205
FXUS63 KLOT 171859
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
159 PM CDT FRI APR 17 2026
KEY MESSAGES
- ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS IS EXPECTED LATER
THIS EVENING INTO TONIGHT. SOME OF THE STORMS COULD BE
SEVERE. LOCALLY HEAVY RAINFALL COULD RESULT IN FLOODING
ISSUES.
- WINDY AND UNSEASONABLY WARM WEATHER TODAY WILL BE REPLACED BY
MUCH COLDER WEATHER OVER THE WEEKEND INTO EARLY NEXT WEEK.
- FROST/FREEZE CONDITIONS ARE EXPECTED MANY AREAS SUNDAY AND
ESPECIALLY MONDAY MORNINGS.
MESOSCALE
ISSUED AT 1219 PM CDT FRI APR 17 2026
THE OPEN WARM SECTOR HAS REMAINED STABLE BASED ON RECENT
SATELLITE TRENDS, WITH LITTLE CUMULUS SO FAR ACROSS ILLINOIS,
AND MAINLY BILLOWS STREETS OVER NORTHERN MISSOURI AND EASTERN
IOWA. HOWEVER, THERE HAVE BEEN RECENT HINTS THAT THE REMAINING
INHIBITION IS QUICKLY ERODING WEST OF THE MISSISSIPPI RIVER WITH
POCKETS OR OPEN CELLED CUMULUS CLUSTERS BEGINNING TO APPEAR.
ATTEMPTS AT CONVECTIVE INITIATION HAVE ALREADY BEEN OBSERVED,
WITH MORE IMMINENT ORGANIZED DEEP CONVECTION LIKELY WITHIN THE
NEXT HOUR OVER FAR NORTH-CENTRAL MISSOURI INTO SOUTHEAST IOWA.
INITIAL DISCRETE SUPERCELLS ARE EXPECTED IN THIS AREA, WITH AN
ULTIMATE TRANSITION INTO A LINEAR QLCS SEGMENT WITH NORTHEAST
EXTENT. WHAT REMAINS LESS CLEAR IS HOW QUICK THIS MODE
TRANSITION OCCURS, WHICH FURTHER DICTATES CONVECTIVE EVOLUTION
FARTHER EAST. THERE APPEARS TO BE TWO MAIN SCENARIOS AT PLAY:
1) CONVECTION OVER IOWA REMAINS MORE DISCRETE OR SEMI-DSICRETE
ACROSS THE MISSISSIPPI RIVER, GREATLY INCREASING THE SEVERE RISK
FOR ALL HAZARDS, INCLUDING TORNADOES AND VERY LARGE HAIL,
AROUND AND WEST OF THE I-39 CORRIDOR IN THE 3-5PM WINDOW. IN THE
ABSENCE OF A MORE FORMIDABLE COLD POOL AND WITH RESIDUAL LOW-
LEVEL CAPPING AND/OR MIXING REMAINING FARTHER EAST, CONVECTION
WOULD LIKELY STRUGGLE TO REMAIN ORGANIZED INTO THE CHICAGO METRO
THROUGH THIS AFTERNOON.
2) CONVECTION CONGEALS INTO A MORE ORGANIZED LINE WITH A
DEVELOPING COLD POOL ACROSS NORTHWEST ILLINOIS, PROSPECTS OF
CONVECTION EXTENDING WELL EASTWARD ACROSS NORTHERN ILLINOIS
WOULD THEN INCREASE SUBSTANTIALLY THIS AFTERNOON. IN THIS CASE,
A GROWING DCAPE RESERVOIR FROM AFOREMENTIONED MIXING FARTHER TO
THE EAST FAVOR AN OUTFLOW DOMINANT LINE OF CONVECTION WITH
PRIMARILY DAMAGING WIND SHIFTING EASTWARD AROUND AND NORTH OF
THE I-80 CORRIDOR THROUGH LATE AFTERNOON.
BEYOND THIS INITIAL ROUND, A SECOND ROUND OF FRONTAL-FORCED
CONVECTION IS EXPECTED ACROSS THE AREA THIS EVENING. COVERAGE OF
THE INITIAL ROUND WILL GREATLY AFFECT THE AREAL EXTENT OF
POTENTIAL SEVERE CONVECTION OVER OUR AREA, BUT A QLCS MODE WITH
DAMAGING WINDS AND EMBEDDED TORNADOES REMAINS A FOCUS WHEREVER
THE WARM SECTOR REMAINS UNCONTAMINATED.
