260
FXUS63 KLOT 132323
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 PM CDT Sun Sep 13 2026

.KEY MESSAGES...

- Waves of heavy rain producing thunderstorms possible in the
  general region Tuesday through Thursday.

&&

.DISCUSSION...
Issued at 230 PM CDT Sun Sep 13 2026

Modest forcing from an axis of mid-level frontogenesis within
the right entrance of a 90 knot mid-level jet streak over
southern Wisconsin into extreme northern Illinois is supporting
a band of rain dragging across northern Illinois early this
afternoon. Precip intensity aloft has been strong enough to
overcome a fairly dry low-level airmass, but is slowly eroding
the rain shield with southeast extent. As better forcing remains
north before shifting eastward early this evening, the precip
band should gradually weaken.

In the wake of this afternoon/evening`s precip, high pressure
will build across Wisconsin and Lower Michigan tonight. With
clearing skies, light winds, and PWATs under 0.5" tonight across
northern Illinois, temps are expected to fall into the mid to
upper 40s across interior northern Illinois. Farther south where
mid-level cloud cover and slightly stronger low-level gradient
flow lingers well into the night, temps will settle in the low
to mid 50s. Precip is not expected, though shallow lake effect
clouds may advect across northwest Indiana tonight and northeast
Illinois late tonight into Monday morning.

As a broad mid-level trough lifts northeastward across the
Upper Mississippi River Valley, a weak disturbance currently
over northeast New Mexico will phase into the far southeastern
extent of the northern trough late tonight into Monday. Though
the air mass will remain quite dry in the lowest 7-9kft,
marginal lapse rates in the 10-20kft layer and modest mid-level
moisture advecting northeastward ahead of a building ridge could
yield a few high-based showers during the late afternoon across
primarily the northwest half of the CWA.

An expansive surface low across far southern Canada will drag a
slowing cold front across northern Illinois late Monday night
through Tuesday, with consensus guidance stalling the front
somewhere across the southern half of Illinois by Tuesday night.
Ahead of the front on Tuesday, temps may push well into the 80s
and even lower 90s, with increasing low-level moisture yielding
heat index values nearing 100 across the far southern CWA.
Strong WSW flow aloft and generally uncapped conditions south of
the front in the afternoon would support organized strong to
severe convection, but with no clear mid/upper level forcing
aligning atop the surface front, coverage of convection may
remain low. Torrential rainfall is also possible with any
convection as pooling low-level moisture pushes PWATs to near or
possibly over 2 inches.

With a broad subtropical high settling over the southeast CONUS
mid to late week, the overall mid and upper-level flow over the
western Great Lakes will remain generally zonal. The front will
remain draped across Illinois during this period, though
whether it drifts back north over the forecast area at any point
remains unclear. Either way, periods of scattered showers and
possibly some thunderstorms can be expected in the general
vicinity of the low-level front through at least Thursday.

By Friday, and especially into next weekend, weak mid-level
blocking will develop as a sub-tropical low over the Gulf
slows/stalls south of the deamplifying ridge. A trough crossing
the northern Great Plains on Friday may extend far enough south
to yield showers and storms in the area on Saturday.

Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 623 PM CDT Sun Sep 13 2026

Only aviation forecast concerns are the potential for some very
patchy shallow fog mainly west/northwest of Chicago toward
morning, and the potential for a few scattered showers Monday
afternoon mainly along/north of the IL/WI border. Southwest
winds also look to increase later Monday afternoon/evening,
likely with some gusts 20-25 kts.

Band of showers/light rain was dissipating across the area as of
early evening, with VFR cloud cover also clearing from north to
south. Surface high pressure will build southeast across the
area this evening, allowing winds to become generally light
north-northwest or just light/variable overnight. Can`t rule
out a little shallow patchy ground fog toward morning away from
the Chicago metro area, primarily to the west/northwest, though
model guidance suggests coverage would be quite spotty and will
just continue to monitor trends tonight in case a mention may
be needed for a few hours toward sunrise mainly for RFD.

The surface high will shift east of the area on Monday, allowing
winds to shift southeast and gradually increase during the day.
It does appear these winds will become gusty later in the
afternoon/evening (20-25 kts) in response to deepening surface
low pressure tracking northeast across the Northern Plains. The
pressure gradient is expected to remain fairly tight Monday
night as well, with a 45-50 kt southerly low-level jet
developing. Would expect our typically warmer more urban
ORD/MDW terminals would see at least sporadic/occasional
surface gusts so have not included LLWS mention in their 30-hour
forecasts at this time.

In addition, there does look to be a brief window Monday
afternoon for some VFR showers clip RFD, though HREF ensemble
pops are only 10-20% at this time and have not included in the
TAF. Might be worth a PROB30 mention in the 19-22Z-ish window
if coverage ends up being higher. Chicago terminals, farther to
the southeast, would appear to remain dry at this time.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement until 10 PM CDT this evening for
     INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for Northerly
     Is. to Gary IN.

     Small Craft Advisory until 10 PM CDT this evening for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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