to 30 to 35kt of westerly upper-level flow on
the southern edge of the aforementioned jet streak, any
established storm would be prone to developing into an eastward-
racing cluster with a threat for damaging winds (again,
sweeping across much of our area). Timing would be from mid-
morning to early afternoon (call it 9 AM to 3 PM from west to
east). At this point, the chance of this outcome seems low
(call it a 10 to 20% probability), but will continue to watch
trends across Iowa this morning in case the odds start to turn.

Will also note there is a threat for thunderstorms to develop
at any point this afternoon when instability is maximized,
though am currently not seeing really any mechanisms to force
ascent beyond the window from this morning within the neutral
mid-level height-tendency regime.

With the more likely scenario appearing to be being no storms
today, will go ahead and expand the Heat Advisory to now
encompass the entire area. While the Heat Advisory will expire
at 9 PM, tonight will still be balmy with overnight lows in the
low to mid 70s.


Wednesday...

On Wednesday, ensemble guidance is in fair agreement that an
upper-level shortwave embedded in the upper-level anticyclonic
flow around the ridge will propagate into the Upper Mississippi
River Valley and upper Great Lakes. Depending on the evolution
of thunderstorms tonight in the northern Plains, can easily
envision an MCS or two riding across Minnesota, central
Wisconsin, and Lower Michigan on Wednesday. With all forcing
remaining well north of our area, the chance for storms on
Wednesday will be even lower than today.

Interestingly, guidance suggests dew points may be a touch
lower on Wednesday compared to today which would lead to heat
index values more in the range of 100 to 105 versus 105 to
110F. However, will continue to lean on evapotranspiration
being a big facor and forecast another day with dangerous heat.

With a subtle signal in guidance for outflow originating in
Wisconsin to creep southward into northern Illinois, am not
quite ready to expand the inherited Heat Advisory for Wednesday
that currently encompasses areas south of Interstate 80 to
instead cover the entire area. Regardless, Wednesday looks like
another hot and humid day.


Thursday and Friday...

The upper-level ridge will finally start to break down Thursday
and Friday, placing our area within reach of upper-level
shortwaves propagating along the northern periphery of the heat
dome. As a result, Thursday and Friday will carry a threat for
severe thunderstorms in the general region (the tail end of the
00Z RRFS is quite a look for our area). Depending on eventual
convective footprints, areas south of I-80 may still be baking
in the heat one or both days possibly requiring an extension of
the Heat Advisory. We`ll be able to sort out those details when
we get closer.


Saturday onward...

Ensemble model guidance favors a surface high pressure system
dropping southward into the Great Lakes region this weekend
finally breaking the hot and humid pattern. With that said,
quasi-northwest flow may develop by early next week bringing a
return of showers and thunderstorms to the general region.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1226 AM CDT Tue Sep 1 2026

Key Aviation Messages for the current TAF period:

- Low potential for thunderstorms Tuesday afternoon, with best
chances near and north of KRFD

Isolated shower activity will fester through the overnight.
However, with the chances of any one shower passing over an
individual terminal, the TAFs were kept dry. Southwest winds
continue to diminish and are expected to remain below 10 knots
through daybreak. By midday, winds will slowly become more west
of south and increase with gusts around 20 knots. Wind gusts are
expected to diminish after 00Z.

There remains a low confidence chance for showers and
thunderstorms tomorrow afternoon. For most terminals, it remains
around a 20 percent chance for thunderstorms and thus kept out
of those TAFs. For KRFD, being closer to the impulse that will
be the driver of any storm development, the 06Z TAFs have
introduced a PROB30 for thunder. If storms develop, it`s
possible that this is converted to a VCTS rather than TEMPO.
Then again if they remain farther north as some guidance
suggests (or don`t develop at all) it is possible that the
PROB30 is removed altogether. Models have been struggling, so it
is a low confidence forecast. However, if storms develops, they
should be fairly strong with deep convective cores featuring
downpours and stronger thunderstorm wind gusts.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory from noon today to 9 PM CDT this evening for
ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-
ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-
ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Heat Advisory from noon to 9 PM CDT Wednesday for ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-
ILZ107-ILZ108.

IN...Heat Advisory from noon CDT /1 PM EDT/ today to 9 PM CDT /10
PM EDT/ this evening for INZ001-INZ002-INZ010-INZ011-
INZ019.

Heat Advisory from noon CDT /1 PM EDT/ to 9 PM CDT /10 PM EDT/
Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Dense Fog Advisory until 4 PM CDT this afternoon for Winthrop
Harbor to Wilmette Harbor IL.

&&

$$

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