771
FXUS63 KILX 290750
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
250 AM CDT TUE SEP 29 2026
KEY MESSAGES
- WARM MIDWEEK TEMPERATURES: HIGHS WILL REACH THE LOW TO MID-80S
TODAY AND WEDNESDAY, RUNNING 5 TO 10 DEGREES ABOVE TYPICAL
LATE-SEPTEMBER AVERAGES.
- INITIAL RAIN CHANCES WEDNESDAY: RAIN CHANCES INCREASE LATE
TONIGHT INTO WEDNESDAY, PARTICULARLY NEAR THE ILLINOIS RIVER
VALLEY, THOUGH CONFIDENCE IN WIDESPREAD 1-INCH TOTALS REMAINS
LOW BEFORE THURSDAY.
- HEAVY RAIN POTENTIAL THURSDAY TO FRIDAY: WIDESPREAD HEAVY
RAIN IS EXPECTED THURSDAY THROUGH FRIDAY. AREAS NORTH OF
INTERSTATE 70 HAVE A 50 TO 80 PERCENT CHANCE OF EXCEEDING 1
INCH OF RAIN, WITH A 40 TO 60 PERCENT CHANCE OF OVER 2 INCHES
NORTH OF A JACKSONVILLE TO RANTOUL LINE.
- COOL AND DRY WEEKEND: DRIER, AUTUMN-LIKE CONDITIONS MOVE IN
LATE FRIDAY INTO SATURDAY, DROPPING WEEKEND HIGHS INTO THE
UPPER 60S AND LOWS INTO THE 40S.
DISCUSSION
ISSUED AT 247 AM CDT TUE SEP 29 2026
TODAY THROUGH WEDNESDAY
A RAPID TRANSFORMATION IN THE SYNOPTIC PATTERN UNFOLDS TODAY
ACROSS THE CENTRAL CONUS AS AN EXPANSIVE WESTERN TROUGH ABSORBS
THE REMNANTS OF TROPICAL DEPRESSION POLO. DOWNSTREAM SHORTWAVE
ENERGY EJECTING OUT OF THE PLAINS INTO THE MID AND UPPER-
MISSISSIPPI VALLEYS WILL FLATTEN THE AMPLIFIED RIDGE OVERHEAD,
SIMULTANEOUSLY ACCELERATING THE DEPARTURE OF AN EAST COAST UPPER
LOW INTO THE ATLANTIC BASIN. ASSOCIATED RETURN FLOW AND ROBUST
THERMAL/MOISTURE ADVECTION WILL SEND AFTERNOON HIGHS INTO THE
LOW TO MID 80S BOTH TODAY AND WEDNESDAY—NEARLY 5 TO 10 DEGREES
ABOVE LATE-SEPTEMBER CLIMATOLOGY.
RECENT MODEL GUIDANCE CONTINUES TO DISPLAY SIGNIFICANT
VARIABILITY REGARDING THE PRECISE TIMING AND MAGNITUDE OF
INCOMING PRECIPITATION, MAINTAINING A FAIR DEGREE OF UNCERTAINTY
ALONG THE HEAVY RAINFALL AXIS. HOWEVER, NOTABLE TRENDS IN THE
00Z MODEL SUITE—PARTICULARLY ACROSS THE ECMWF AND RRFS-MPAS—HAVE
SHIFTED THE INITIAL PUSH OF RAIN FARTHER SOUTH FOR WEDNESDAY.
THIS REFLECTS A DISTINCT SHIFT FROM YESTERDAY'S GLOBAL
CONSENSUS, WHICH MAINTAINED THE PREDECESSOR RAIN EVENT WELL TO
OUR NORTHWEST ACROSS IOWA AND WISCONSIN.
