845
FXUS63 KLOT 260721
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
221 AM CDT SAT JUL 26 2025
KEY MESSAGES
- TORRENTIAL RAIN PRODUCING THUNDERSTORMS (WITH RATES GREATER
THAN 2 INCHES PER HOUR) ARE LIKELY AGAIN AT TIMES SATURDAY
- STRONG TO SEVERE THUNDERSTORMS MAY DEVELOP SATURDAY AFTERNOON,
MAINLY NEAR AND SOUTH OF I-80 (SEVERE THREAT LEVEL 1 OF 5).
- DANGEROUS HEAT AND HUMIDITY RETURNS SUNDAY AND ESPECIALLY ON
MONDAY, FOLLOWED BY PERIODS OF THUNDERSTORMS TUESDAY INTO
WEDNESDAY.
- PATTERN CHANGE LATE WEEK TO BRING SEASONAL AND DRY WEATHER.
DISCUSSION
ISSUED AT 221 AM CDT SAT JUL 26 2025
THROUGH SUNDAY:
THE ATMOSPHERE REMAINS PRIMED FOR CONVECTION AND ALL IT TAKES ARE
SUBTLE BOUNDARIES IN CONJUNCTION WITH MID LEVEL VORT
MAXES/MCVS.THE FIRST WAVE IS MARCHING ON THROUGH WELL SOUTH OF
CHICAGO AND EXTENDING INTO NORTHERN INDIANA, WITH A SLIGHTLY MORE
ORGANIZED COMPLEX ACROSS EAST-CENTRAL ILLINOIS. MEANWHILE THE
SLOWER MOVING AREA OF EARLIER HEAVY RAINFALL PRODUCING SHOWERS
AND EMBEDDED THUNDER WILL SLOWLY MEANDER ALONG AND NORTH OF
I-90 AND INTO WISCONSIN. BEYOND THIS FIRST SURGE THERE IS
REALLY NO CLEAR DEFINED BREAK THAT WOULD EASILY POINT OUT AS THE
MAIN TROUGH AXIS AND ADDITIONAL MID LEVEL PERTURBATIONS SHOULD
EASILY BE ABLE TO INTERACT WITH ANY LOW LEVEL BOUNDARIES AND
WRING OUT ADDITIONAL HEAVY RAINFALL.
THE NEXT AREA OF FOCUS WILL THEN BE THE PLETHORA OF WAVES
UPSTREAM ACROSS MISSOURI INTO SOUTH CENTRAL IA. WE ARE STARTING
TO FEEL THE EFFECTS OF THIS WITH SCATTERED SHOWERS AND ISOLATED
THUNDER BEGINNING TO FILL IN FROM LASALLE COUNTY NORTHEAST
TOWARD CHICAGO. IT LOOKS LIKE THE TIMING THE MORE ORGANIZED
FORCING AND ASSOCIATED UPTICK IN CONVECTION WILL BE INTO THE
MORNING ON SATURDAY. STILL, THERE ARE NUMEROUS LITTLE
DISTURBANCES THAT WILL MAINTAIN SHOWERS AND EMBEDDED STORMS
THROUGH DAYBREAK.
FORECAST SOUNDINGS CONTINUE TO FEATURE VERY HIGH FREEZING LEVELS,
AND NEAR RECORD PRECIPITABLE WATER VALUES. WHILE THERE IS SOME
INCREASING LOW-MID LEVEL FLOW TO AID STORM MOTION AND ORGANIZATION,
THE LOW LEVEL JET ORIENTATION STILL CONTINUES TO SUGGEST OVERALL
SLOW STORM PROPAGATIONS.
ADMITTEDLY INSTABILITY IS NOT SUPER HIGH AS YOU GET FARTHER
NORTH/EAST WHICH WILL CONTINUE GIVEN THE AMOUNT OF CLOUD COVER THAT
WILL REMAIN IN PLACE THROUGH THE DAY, THUS THUNDER MAY BE
INITIALLY LIMITED NORTH OF I088. SOME DIURNAL WARNING MAY BE
ALL THAT'S NEEDED TO REINVIGORATE MORE ROBUST SHOWERS AND STORMS
THIS AFTERNOON. THIS WILL ALSO BE ABLE TO PUMP UP INSTABILITY
JUST ENOUGH, ESPECIALLY ALONG AND SOUTH OF I-80 IN CONJUNCTION
WITH ABOUT 30 KT OF DEEP LAYER SHEAR TO SUPPORT A LIMITED RISK
OF GUSTY STORMS. ALL THAT BEING SAID, PERIODIC HEAVY RAINFALL
SHOWERS AND THUNDERSTORMS WILL BE THE RULE THROUGH MUCH OF
TODAY, POSSIBLY EVEN INTO THE EVENING.
