577
FXUS63 KILX 091758
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
1258 PM CDT SUN AUG 9 2026
KEY MESSAGES
- INCREASING HEAT AND HUMIDITY TODAY AND MONDAY WILL YIELD HEAT
INDEX VALUES BETWEEN 100 AND 105 DEGREES, PARTICULARLY IN
AREAS SOUTH OF THE INTERSTATE 74 CORRIDOR.
- LATE-NIGHT STRONG TO SEVERE THUNDERSTORMS MAY DIVE SOUTH INTO
THE REGION TONIGHT AND MONDAY NIGHT, BRINGING A 15% TO 30%
CHANCE FOR DAMAGING WINDS.
- A BROADER THREAT FOR SEVERE WEATHER DEVELOPS TUESDAY INTO
WEDNESDAY, WITH DAMAGING WINDS AND HAIL FORECAST ALONG AN
ADVANCING COLD FRONT.
- COOLER CONDITIONS ARRIVE THURSDAY, BRINGING HIGH TEMPERATURES
BACK INTO THE UPPER 70S AND LOWER 80S.
DISCUSSION
ISSUED AT 341 AM CDT SUN AUG 9 2026
NEAR TERM
(TODAY THROUGH TONIGHT)
AGGRESSIVE LOW-LEVEL WARM AIR ADVECTION BLEEDING EASTWARD FROM
THE CENTRAL PLAINS WILL FORCE A LINGERING SURFACE BOUNDARY TO
RAPIDLY RETREAT NORTHWARD AS A WARM FRONT TODAY, PLACING CENTRAL
ILLINOIS FIRMLY WITHIN A BROADENING, UNSTABLE WARM SECTOR.
HIGHS WILL SURGE INTO THE UPPER 80S TO LOWER 90S AREAWIDE. WHEN
PAIRED WITH MID-70S DEWPOINTS, PEAK HEAT INDEX VALUES WILL REACH
100-105 DEGREES. GIVEN THESE CONDITIONS, A HEAT ADVISORY HAS
BEEN ISSUED FOR PORTIONS OF THE LOWER ILLINOIS AND SANGAMON
RIVER VALLEYS.
THE CONVECTIVE EVOLUTION THROUGH TONIGHT PRESENTS A CLASSIC,
LOW-PREDICTABILITY NORTHWEST FLOW SETUP. WHILE PRIMARY DAYTIME
CONVECTION SHOULD BYPASS OUR CWA TO THE NORTH NEAR THE WI/IL
BORDER, A FEW MORNING CELLS MAY GRAZE NORTHERN COUNTIES ALONG
AND NORTH OF A MACOMB TO BLOOMINGTON LINE.
AFTERNOON MESOSCALE DETAILS WILL DEPEND HEAVILY ON RESIDUAL
OUTFLOW BOUNDARIES. IF A COMPOSITE OUTFLOW BOUNDARY SETTLES NEAR
OR SOUTH OF I-80 BY LATE AFTERNOON, IT COULD ACT AS A FOCAL
POINT FOR EVENING CONVECTIVE INITIATION.
WITH A VEERING LOW-LEVEL JET (LLJ) ACCELERATING TONIGHT ATOP A
HEALTHY THERMODYNAMIC RESERVOIR (MUCAPE EXCEEDING 2000 J/KG AND
0-6KM BULK SHEAR OF 30-40 KT), A MATURING CONVECTIVE SYSTEM
(MCS) IS EXPECTED TO ORGANIZE AND DIVE SOUTHWARD INTO CENTRAL
ILLINOIS. SPC MAINTAINS A MARGINAL RISK (LEVEL 1 OF 5) ACROSS
MUCH OF THE AREA, WITH A SLIGHT RISK (LEVEL 2 OF 5) EXTENDING
SOUTH TO THE I-74 CORRIDOR TO ACCOUNT FOR THIS LATE-NIGHT
DAMAGING WIND AND HAIL THREAT.
SHORT TERM
(MONDAY THROUGH WEDNESDAY)
MONDAY WILL MIRROR SUNDAY'S CONVECTIVE POTENTIAL, BUT WITH
ENHANCED THERMODYNAMICS. DEEP SYNOPTIC SUBSIDENCE AND A STOUT
ELEVATED MIXED LAYER (EML) SHOULD KEEP THE BOUNDARY LAYER CAPPED
THROUGH THE AFTERNOON, YIELDING CLEARING SKIES AND EXTREME
BUOYANCY. AFTERNOON HIGHS IN THE LOWER 90S COMBINED WITH MID-70S
DEWPOINTS WILL PUSH HEAT INDICES TO 100-105 DEGREES SOUTH OF
I-74, WHERE ANOTHER HEAT ADVISORY IS CURRENTLY DRAWN. MUCAPE IN
EXCESS OF 3000 J/KG COULD SET THE STAGE FOR ANOTHER NOCTURNAL
SEVERE THREAT MONDAY NIGHT AS AN UPSTREAM MCS, AIDED BY THE LLJ,
TRACKS ALONG THE UPPER RIDGE PERIPHERY AND PERHAPS INTO OUR
CWA.
