806
FXUS63 KILX 170522
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
1222 AM 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.
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 90TH
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 /06Z TAFS THROUGH 06Z FRIDAY/
ISSUED AT 1209 AM CDT THU SEP 17 2026
LATE THIS EVENING AND OVERNIGHT, SCATTERED SHOWERS AND STORMS
ARE EXPECTED TO DEVELOP ACROSS PORTIONS OF NORTH CENTRAL
ILLINOIS NEAR OR JUST NORTH OF PIA AND BMI. IF STORMS CONGEAL
INTO A LINE, THEY MAY SPILL SOUTHEAST LATE TONIGHT, THOUGH IF
THEY REMAIN SCATTERED THEY SHOULD CONTINUE TO TRACK EAST JUST
NORTH OF THE TERMINALS. MEANWHILE, IFR CEILINGS OVER IOWA AND
NORTHERN ILLINOIS ARE EXPECTED TO EXPAND SOUTH TO NEAR THE I-74
CORRIDOR WITH PIA/BMI HAVING THE GREATEST CHANCE TO SEE THE IFR
OR LOW END MVFR CEILINGS LATE TONIGHT. LOW CEILINGS SHOULD
RETREAT BACK NORTH DURING THE DAY TODAY, BUT MAY SPREAD SOUTH
AGAIN THURSDAY NIGHT INTO FRIDAY MORNING.
ILX WATCHES/WARNINGS/ADVISORIES
FLOOD WATCH UNTIL 10 AM CDT THIS MORNING FOR ILZ027-028-030.
HEAT ADVISORY UNTIL 7 PM CDT THIS EVENING FOR ILZ049>052-061-
062-066>068-071>073.
KEY MESSAGES...25
DISCUSSION...25
AVIATION...DEUBELBEISS
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page