KMD
SATURDAY NIGHT THROUGH FRIDAY:
SATURDAY NIGHT THE LOW LEVEL FLOW TURNS MORE WESTERLY AND IT LOOKS
LIKE THE MORE ORGANIZED SHORTWAVES WILL PASS EAST OF THE AREA. THERE
WILL STILL BE SUBTLE WAVES IN WESTERLY FLOW MOVING IN, BUT RISING
HEIGHTS ALOFT SHOULD LIMIT STORM COVERAGE ON SUNDAY, AND THUS
STORMS MAY BE FAVORED CLOSER TO CENTRAL ILLINOIS.
CONCERN WILL THEN SHIFT TO A RETURN TO HEAT AND HUMIDITY. DEWPOINTS
WILL AGAIN BE ON THE RISE ON SUNDAY AND A PORTION OF THE THERMAL
RIDGE ACROSS THE PLAINS WILL LOOK TO SHIFT BACK OVERHEAD EARLY NEXT
WEEK, ESPECIALLY INTO MONDAY AS AN EXPANSIVE HIGH WILL SETUP FROM
THE SOUTHEAST AND INTO THE CENTRAL US. SUNDAY LOOKS VERY WARM AND
HUMID, BUT MONDAY LOOKS OPPRESSIVELY HOT AND HUMID. DEWPOINTS WILL
JUMP BACK INTO UPPER 70S TO NEAR 80, WHICH WOULD PLACE HEAT INDICES
CLOSE TO 110, POSSIBLY AS HIGH AS 115 IN RURAL AREAS OUTSIDE OF
CHICAGO. THE QUESTION MARKS WILL REVOLVE AROUND CONVECTIVE
CHANCES. THE PATTERN IS ALOFT IS THE CLASSIC RING OF FIRE WITH
SYSTEMS MAINLY REMAINING TO THE NORTH, BUT AS THE WEEK GOES ON,
ESPECIALLY INTO TUESDAY, WE WILL SEE IF THE RIDGE HOLDS.
GUIDANCE IS SUGGESTING THAT THE UPPER FLOW MAY TURN
NORTHWESTERLY AND STEER ANY UPSTREAM CONVECTIVE COMPLEXES TOWARD
NORTHERN ILLINOIS, OR ALLOW FOR THE CAP TO BREAK AS EARLY AS
MONDAY. NBM HAS SOME LOW POPS FOR MONDAY AFTERNOON, GIVEN WHAT
LOOKS LIKE EXTREME INSTABILITY, BUT QUESTIONS ON FORCING EXIST.
EXTENDED GUIDANCE DIVERGES AS TO HOW QUICKLY ANY SYSTEMS CAN
BREAK INTO THE RIDGE, AS EARLY AS TUESDAY MORNING IS IN THE
REALM OF POSSIBILITIES. THERE IS BETTER AGREEMENT THAT A
LONGWAVE TROUGH ACROSS CANADA WILL LOOK TO SQUASH THE RIDGE AXIS
SOUTH MID WEEK, OPENING THE DOOR TO HEAVY RAINFALL PRODUCING
STORMS. SEVERE WEATHER WOULD ALSO BE ON THE TABLE WITH AN UPPER
JET NEARBY, SOUTHWARD SURGING COLD FRONT, AND WEST- NORTHWEST
FLOW IN PLACE. A MUCH COOLER AND DRIER AIRMASS TOWARD THE LATTER
HALF OF THE WEEK.
KMD
AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
ISSUED AT 108 AM CDT SAT JUL 26 2025
FORECAST CONCERNS FOR THE TERMINALS...
- WAVES OF SHOWERS AND THUNDERSTORMS TONIGHT THROUGH SATURDAY
EVENING.