KLUBER
DISCUSSION
ISSUED AT 136 PM CDT FRI APR 17 2026
FOR NEAR TERM DETAILS ON THE CONVECTIVE THREATS THROUGH THIS
EVENING, SEE THE MESOSCALE DISCUSSION (AND/OR ADDITIONAL FUTURE
DISCUSSIONS) ABOVE. ADDITIONAL CONCERNS FOR TONIGHT WILL BE
CENTERED ON HEAVY RAINFALL AND LOCALIZED FLASH FLOODING.
PRECIPITABLE WATER VALUES STILL LIE AT THE UPPER ECHELON OF
TYPICALLY OBSERVED VALUES FOR MID APRIL, AND THIS COUPLED WITH
SOME ENHANCED FORCING SHOULD LEAD TO AREAS OF 1-1.5" RAINFALL
IN A RELATIVELY SHORT WINDOW OF TIME. WE CANNOT RULE OUT ANY
HIGHER OVERALL TOTALS (PUSHING 3"), GIVEN THAT WE HAVE SEVERAL
WINDOWS OF FORCING BOTH IN THE OPEN WARM SECTOR AND ALONG THE
COLD FRONT. MUCH OF THIS RAIN WILL GO IMMEDIATELY INTO RUNOFF.
IN ADDITION TO ANY URBAN LOCATIONS, IT IS HARD TO PINPOINT THE
MORE FAVORED AREAS IN THE WATCH AREA, HOWEVER THE MOST
SUSCEPTIBLE AREAS OVERLAPPING WITH HIGHER CONFIDENCE RAINFALL
WOULD BE IN THE ROCK RIVER BASIN, AND THEN EXTENDING EASTWARD
INTO THE FOX AND DES PLAINES RIVER BASINS ACROSS NORTHERN
ILLINOIS. A SECONDARY HEAVY RAIN AXIS MAY SETUP SOUTH, BUT IF WE
HAVE A FORWARD PROPAGATING SYSTEM, THAT MAY LIMIT THE DURATION
OF HEAVY
A TASTE OF REALITY WILL RETURN TO THE LOWER GREAT LAKES REGION
BEHIND TONIGHT'S STRONG COLD FRONT. HIGH TEMPERATURES WILL
REACHED AT MIDNIGHT. WHILE SOME TEMPERATURE RECOVERIES WILL
OCCUR AS CLOUDS BREAK THROUGH THE MORNING, RATHER STOUT COLD AIR
ADVECTION THROUGH THE DAY WILL PREVENT TEMPERATURES FROM
PLATEAUING HIGHER THAN THE MID 40S TO LOW 50S AREA WIDE. NORMAL
HIGHS FOR MID APRIL ARE RIGHT AT 60. A STIFF NORTHWEST BREEZE
WILL MAKE IT FEEL EVEN LESS PLEASANT, WITH "WIND CHILL" READINGS
IN THE LOWER 40 IN MANY AREAS IN THE AFTERNOON. THIS WILL SET
THE STAGE FOR A COLD NIGHT SATURDAY NIGHT.
COLD ADVECTION GETS REINFORCED AS THE UPPER TROUGH AXIS WILL BE
SHIFTING OVERHEAD. THE CENTER OF THE SURFACE HIGH WILL BE OFF TO OUR
SOUTHWEST SATURDAY NIGHT, THUS WE WILL MAINTAIN A BIT OF A WIND
COMPONENT, HOWEVER THINGS LOOK TO DECOUPLE ENOUGH TO SUGGEST SOME
AREAS OF FROST.
REINFORCED NORTHWEST FLOW ON THE BACKSIDE OF THE UPPER TROUGH WILL
MAINTAIN A SIMILAR AIRMASS ON SUNDAY AND PROVIDE A CARBON COPY DAY
FROM SATURDAY. WITH HIGH PRESSURE NEARBY SUNDAY NIGHT, A WIDESPREAD
FROST APPEARS INCREASINGLY LIKELY, WITH SOME AREA IN OUTLYING AREAS
HEADING FOR A POTENTIAL HARD FREEZE.
