905
FXUS63 KILX 131436
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
936 AM CDT THU AUG 13 2026
KEY MESSAGES
- MIDDAY THUNDERSTORM COMPLEX: A CLUSTER OF THUNDERSTORMS TRACKS
ACROSS IOWA WILL LIKELY IMPACT PARTS OF THE FORECAST AREA,
WITH INCREASING CONCERNS FOR AREAS NEAR THE ILLINOIS RIVER
VALLEY. ISOLATED SEVERE THUNDERSTORMS AND EFFICIENT, HEAVY
RAINFALL ARE A CONCERN.
- HEAT CONCERNS SOUTH: SOUTH OF THE IMPENDING THUNDERSTORM
DEBRIS, HOT AND HUMID CONDITIONS YIELD HEAT INDICES NEAR 105F.
A HEAT ADVISORY IS ACTIVE THROUGH EARLY EVENING FOR SOUTHEAST
ILLINOIS, AND WILL NEED TO BE EXPANDED NORTHWARD THIS WEEKEND.
- OVERNIGHT FLOOD WATCH: REPEATED THUNDERSTORMS WILL HEIGHTEN
FLASH FLOODING RISKS. A FLOOD WATCH IS IN EFFECT FOR COUNTIES
ALONG/NORTH OF I-74 THROUGH FRIDAY MORNING.
UPDATE
ISSUED AT 935 AM CDT THU AUG 13 2026
MCS CONTINUES TO TRACK ACROSS SOUTHEAST IOWA THIS MORNING, WITH
COLD CLOUD TOPS NEAR -75C. CONVECTIVE LINE HAS MAINTAINED ITSELF
AND WARNINGS CONTINUE TO BE ISSUED IN THAT AREA. AHEAD OF THE
SYSTEM IN CENTRAL ILLINOIS, CONDITIONS ARE FAIRLY STABLE DUE TO
THE EXTENSIVE LOW CLOUDINESS AND EARLIER FOG ACROSS THE
NORTHERN CWA. HOWEVER, A NW-SE CAPE GRADIENT MAY ALLOW FOR A
PATH TOWARD THE ILLINOIS RIVER VALLEY IN WEST CENTRAL ILLINOIS.
NONE OF THE REFLECTIVITY GUIDANCE IN THE MORNING CAM'S FEATURE
SUCH A SCENARIO AT THE MOMENT, THOUGH VARIOUS RAP CAPE PROFILES
DO SHOW THE GRADIENT ASPECT. OVERALL, LITTLE CHANGE WAS MADE FOR
TODAY'S POP'S IN THAT AREA.
LOOKING AHEAD TO TONIGHT, THE NEXT MCS STILL APPEARS TO SET UP
NEAR THE WARM FRONT ACROSS NORTHERN ILLINOIS THIS EVENING,
GRADUALLY SINKING TOWARD THE I-74 CORRIDOR TOWARD MIDNIGHT. SOME
MINOR POP ADJUSTMENTS WERE DONE TO INCREASE RAIN CHANCES NEAR
AND NORTH OF I-74. NO CHANGES WERE MADE TO THE FLOOD WATCH AT
THIS TIME.
DISCUSSION
ISSUED AT 441 AM CDT THU AUG 13 2026
MORNING DENSE FOG
A STRONG RADIATIONAL COOLING REGIME EARLY THIS MORNING HAS
FOSTERED PATCHY DENSE FOG DEVELOPMENT ACROSS CENTRAL ILLINOIS.
DESPITE HIGH-LEVEL ANVIL DEBRIS STREAMING OVERHEAD NEAR/NORTH
OF THE I-74 CORRIDOR AROUND MIDDAY, BRINGING A THREAT FOR
UPSTREAM CONVECTION, LINGERING NEAR-SURFACE MOISTURE AND WEAK
BOUNDARY- LAYER FLOW (WINDS < 10 KT THROUGH 3 KFT) WILL KEEP FOG
GROUNDED PAST DAYBREAK. SHALLOW WARM AIR ADVECTION (WAA)
OVERRUNNING THE EFFECTIVE SURFACE BOUNDARY CURRENTLY DRAPED JUST
SOUTH OF THE FORECAST AREA WILL ALSO AID IN MAINTAINING LOW
STRATUS/FOG WELL INTO THE MID-MORNING HOURS. A DENSE FOG
ADVISORY IS IN EFFECT THROUGH 9 AM CDT FOR LOCATIONS ROUGHLY
NORTH OF A QUINCY TO TERRE HAUTE LINE.
