113
FXUS63 KILX 171745
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1245 PM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Shower and thunderstorm chances will be highest north of I-74
  this morning. Some areas could see locally heavy rain and a
  Flood Watch remains in effect for Knox, Stark and Marshall
  Counties through 10 AM.

- High heat and humidity will build over portions of central
  and southeast IL today. A Heat Advisory is in effect until 7
 PM south of a Springfield to Robinson line, where afternoon
  heat indices will peak near 105 degrees.

&&

.UPDATE...
Issued at 1030 AM CDT Thu Sep 17 2026

Isolated to scattered showers continue to bubble up form the
outflow/frontal boundary that has been sliding south this
morning. Will be making some adjustments to the short term grids
to better depict pop/wx grids for the rest of the day. Lower
clouds are also dropping into the area from the north behind
the front, but do not think these lower clouds will reach the
I-72 corridor as satellite loops already showing lower clouds
dissipating across northern parts of the CWA with decent cloud
cover. Heat Advisory will continue into this evening south of
the boundary. Update will be forthcoming.

&&

.DISCUSSION...
Issued at 1222 AM CDT Thu Sep 17 2026

A stationary frontal boundary currently bisects central
Illinois just south of the I-72 corridor. Moisture transport
along this boundary remains robust, with observed precipitable
water values around 1.8 to 2.0 inches, exceeding the the
percentile for mid-September. Regional radar demonstrates
elevated convection rooted above the nocturnal boundary layer
well north of the front, north of the I-74 corridor.

Heavy rainfall across the far northern CWA is the primary
forecast concern through this morning as the 850 mb jet
increases to around 35 kt. A small, but notable shift northward
in QPF swaths has been seen in the 00z runs of the HREF and
REFS. This places the highest probability of heavy rain just
north of the local area near the I-80 corridor through mid
morning. In these areas swaths of 2-4" are shown in PMM
products. Will leave the Flood Watch in place through 15z for
Knox, Stark, and Marshall Counties in case the band of heavier
rain ends up shifting farther south, but overall that risk is
now lower.

The front is forecast to remain stationary through this
afternoon. South of the front, highs will reach the mid 90s,
with some upper 90s south of I-70. These temps combined with
dewpoints in the lower 70s will yield heat indices peaking
around 105 degrees. Have expanded the Heat Advisory a row of
counties north to include the Springfield and Jacksonville
areas. Isolated pulse convective development is possible this
afternoon and evening near the front, or outflow from morning
storms. Without much of a trigger have kept PoPs low at 20-30%.

A sharp north to south temperature gradient remains over the
area Friday with the front laid out west-to east near the middle
of the state. This will result in highs ranging from the lower
80s north of I-74 to the mid 90s south of I-70. Dewpoints are
forecast to be a few degrees lower which should keep peak heat
indices at or below 100 degrees south of the front. Will keep
low chance PoPs in the forecast with the front in the area,
though with a lack of upper support coverage should be low.

Guidance shows good agreement that the front temporarily lifts
just north of the area on Saturday, as the flow buckles
southwest ahead of a mid level trough digging through the
central Rockies. As a result PoPs have been lowered and highs
raised into the 90s CWA-wide.

Shower and thunderstorm coverage is forecast to increase for
the Saturday night through Monday period, when forcing for
ascent increases ahead of the slow-moving upper level wave.
Latest NBM shows likely (60%) PoPs for this timeframe north of
I-70.

Medium range ensemble guidance shows good agreement of a major
pattern change for early next week. Upper flow transitions to
northwesterly as the trough digs across the Great Lakes. This is
forecast to bring a surge of cooler and drier air into the
region, with the NBM advertising highs in the 70s north of I-70
Mon-Wed.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1245 PM CDT Thu Sep 17 2026

All sites will be VFR the rest of the afternoon and into this
evening. Then as winds become stronger from the north to
northeast, lower MVFR and IFR clouds will advect into the area
and affect the TAFS from north to south. Expecting IFR clouds at
PIA, BMI and CMI at less than 1kft, while MVFR clouds are
expected at SPI and DEC, at less than 1.5kft. The conditions
will continue in the morning but then improve during the late
morning hours at all sites. PIA, BMI, and CMI will only improve
to above 1kft by late morning, while SPI and DEC rise to VFR by
late morning. Predominate showers are expected at PIA and BMI
with only VCSH at SPI and DEC. Will leave precip out for CMI as
the precip should remain well west of CMI tomorrow. Winds will
be variable this afternoon but predominately less than 10kts in
speed through the period. Wind direction will mainly be variable
or northeast through the period.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for ILZ049>052-061-
062-066>068-071>073.

&&

$$

KEY MESSAGES...25
UPDATE...Auten
DISCUSSION...25
AVIATION...Auten