643
FXUS63 KILX 282347
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
304 PM CDT MON JUL 28 2025
KEY MESSAGES
- A HEAT ADVISORY REMAINS IN EFFECT FOR PORTIONS OF CENTRAL AND
SOUTHEAST ILLINOIS THROUGH TUESDAY AFTERNOON FOR HEAT INDEX
VALUES BETWEEN 100-110 F.
- A STRONG COMPLEX OF THUNDERSTORMS WILL SHIFT SOUTHEAST TOWARD
WESTERN ILLINOIS OVERNIGHT, BRINGING A POTENTIAL FOR DAMAGING
STRAIGHT LINE WINDS.
- ADDITIONAL THUNDERSTORMS WILL DEVELOP ALONG A SLOW-MOVING COLD
FRONT WEDNESDAY, BRINGING A RISK OF LARGE HAIL, DAMAGING WINDS,
TORRENTIAL RAINFALL, AND FREQUENT LIGHTNING.
DISCUSSION
ISSUED AT 255 PM CDT MON JUL 28 2025
EARLY AFTERNOON SATELLITE AND SFC ANALYSIS DEPICTS A FEW KEY
FEATURES THAT WILL DRIVE THE SYNOPTIC PATTERN OVER THE NEXT 24
HOURS. OF IMMEDIATE IMPORTANCE, A REMNANT OUTFLOW BOUNDARY HAS
PUSHED INTO SOUTHERN ILLINOIS. THIS BOUNDARY MAY DRIFT A LITTLE
FURTHER NORTH THROUGHOUT THE REMAINDER OF THE AFTERNOON AS
SOUTHERLY LOW-LEVEL FLOW IMPINGES ON IT. A QUICK GLANCE AT
SATELLITE IMAGERY DEPICTS A GROWING CUMULUS FIELD ON THE SOUTH
SIDE OF THIS BOUNDARY, WITH THE NORTHERN EDGE BUTTING UP AGAINST
I-70. ROBUST INSTABILITY (MLCAPE > 4000 J/KG) AND JUST ENOUGH DEEP
LAYER SHEAR (20-25 KTS) IS BEING ANALYZED BY RECENT RAP GUIDANCE
ACROSS SE ILLINOIS AMID MODEST MID-LEVEL LAPSE RATES (6-6.6 C/KM)
AND DRY MID-LEVEL AIR. UPDRAFTS SHOULD STRUGGLE TO GET GOING IN
THIS ENVIRONMENT, BUT WE WILL NEED TO MONITOR FOR THE POTENTIAL
FOR A FEW STRONG MULTI-CELL CLUSTERS TO DEVELOP IN THE VICINITY OF
THE OUTFLOW BOUNDARY SHOULD A FEW UPDRAFTS OVERPERFORM. POTENTIAL
HAZARDS THIS AFTERNOON AND EVENING WOULD FAVOR DOWNBURST WINDS
(50-60 MPH), SMALL HAIL, TORRENTIAL DOWNPOURS AND CLOUD-TO-GROUND
LIGHTNING.
ANOTHER SYNOPTIC FEATURE WE ARE WATCHING CLOSELY IS A SHORTWAVE
TROUGH CRESTING THE TOP OF THE HEAT RIDGE. THIS SHORTWAVE IS
CURRENTLY OVER THE DAKOTAS AND IS FORECAST TO PUSH SOUTHEASTWARD
INTO THE UPPER-MISSISSIPPI VALLEY OVERNIGHT. MOST MODEL GUIDANCE
DEPICTS A STRENGTHENING MCS FOLLOWING THE PATH OF THE UPPER
FORCING (SHORTWAVE), EVENTUALLY PUSHING INTO PORTIONS OF EASTERN
IOWA AND SOUTHWEST WISCONSIN AFTER 06Z/1AM. SOME UNCERTAINTY
REMAINS WITH EXACTLY HOW THIS MCS EVOLVES IN TIME AND SPACE. WHILE
IT MAKES GOOD CONCEPTUAL SENSE FOR THE MCS TO FOLLOW THE UPPER
FORCING, WE DO OCCASIONALLY SEE THE CONVECTIVE ACTIVITY SHOW A
TENDENCY TO WANT TO FOLLOW THE MUCAPE GRADIENT. RECENT GUIDANCE
FROM THE HREF AND WOFS POSITIONS A 2000-4000 J/KG MUCAPE GRADIENT
ALONG A LINE EXTENDING FROM DES MOINES TO MACOMB. AND IT'S HERE THAT
THE FORECAST UNCERTAINTY LIES.