IN LIGHT OF THESE TRENDS, POPS HAVE BEEN NUDGED UPWARD FROM
LATE TONIGHT THROUGH MIDDAY WEDNESDAY TO CAPTURE THE ROBUST
SHORTWAVE ENERGY TRAVERSING THE ILLINOIS RIVER VALLEY IN TANDEM
WITH FORCED WAA AND ISENTROPIC LIFT. FORECAST SOUNDINGS
ILLUSTRATE THAT LOW-LEVEL SATURATION WILL OCCUR GRADUALLY VIA
TOP-DOWN MOISTENING. CONSEQUENTLY, CONFIDENCE REMAINS LOW
REGARDING DETERMINISTIC MODEL DEPICTIONS OF WIDESPREAD 1-INCH
RAINFALL TOTALS PRIOR TO THURSDAY, OWING TO THE PREDOMINANTLY
NON-CONVECTIVE NATURE OF THE RAIN AND WEAK FRONTOGENETICAL
FORCING DURING THIS EARLY PHASE. EVEN SO, AN ANOMALOUSLY RICH
MOISTURE PLUME—WITH PWATS EXCEEDING THE 99TH PERCENTILE FOR LATE
SEPTEMBER (>1.75 INCHES)—UNDERSCORES THE POTENTIAL FOR HIGHLY
EFFICIENT RAINFALL DURING THIS INITIAL ROUND SHOULD IT FULLY
MATERIALIZE ON WEDNESDAY.
THURSDAY THROUGH MONDAY
THE HIGHEST CONFIDENCE FOR WIDESPREAD, HEAVY RAINFALL REMAINS
CENTERED IN THE THURSDAY THROUGH FRIDAY TIMEFRAME. DURING THIS
PERIOD, THE MAIN WESTERN SHORTWAVE OPENS UP AND TRACKS EASTWARD
ALONG A SLOW-MOVING SURFACE COLD FRONT. HIGH CONFIDENCE EXISTS
IN WIDESPREAD MEASURABLE QPF ACROSS CENTRAL AND EASTERN
ILLINOIS; HOWEVER, EXACT STORM-TOTAL ACCUMULATIONS WILL HINGE
HEAVILY ON THE FRONTAL SPEED AND THE SHARPNESS OF THE BAROCLINIC
ZONE. BLENDED NBM GUIDANCE CURRENTLY HIGHLIGHTS A 50 TO 80
PERCENT PROBABILITY OF RAINFALL EXCEEDING 1.0 INCH NORTH OF
INTERSTATE 70 THROUGH FRIDAY NIGHT. THE POTENTIAL FOR HEAVY RAIN
EXCEEDING 2.0 INCHES IS HIGHEST (40 TO 60 PERCENT CHANCE) NORTH
OF A JACKSONVILLE TO RANTOUL LINE, WHERE LOCALIZED MESOSCALE
BANDING AND TRAINING ECHOES COULD READILY YIELD HIGHER SWATHS.
WHILE LOW-END PRECIPITATION CHANCES (15 TO 25 PERCENT) LINGER
THROUGH MIDDAY SATURDAY TO ACCOUNT FOR A POTENTIAL DELAY IN THE
NORTHERN STREAM SHORTWAVE, CONDITIONS WILL DRY OUT RAPIDLY FROM
NORTHWEST TO SOUTHEAST AS A BROAD CONTINENTAL POLAR (CP) HIGH
EXPANDS ACROSS THE MIDWEST AND GREAT LAKES. THIS AIRMASS
TRANSFORMATION WILL USHER IN A CLASSIC AUTUMN COOL-DOWN RELATIVE
TO OUR MIDWEEK WARMTH, HOLDING DAILY HIGHS IN THE UPPER 60S AND
LOWS IN THE 40S TO CLOSE OUT THE WEEK. EXTENDED ENSEMBLE
GUIDANCE SUGGESTS THIS EXPANSIVE CANADIAN HIGH WILL REMAIN
ENTRENCHED ACROSS THE REGION THROUGH AT LEAST MIDWEEK,
DELIVERING A PROLONGED STRETCH OF DRY, IDEAL FALL WEATHER.
AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 1151 PM CDT MON SEP 28 2026
VFR CONDITIONS ARE EXPECTED TO PERSIST THROUGH THE
PERIOD. SURFACE WINDS WILL VEER SOUTH TUESDAY MORNING AS HIGH
PRESSURE DEPARTS. INCOMING HIGH CLOUDS WILL GRADUALLY THICKEN
AND LOWER THROUGHOUT THE DAY AHEAD OF AN INCOMING FRONTAL
SYSTEM.
ILX WATCHES/WARNINGS/ADVISORIES
NONE.
KEY MESSAGES...MJA
DISCUSSION...MJA
AVIATION...MJA
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