163
FXUS63 KILX 130946
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
446 AM CDT Thu Aug 13 2026

.KEY MESSAGES...

- Dense Fog Advisory in effect until 9 AM CDT for areas
  generally north of a Quincy to Terre Haute line.

- Midday Convective Complex: A decaying Mesoscale Convective
  Complex (MCC) tracks near/north of the I-74 corridor around
  midday, bringing a threat for isolated severe thunderstorms
  and efficient, heavy rainfall.

- Heat Concerns South: South of the effective boundary, hot and
  humid conditions yield heat indices near 100F. A Heat Advisory
  is active through early evening for southeast Illinois.

- Overnight Flood Watch: repeated thunderstorms will heighten
  flash flooding risks. A Flood Watch is in effect for counties
  along/north of I-74 through Friday morning.

&&

.DISCUSSION...
Issued at 441 AM CDT Thu Aug 13 2026

.Morning Dense Fog...

A strong radiational cooling regime early this morning has
fostered patchy dense fog development across central Illinois.
Despite high-level anvil debris streaming overhead from upstream
convection, lingering near-surface moisture and weak boundary-
layer flow (winds < 10 kt through 3 kft) will keep fog grounded
past daybreak. Shallow warm air advection (WAA) overrunning the
effective surface boundarycurrently draped just south of the
forecast areawill also aid in maintaining low stratus/fog well
into the mid-morning hours. A Dense Fog Advisory is in effect
through 9 AM CDT for locations roughly north of a Quincy to
Terre Haute line.


.Midday Convective Complex & Heavy Rain Potential...

Attention then shifts to the afternoon as the first of multiple
convective rounds approaches from the west. Early morning radar
trends reveal a decaying MCC positioned near the NE/IA border.
Short-term high-resolution guidance remains in solid consensus
that this complex will track near and north of the I-74 corridor
by midday, riding along the nose of a receding low-level jet
(LLJ), a convectively-augmented shortwave trough, and the
northern edge of the instability gradient.

Severe potential with this initial wave remains conditional.
While cold pool dynamics and potential Rear Inflow Jet (RIJ)
organization bear watching, poor mid-level lapse rates,
persistent low-level temperature inversions, and lingering
morning fog/stratus will likely keep this incoming activity
elevated and predominantly sub-severe.

Regardless of severe risk, this initial round will be an
efficient heavy rainfall producer. Atmospheric soundings display
classic tropical-like characteristics: (a) anomalous PWAT
values of 2-2.2 inches; (b) low- to mid-level RH exceeding 80%;
(c) a deep warm cloud layer (> 4km); and (d) a long, skinny CAPE
profile.

Latest HREF 6-hour Probability-Matched Mean (PMM) QPF targets
0.5 to 1.5 inches across the northern forecast area with this
initial midday push.


.Heat & Humidity Across the South

South of the convective footprint and the effective surface
boundary (primarily south of a Quincy to Terre Haute line),
strong surface heating and rich moisture will persist. Highs in
the upper 80s paired with dewpoints in the mid 70s will yield
heat index values around 100F. A Heat Advisory remains in effect
through early evening for portions of southeast Illinois.


.Overnight Flash Flood Threat...

The threat for flash flooding escalates significantly tonight
across central and northern Illinois. The effective surface
boundary will begin to retreat northward as a sharpening warm
front. An intensifying nocturnal LLJ impinging directly onto
this elevated baroclinic zone will fuel repeated convective
initiation and training thunderstorm complexes near and north of
the I-74 corridor.

With antecedent conditions primed by the midday convective
round, susceptibility to rapid runoff will be high. HREF QPF PMM
guidance indicates an additional 1-2 inches overnight, with
mesoscale training corridors capable of producing localized
totals exceeding 4 inches. Given the high-efficiency airmass and
high potential for training, a Flood Watch is in effect for
counties along and north of I-74 through Friday morning.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1238 AM CDT Thu Aug 13 2026

A challenging forecast over the next 24 hours characterized by fog,
low stratus, and periods of showers and storms. Cirrus continues to
clear from west to east late tonight with light surface winds in
place north of a surface boundary. MVFR to IFR fog/low stratus
will expand through early morning with the worst conditions
expected near and north of the I-74 corridor. Visibility should
improve between 12- 14Z, though ceilings at northern terminals
will remain MVFR for at least the first half of the day.

A thunderstorm complex will race through north-central parts of IL
from mid-morning to early afternoon, with the highest probabilities
of TSRA at KPIA and KBMI. PROB30 groups are noted at the remaining
sites where confidence remains lower. Additional storms are possible
near/north of I-74 late Thursday night with PROB30 groups added for
this potential. Easterly winds will largely remain in place through
the period, with the direction more variable with any storm
activity.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Flood Watch from 10 AM CDT this morning through Friday morning
for ILZ027>031-037-038-045-046.
Heat Advisory from noon today to 8 PM CDT Sunday for
ILZ066>068-071>073.

&&

$$

KEY MESSAGES...MJA
DISCUSSION...MJA
AVIATION...NMA