584
FXUS63 KILX 090519
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
1219 AM CDT WED SEP 9 2026
KEY MESSAGES
- HEAT INDICES WILL TOP OUT AROUND 100 DEGREES WEDNESDAY
AFTERNOON SOUTH OF I-72 DUE TO HIGH TEMPERATURES IN THE 90S
AND RISING HUMIDITY LEVELS.
- SCATTERED THUNDERSTORMS ARE FORECAST WEDNESDAY AFTERNOON,
BECOMING MORE WIDESPREAD WEDNESDAY NIGHT ALONG A STALLED
FRONT; LOCALIZED HEAVY RAINFALL OF 1 TO 3 INCHES COULD CAUSE
LOCALIZED FLOODING, PARTICULARLY SOUTH OF I-70. THERE IS A
MARGINAL RISK (LEVEL 1 OF 5) FOR SEVERE THUNDERSTORMS FOR
MUCH OF CENTRAL AND SOUTHEAST ILLINOIS.
DISCUSSION
ISSUED AT 1256 PM CDT TUE SEP 8 2026
SHORT TERM (TODAY THROUGH WEDNESDAY NIGHT)...
THIS AFTERNOON, A POTENT 500MB LOW IS CURRENTLY LIFTING
NORTHEASTWARD ACROSS MANITOBA, WITH AN ASSOCIATED SURFACE LOW
ALSO OVER MANITOBA AND A COLD FRONT TRAILING SOUTHWARD ACROSS
THE MIDDLE MISSOURI VALLEY. AHEAD OF THIS SYSTEM, CENTRAL
ILLINOIS RESIDES WITHIN A WELL MIXED SOUTHWESTERLY FLOW REGIME.
THIS IS FACILITATING THE ADVECTION OF AN 850MB THERMAL RIDGE
(WITH TEMPERATURES OF 22-24C) ACROSS THE REGION. CONSEQUENTLY,
TEMPERATURES ARE WARMING INTO THE LOW TO MID-90S THIS AFTERNOON.
DEW POINTS REMAIN IN CHECK--IN THE LOW TO MID-60S--DUE TO DEEP
BOUNDARY LAYER MIXING, KEEPING PEAK HEAT INDICES NEAR THE
AMBIENT TEMPERATURES IN THE LOW TO MID-90S.
ON WEDNESDAY, LOW LEVEL FLOW WILL CONTINUE TO STRENGTHEN,
TRANSPORTING A MORE HUMID AIR MASS INTO THE REGION, WITH SURFACE
DEW POINTS RISING INTO THE LOW TO MID-70S. THE COMBINATION OF
HIGH AMBIENT TEMPERATURES IN THE 90S AND THIS RICH MOISTURE
PROFILE WILL DRIVE AFTERNOON HEAT INDICES TO AROUND OR JUST PAST
100F, PARTICULARLY FOR AREAS SOUTH OF THE I-72 CORRIDOR. THE
COLD FRONT IS EXPECTED TO SLOWLY PUSH INTO CENTRAL ILLINOIS BY
MID-TO-LATE WEDNESDAY MORNING. GIVEN ITS ARRIVAL NEAR THE
DIURNAL INSTABILITY MINIMUM, INITIAL CONVECTIVE ACTIVITY IS
PROJECTED TO BE IN A DECAYING PHASE. HOWEVER, AS THE FRONT SAGS
SOUTH, IT IS FORECAST TO SLOW AND ULTIMATELY STALL ACROSS THE
I-72 AND I-70 CORRIDORS DURING THE AFTERNOON AND EVENING.
SCATTERED STORM DEVELOPMENT REMAINS POSSIBLE ALONG THIS STALLED
BOUNDARY. INSTABILITY IS PROGGED TO REMAIN MODEST WEDNESDAY
AFTERNOON WITH MLCAPE VALUES PROGGED TO PEAK AROUND 500-750
J/KG. DEEP LAYER SHEAR WILL BE SEASONABLY STRONG, HOWEVER,
AROUND 35-40 KT WHICH MAY SUPPORT A FEW STRONG TO SEVERE STORMS.
DAMAGING WIND GUSTS WILL BE THE PRIMARY HAZARD.