- PERIODS OF MVFR CEILINGS WITH SHOWERS AND STORMS, OTHERWISE
VFR CONDITIONS.
THE FORECAST THROUGH SATURDAY EVENING WILL BE QUITE MESSY AS
SEVERAL DISTURBANCES MOVE THROUGH RESULTING IN OFF AND ON
PERIODS OF SHOWERS AND THUNDERSTORMS. THE FIRST DISTURBANCE WILL
CONTINUE TO MOVE OVERHEAD THROUGH 07-08Z WHICH WILL MAINTAIN
SCATTERED SHOWERS OVER THE TAF SITES WITH MORE ORGANIZED SHOWERS
AND STORMS ALONG AND SOUTH OF A PNT TO VPZ LINE. AS THIS FIRST
WAVE EXITS A BRIEF LULL SHOULD MATERIALIZE AS SOME SINKING AIR
BEHIND THE SYSTEM LIMITS THE FORCING FOR REDEVELOPMENT, THOUGH
SOME ISOLATED SHOWERS MAY FESTER THROUGH THE NIGHT DUE TO THE
HUMID AIR MASS.
THE SECOND DISTURBANCE IS EXPECTED TO PIVOT INTO THE AREA
TOWARDS DAYBREAK BRINING WITH IT ANOTHER ROUND OF SHOWERS AND
THUNDERSTORMS. THIS DISTURBANCE IS CURRENTLY (AS OF 557Z)
TRACKING ACROSS NORTHERN MO WITH JUST SOME ISOLATED SHOWERS
ONGOING, BUT AS WE NEAR DAYBREAK THE INCREASING INSTABILITY
SHOULD ALLOW FOR THE COVERAGE OF SHOWERS AND STORMS TO INCREASE
ESPECIALLY AT THE CHICAGO AREA TERMINALS AND POINTS SOUTH. ONCE
THE SHOWERS/STORMS ARRIVE THEY SHOULD PERSIST THROUGH MID-
MORNING BEFORE EXITING AND LEAVING US WITH ANOTHER PERIOD OF OFF
AND ON PASSING SHOWERS THROUGH THE MORNING.
THE THIRD AND FINAL DISTURBANCE (CURRENTLY OVER EASTERN KS) IS
EXPECTED TO PIVOT INTO THE AREA THIS AFTERNOON WHICH WILL ONCE
AGAIN LEAD TO THE DEVELOPMENT OF SHOWERS AND THUNDERSTORMS
MAINLY ALONG AND SOUTH OF A VYS TO VPZ LINE. HOWEVER, SOME
ISOLATED TO WIDELY SCATTERED SHOWERS AND STORMS MAY STILL PASS
OVER THE CHICAGO AREA TERMINALS SATURDAY AFTERNOON (MAINLY AFTER
19Z). STORMS THAT DEVELOP WITH THIS THIRD WAVE WILL HAVE THE
POTENTIAL TO BECOME STRONG TO SEVERE WITH A THREAT FOR DAMAGING
WINDS (IN EXCESS OF 40KTS) AND HEAVY RAINFALL WHICH MAY RESULT
IN LOCALIZED FLOODING. SHOWERS AND STORMS WILL FINALLY BEGIN TO
EXIT THE AREA AFTER 00Z SATURDAY EVENING WITH DRY WEATHER
EXPECTED TO CLOSE OUT THE TAF PERIOD.
OUTSIDE OF THE RAIN, VFR CONDITIONS CAN GENERALLY BE EXPECTED
THOUGH PERIODS OF MVFR CEILINGS WILL BE POSSIBLE WITH ANY
SHOWERS AND/OR STORMS. LIGHT SOUTHEAST WINDS TONIGHT WILL BECOME
SOUTHWESTERLY ON SATURDAY AND INCREASE INTO THE 8-10 KT RANGE
THROUGH THE AFTERNOON. WHILE SPEEDS WILL EASE SATURDAY EVENING,
DIRECTIONS WILL REMAIN SOUTH-SOUTHWEST THROUGH THE END OF THE
PERIOD.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...FLOOD WATCH THROUGH THIS EVENING FOR ILZ003-ILZ004-ILZ005-
ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...FLOOD WATCH THROUGH THIS EVENING FOR INZ001.
LM...NONE.
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