THE MOST LIKELY SCENARIO IS FOR THE MCS TO FOLLOW THE UPPER
FORCING (SHORTWAVE) INTO SOUTHERN WISCONSIN WHERE IT WILL RAPIDLY
DECAY AMID ROBUST MID-LEVEL DRY AIR AND SLIGHTLY LESS UNSTABLE
CONDITIONS. A REASONABLE ALTERNATIVE WOULD BE FOR THE MCS TO FAN
OUT, WITH SOME HEALTHY DEVELOPMENT ON THE SOUTHERN PERIPHERY (SE
IA/NE MO/W IL) OF THE UPPER FORCING TOWARD THE MUCAPE GRADIENT,
BUT DECAYING ON THE EASTERN PERIPHERY (S WI/N IL). THE OUTLYING
SOLUTION WOULD BE FOR THE MATURE MCS TO NOT DECAY, BUT INSTEAD
ACCELERATE AND MAINTAIN ITSELF FOR A LONG-DURATION BOW ECHO OR
DERECHO. NO CAM CURRENTLY SHOWS THE OUTLIER, BUT THAT ISNT
UNCOMMON FOR DERECHOS THIS TIME OF YEAR (SEE AUG 10 2020, JUN 29
2023, AND JUL 15 2024).
REGARDLESS OF THE EVOLUTION OF UPSTREAM CONVECTION OVERNIGHT, IT
WILL LIKELY THROW SOME CONVECTIVE DEBRIS INTO CENTRAL ILLINOIS.
THIS WOULD THEORETICALLY TAMP DOWN TEMPERATURES AND ATTENDANT HEAT
INDEX VALUES FOR WEDNESDAY AFTERNOON, PARTICULARLY ACROSS OUR
NORTHERN COUNTIES NEAR AND NORTH OF I-74. AS A RESULT, WE HAVE
TRIMMED OUR CURRENT HEAT ADVISORY BUT DID EXTEND IT INTO WEDNESDAY
AFTERNOON.
ATTENTION THEN TURNS TOWARD TUESDAY NIGHT AND WEDNESDAY AFTERNOON
AS A COLD FRONT SLOWLY SAGS INTO CENTRAL ILLINOIS FROM THE NORTH.
CAPE/SHEAR PROFILES IN THE VICINITY OF THE FRONT WILL BE CONDUCIVE
FOR SEVERE WEATHER, WITH A LARGE HAIL AND DAMAGING WIND THREAT
EVOLVING OVERNIGHT IN AREAS NORTH OF I-72, AND A CONTINUED THREAT
RE-DEVELOPING LATER WEDNESDAY AFTERNOON IN AREAS NEAR AND SOUTH OF
I-72. LOCALIZED FLASH FLOODING MAY ALSO BE OF CONCERN AS PWATS
EXCEED 99TH PERCENTILE OF LATE JULY CLIMATOLOGY, AND WITH MUCH OF
THE REGION EXCEEDING 150% OF NORMAL RAINFALL OVER THE PAST 30 DAYS.
A MUCH DRIER AND COOLER PATTERN WILL TAKE SHAPE THURSDAY ONWARD IN
THE WAKE OF THE PASSING COLD FRONT. A MODIFIED AREA OF CANADIAN
HIGH PRESSURE WILL SLOWLY BUILD ACROSS PORTIONS OF THE MIDWEST AND
GREAT LAKES, PUSHING DAILY TEMPERATURES 5-10 DEGREES BELOW NORMAL
FOR LATE JULY AND EARLY AUGUST.
MJA
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z TUESDAY AFTERNOON)
ISSUED AT 1151 AM CDT MON JUL 28 2025
LARGE SCALE SUBSIDENCE HAS ALLOWED FOR SKIES TO CLEAR THIS
AFTERNOON. VFR CONDITIONS HAVE RETURNED ACROSS THE BOARD AND WILL
PERSIST THROUGH MUCH OF THE LATE EVENING PERIOD. SOME UNCERTAINTY
STILL REMAINS REGARDING THUNDERSTORM POTENTIAL OVERNIGHT INTO
TUESDAY MORNING. RECENT MODEL GUIDANCE PAINTS A LOW-PROBABILITY,
HIGH-IMPACT THUNDERSTORM EVENT EVOLVING ACROSS THE UPPER-
MISSISSIPPI VALLEY OVERNIGHT, WITH SOME MEMBERS OF GUIDANCE
PUSHING THE THUNDERSTORM COMPLEX SOUTH INTO CENTRAL ILLINOIS
BETWEEN 08Z-10Z. GIVEN LOW CONFIDENCE, HAVE ONLY INTRODUCED A
PROB30 GROUP FOR TSRA ALONG OUR I-74 TERMINALS (KPIA, KBMI, AND
KCMI). OTHERWISE, LOW-LEVEL WINDS WILL REMAIN LIGHT THIS AFTERNOON
(5-10 KTS), BECOMING LIGHT AND VARIABLE OVERNIGHT BUT WITH THE
CAVEAT THAT THUNDERSTORM OUTFLOW MAY PRODUCE A SUDDEN SHIFT IN
DIRECTION AND MAGNITUDE.
MJA
ILX WATCHES/WARNINGS/ADVISORIES
HEAT ADVISORY UNTIL 8 PM CDT THIS EVENING FOR ILZ027>031-036>038-
040>057-061>063-066>068-071>073.
HEAT ADVISORY FROM NOON TO 8 PM CDT TUESDAY FOR ILZ027-029-036-
037-040>057-061>063-066>068-071>073.
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