MIDDAY CONVECTIVE COMPLEX & HEAVY RAIN POTENTIAL
ATTENTION THEN SHIFTS TO THE AFTERNOON AS THE FIRST OF MULTIPLE
CONVECTIVE ROUNDS APPROACHES FROM THE WEST. EARLY MORNING RADAR
TRENDS REVEAL A MCC POSITIONED OVER WESTERN IA. SHORT- TERM
HIGH- RESOLUTION GUIDANCE REMAINS IN SOLID CONSENSUS THAT THIS
COMPLEX WILL TRACK NEAR AND NORTH OF THE I-74 CORRIDOR BY
MIDDAY, RIDING ALONG THE NOSE OF A RECEDING LOW-LEVEL JET (LLJ),
A CONVECTIVELY-AUGMENTED SHORTWAVE TROUGH, AND THE NORTHERN
EDGE OF THE INSTABILITY GRADIENT.
SEVERE POTENTIAL WITH THIS INITIAL WAVE REMAINS CONDITIONAL.
WHILE COLD POOL DYNAMICS AND POTENTIAL REAR INFLOW JET (RIJ)
ORGANIZATION BEAR WATCHING, POOR LAPSE RATES, LINGERING MORNING
FOG/STRATUS, AND A PERSISTENT LOW-LEVEL TEMPERATURE INVERSION
COULD KEEP THIS INCOMING ACTIVITY ELEVATED AND PREDOMINANTLY
SUB-SEVERE.
REGARDLESS OF SEVERE RISK, THIS INITIAL ROUND WILL BE AN
EFFICIENT HEAVY RAINFALL PRODUCER. ATMOSPHERIC SOUNDINGS DISPLAY
CLASSIC TROPICAL-LIKE CHARACTERISTICS: (A) ANOMALOUS PWAT
VALUES (2-2.2 INCHES); (B) LOW- TO MID-LEVEL RH EXCEEDING 80%;
(C) A DEEP WARM CLOUD LAYER (> 4KM); AND (D) A LONG, SKINNY
CAPE PROFILE.
LATEST HREF 6-HOUR PROBABILITY-MATCHED MEAN (PMM) QPF TARGETS
0.5 TO 1.5 INCHES ACROSS THE NORTHERN FORECAST AREA WITH THIS
INITIAL MIDDAY PUSH.
HEAT & HUMIDITY ACROSS THE SOUTH
SOUTH OF THE CONVECTIVE FOOTPRINT AND THE EFFECTIVE SURFACE
BOUNDARY (PRIMARILY SOUTH OF A QUINCY TO TERRE HAUTE LINE),
STRONG SURFACE HEATING AND RICH MOISTURE WILL PERSIST. HIGHS
NEAR 90 PAIRED WITH DEWPOINTS IN THE MID 70S WILL YIELD HEAT
INDEX VALUES BETWEEN 100-105F. A HEAT ADVISORY IS IN EFFECT
THROUGH EARLY EVENING FOR PORTIONS OF SOUTHEAST ILLINOIS.
OVERNIGHT FLASH FLOOD THREAT
THE THREAT FOR FLASH FLOODING ESCALATES SIGNIFICANTLY TONIGHT
ACROSS CENTRAL AND NORTHERN ILLINOIS. THE EFFECTIVE SURFACE
BOUNDARY WILL BEGIN TO RETREAT NORTHWARD AS A SHARPENING WARM
FRONT. AN INTENSIFYING NOCTURNAL LLJ IMPINGING DIRECTLY ONTO
THIS ELEVATED BAROCLINIC ZONE WILL FUEL REPEATED CONVECTIVE
INITIATION AND TRAINING THUNDERSTORM COMPLEXES NEAR AND NORTH OF
THE I-74 CORRIDOR.