UPPER HEIGHTS RECOVER NICELY AND WARM ADVECTION ON THE BACK SIDE FOR
THE DEPARTING SURFACE HIGH WILL ENCOURAGE TEMPERATURES TO REBOUND
BACK ABOVE NORMAL THROUGH MID WEEK WITH GENERALLY DRY
CONDITIONS. THERE WILL BE PERIODS OF ONSHORE FLOW KEEPING COOL
READINGS NEAR THE SHORE.
KMD
AVIATION /18Z TAFS THROUGH 00Z SUNDAY/
ISSUED AT 116 PM CDT FRI APR 17 2026
KEY MESSAGES:
- HIGH CONFIDENCE IN A LINE OF THUNDERSTORMS IMPACTING THE
TERMINALS LATE THIS AFTERNOON AND EVENING. LOCALIZED WIND
GUSTS IN EXCESS OF 50 KTS MAY OCCUR IN SOME PORTIONS OF THE
LINE OF STORMS.
- SOUTHERLY WINDS THIS AFTERNOON WILL GUST TO AROUND 30 KTS AT
TIMES BEFORE A GUSTY WESTERLY TO NORTHWESTERLY WIND SHIFT
OCCURS WITH BOTH THE LINE OF STORMS AND AGAIN WITH A TRAILING
COLD FRONT LATER TONIGHT.
- POST-FRONTAL MVFR CEILINGS ARE EXPECTED TONIGHT.
CONFIDENCE REMAINS HIGH IN ALL TERMINALS BEING AFFECTED BY
THUNDERSTORMS TODAY. UNCERTAINTY REMAINS WITH THE PRECISE TIMING
OF THE STORMS, BUT THIS EVENING STILL APPEARS TO THE MOST LIKELY
TIME FRAME FOR STORMS AT THE CHICAGO METRO TERMINALS, WHILE RFD
COULD SEE STORMS AS EARLY AS THE MID-LATE AFTERNOON. THE STORMS
WILL LIKELY AFFECT THE TERMINALS AS A SOLID LINE, THOUGH SOME
POTENTIAL EXISTS FOR RFD TO BE AFFECTED BY MORE CELLULAR OR
SUPERCELLULAR CONVECTION BEFORE STORMS GROW UPSCALE INTO A LINE
WITH TIME. A GUSTY WESTERLY TO NORTHWESTERLY WIND SHIFT AND
LOCALIZED WIND GUSTS IN EXCESS OF 50 KTS MAY OCCUR IN SOME
PORTIONS OF THE THUNDERSTORM LINE, AND SOME HAIL AND TORNADO
THREAT EXISTS TOO -- PRIMARILY AT RFD. THE STORMS SHOULD LAST
FOR ABOUT 2-3 HOURS AT ANY ONE LOCATION (POSSIBLY LONGER AT RFD
IF MULTIPLE ROUNDS OF STORMS MATERIALIZE THERE) WITH RAIN THEN
CONTINUING FOR A BIT LONGER UNTIL A COLD FRONT PUSHES ACROSS
THE AREA.
PRIOR TO THE ARRIVAL OF THE STORMS, SOUTHERLY WINDS WILL GUST
TO AROUND 30 KTS AT TIMES THIS AFTERNOON. ANOTHER GUSTY
NORTHWESTERLY WIND SHIFT IS EXPECTED BEHIND THE AFOREMENTIONED
COLD FRONT WITH BLUSTERY POST-FRONTAL WEST-NORTHWESTERLY WINDS
THEN CONTINUING THROUGH THE REMAINDER OF THE TAF PERIOD.
LASTLY, MVFR (AND POTENTIALLY IFR) CEILINGS ARE LIKELY TO OCCUR
DURING THE THUNDERSTORMS AND AGAIN BEHIND THE COLD FRONTAL
PASSAGE. A RETURN TO VFR CONDITIONS IS THEN LIKELY TO OCCUR
SOMETIME TOMORROW MORNING.
OGOREK
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH THROUGH LATE TONIGHT FOR ILZ003-ILZ004-ILZ005-
ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-
ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH THROUGH LATE TONIGHT FOR INZ001-INZ002-INZ010-
INZ011-INZ019.
LM...SMALL CRAFT ADVISORY UNTIL 10 PM CDT SATURDAY FOR THE IL AND
IN NEARSHORE WATERS.
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