BY TUESDAY INTO WEDNESDAY, THE SEVERE WEATHER POTENTIAL BECOMES
BROADER AND MORE ORGANIZED. A DEEPER UPPER-LEVEL TROUGH DIGGING
INTO THE GREAT LAKES WILL BUCKLE UPPER FLOW AND ENHANCE MID-
LEVEL FLOW, STRENGTHENING 0-6 KM BULK SHEAR PROFILES TO 30-40+
KNOTS. OVERLAID UPON MODERATE BOUNDARY LAYER INSTABILITY AND
RICH LOW-LEVEL MOISTURE, THIS KINEMATIC BOOST WILL FAVOR
ORGANIZED CONVECTIVE CLUSTERS AND BOWING SEGMENTS CAPABLE OF
WIDESPREAD DAMAGING WINDS AND LARGE HAIL. THE PRIMARY COLD FRONT
WILL SLOWLY SINK SOUTHWARD INTO CENTRAL ILLINOIS LATE TUESDAY
NIGHT INTO WEDNESDAY, CONTINUING THE THREAT FOR ACTIVE WEATHER.
LONG TERM
(THURSDAY THROUGH FRIDAY)
THE COLD FRONT IS FORECAST TO PUSH SOUTH OF THE REGION BY
THURSDAY, BRINGING NOTABLE RELIEF AS HIGH TEMPERATURES DROP INTO
THE UPPER 70S AND LOWER 80S. HOWEVER, MODEL GUIDANCE CONTINUES
TO DIVERGE REGARDING THE POST-FRONTAL AIR MASS EVOLUTION. THE
GFS/GEFS SUITE DRIVES THE FRONT CLEANLY SOUTH, USHERING IN A
COOLER, DRIER CONTINENTAL POLAR (CP) AIR MASS THROUGH SATURDAY.
CONVERSELY, THE ECMWF/EPS MODELS FAVOR A QUICKER PROGRESSION,
LIFTING THE FRONT BACK NORTHWARD ACROSS ILLINOIS BY FRIDAY AND
REINTRODUCING STIFLING HUMIDITY AND THUNDERSTORM CHANCES. GIVEN
THE UNCERTAINTY, RAIN CHANCES REMAIN IN THE FORECAST THROUGH
LATE WEEK DUE TO THE CWA'S PROXIMITY TO THE STALLED BAROCLINIC
ZONE.
AVIATION /18Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 1258 PM CDT SUN AUG 9 2026
1745Z/1245PM RADAR IMAGERY SHOWS A LINE OF THUNDERSTORMS
TRACKING EASTWARD ACROSS NORTHERN ILLINOIS TOWARD LAKE MICHIGAN.
MEANWHILE FURTHER SOUTHWEST ALONG/AHEAD OF THE TRAILING OUTFLOW
BOUNDARY, A BROKEN LINE OF LIGHT SHOWERS CONTINUES TO DROP
SOUTHEASTWARD THROUGH THE ILLINOIS RIVER VALLEY. BASED ON
MESOANALYSIS SHOWING CAPPING AND A CONSISTENT SIGNAL FROM MOST
CAMS, HAVE ONLY CARRIED VCSH AT KPIA/KBMI/KSPI THROUGH 20Z/21Z.
ONCE THIS LINE PASSES, THE ACCOMPANYING MID-LEVEL OVERCAST WILL
SCATTER...LEAVING BEHIND MOSTLY CLEAR CONDITIONS EARLY THIS
EVENING. THE NEXT AVIATION FORECAST CHALLENGE WILL REVOLVE
AROUND CONVECTIVE RE-DEVELOPMENT TONIGHT AS THE NOCTURNAL LOW-
LEVEL JET STRENGTHENS. BASED ON MODEL CONSENSUS, IT APPEARS
SCATTERED CELLS WILL FORM ALONG/NORTH OF I-74 BY MID-EVENING,
THEN TRACK E/SE INTO INDIANA TOWARD DAWN MONDAY. AS SUCH, HAVE
INCLUDED PROB30 GROUPS FOR THUNDER AT KPIA/KBMI/KCMI BEGINNING
AT 02Z IN KPIA AND ENDING AT 10Z IN KCMI. AFTER THIS ACTIVITY
DEPARTS, DRY CONDITIONS ARE ANTICIPATED ACROSS THE BOARD THROUGH
18Z MON.
ILX WATCHES/WARNINGS/ADVISORIES
HEAT ADVISORY UNTIL 7 PM CDT THIS EVENING FOR ILZ027-036-040-
041-047>051.
HEAT ADVISORY FROM NOON TO 7 PM CDT MONDAY FOR ILZ036-040-041-
047>052-061>063-066>068-071>073.
KEY MESSAGES...MJA
DISCUSSION...MJA
AVIATION...BARNES
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