WITH ANTECEDENT CONDITIONS POTENTIALLY PRIMED BY THE MIDDAY
CONVECTIVE ROUND, SUSCEPTIBILITY TO RAPID RUNOFF COULD BE HIGH.
HREF QPF PMM GUIDANCE INDICATES AN ADDITIONAL 1-2 INCHES
OVERNIGHT, WITH MESOSCALE TRAINING CORRIDORS CAPABLE OF
PRODUCING LOCALIZED TOTALS EXCEEDING 4 INCHES. GIVEN THE HIGH-
EFFICIENCY AIRMASS AND HIGH POTENTIAL FOR TRAINING, A FLOOD
WATCH HAS BEEN ISSUED FOR COUNTIES ALONG AND NORTH OF I-74
THROUGH FRIDAY MORNING.
THIS WEEKEND THROUGH MID-NEXT WEEK
THE SURFACE WARM FRONT IS PROJECTED TO SURGE NORTH OF THE AREA
FRIDAY NIGHT INTO SATURDAY, RE-INTRODUCING TRIPLE- DIGIT HEAT
INDICES AND SCATTERED ELEVATED WARM-ADVECTION CONVECTION TO
START THE WEEKEND. THE HEAT ADVISORY WILL MOST LIKELY NEED TO BE
EXPANDED NORTHWARD FOR FRIDAY AND SATURDAY.
A STRONGER, MORE POTENT SHORTWAVE TROUGH IS THEN FORECAST TO
DIG OUT OF THE NORTHERN PLAINS SUNDAY INTO MONDAY, DRIVING A
SHARPER SURFACE COLD FRONT THROUGH THE MISSISSIPPI VALLEY. WHILE
SHOWERS AND THUNDERSTORMS MAY ACCOMPANY THE FRONTAL PASSAGE,
DETERMINISTIC MODELS AND GLOBAL ENSEMBLES SHOW REMARKABLY STRONG
CONSENSUS FOR A DEEPER CONTINENTAL POLAR (CP) AIRMASS TO PUSH
INTO THE CENTRAL U.S. EARLY NEXT WEEK. THIS WILL USHER IN A
PERIOD OF BELOW-NORMAL TEMPERATURES, LOWER DEWPOINTS, AND AN
EXTENDED BREAK FROM THE ACTIVE CONVECTIVE PATTERN BY MONDAY AND
TUESDAY.
AVIATION /12Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 538 AM CDT THU AUG 13 2026
MVFR TO IFR FOG/LOW STRATUS CONTINUES TO EXPAND THIS MORNING
WITH THE WORST CONDITIONS NEAR AND NORTH OF THE I-74 CORRIDOR.
VISIBILITY SHOULD IMPROVE BEYOND 13Z, THOUGH CEILINGS AT
NORTHERN TERMINALS WILL REMAIN MVFR FOR AT LEAST THE FIRST HALF
OF THE DAY.
A DECAYING THUNDERSTORM COMPLEX WILL APPROACH THE CENTRAL
ILLINOIS TERMINALS BY EARLY AFTERNOON, WITH THE HIGHEST
PROBABILITIES OF TSRA AT KPIA AND KBMI. PROB30 GROUPS ARE NOTED
AT THE REMAINING SITES WHERE CONFIDENCE REMAINS LOWER.
ADDITIONAL STORMS ARE POSSIBLE NEAR/NORTH OF I-74 LATE TONIGHT
WITH PROB30 GROUPS MAINTAINED FOR THIS POTENTIAL. EASTERLY
WINDS WILL LARGELY REMAIN IN PLACE THROUGH THE PERIOD, WITH THE
DIRECTION MORE VARIABLE WITH ANY STORM ACTIVITY.
ILX WATCHES/WARNINGS/ADVISORIES
FLOOD WATCH THROUGH FRIDAY MORNING FOR ILZ027>031-037-038-045-
046.
HEAT ADVISORY FROM NOON TODAY TO 8 PM CDT SUNDAY FOR
ILZ066>068-071>073.
KEY MESSAGES...GEELHART
UPDATE...GEELHART
DISCUSSION...MJA
AVIATION...MJA
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