OVERNIGHT, CONVECTIVE POTENTIAL INCREASES SIGNIFICANTLY AS A
MODEST LOW-LEVEL JET TO OUR WEST INTERACTS WITH THE STALLED
FRONTAL ZONE, TRIGGERING A WIDESPREAD ROUND OF STORMS THAT WILL
TRACK EASTWARD. GIVEN THAT THE STEERING FLOW (850-300MB) IS
ORIENTED NEARLY PARALLEL TO THE SURFACE BOUNDARY, TRAINING OF
STORMS IS A CONCERN. TOTAL RAINFALL THROUGH WEDNESDAY NIGHT WILL
AVERAGE UNDER 0.50 INCHES NORTH OF I-72, BUT FURTHER SOUTH,
WIDESPREAD AMOUNTS OF 0.50 TO 1.25 INCHES ARE EXPECTED, WITH
LOCALIZED SWATHS OF 1 TO 3 INCHES POSSIBLE (PARTICULARLY SOUTH
OF I-70) AS HIGHLIGHTED BY THE HREF LPMM.
LONG TERM (THURSDAY THROUGH NEXT TUESDAY)...
AS THE FRONT SAGS SOUTH OF THE AREA ON THURSDAY, HIGH PRESSURE
WILL BUILD ACROSS THE UPPER MIDWEST, GRADUALLY SHIFTING EASTWARD
TOWARD THE GREAT LAKES BY FRIDAY. THIS WILL USHER IN A MUCH
DRIER AND MORE SEASONABLE AIR MASS. EXPECT QUIET, PLEASANT
CONDITIONS ACROSS CENTRAL ILLINOIS BY FRIDAY WITH HIGH
TEMPERATURES RETURNING TO SEASONAL NORMS IN THE 80S. HEADING
INTO THE WEEKEND AND EARLY NEXT WEEK, THE SYNOPTIC-SCALE PATTERN
IS PROJECTED TO FLATTEN INTO A LOW-AMPLITUDE, ZONAL FLOW
REGIME. A SERIES OF WEAK, PROGRESSIVE WAVES WILL TRAVERSE THIS
FLOW, INTRODUCING PERIODIC CHANCES FOR SHOWERS AND
THUNDERSTORMS. DUE TO PERSISTENT MODEL DISCREPANCIES AND LOW
PREDICTABILITY INHERENT IN THESE LOW-AMPLITUDE SYSTEMS, A
BROAD-BRUSHED 20-40% PROBABILITY OF PRECIPITATION (POP) HAS BEEN
MAINTAINED FROM FRIDAY NIGHT THROUGH NEXT TUESDAY BY THE NBM.
AVIATION /06Z TAFS THROUGH 06Z THURSDAY/
ISSUED AT 1219 AM CDT WED SEP 9 2026
CLUSTER OF THUNDERSTORMS ALONG THE IL/IA BORDER CURRENTLY ON A
TRAJECTORY TO PASS NORTH OF KPIA, THOUGH ADDITIONAL DEVELOPMENT
IS EXPECTED TOWARD DAYBREAK AS A COLD FRONT BEGINS TO MOVE INTO
CENTRAL ILLINOIS. BEST CHANCES OF THUNDER WILL BE AT KPIA/KBMI
THROUGH MID MORNING, THOUGH SOUTHERN EXTENT OF THE RAIN SHIELD
IS MORE QUESTIONABLE AND IT IS UNCERTAIN WHETHER THE RAIN WILL
REACH KDEC/KSPI. WINDS DO TURN AROUND TO THE NORTHWEST AT ALL
SITES DURING THE MORNING WITH FRONTAL PASSAGE, THOUGH DROP OFF
IN SPEED TOWARD 00Z.
VFR CONDITIONS EXPECTED TO PREVAIL WITH CEILINGS GENERALLY AT OR
ABOVE 10KFT, THOUGH SOME LOWER CEILINGS CLOSER TO 4000 FEET WILL
ACCOMPANY ANY THUNDERSTORMS.
ILX WATCHES/WARNINGS/ADVISORIES
NONE.
KEY MESSAGES...DEUBELBEISS
DISCUSSION...DEUBELBEISS
AVIATION...GEELHART
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