845
FXUS63 KLOT 030527
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1227 AM CDT FRI JUL 3 2026
KEY MESSAGES
- HEAT AND HUMIDITY WILL CONTINUE ON FRIDAY, THOUGH THE WORST
CONDITIONS (HEAT INDICES UP TO/AROUND 105F) WILL LIKELY FOCUS
NEAR/SOUTH OF THE KANKAKEE RIVER. ON INDEPENDENCE DAY, PEAK
HEAT INDICES OF ~95-100F ARE EXPECTED NEAR AND SOUTH OF I-80.
- WAVES OF THUNDERSTORMS ARE EXPECTED THROUGH THE WEEKEND,
THOUGH THERE WILL BE MANY DRY HOURS. ANY THUNDERSTORMS ON
FRIDAY WILL BE CAPABLE OF PRODUCING DAMAGING WINDS. THERE WILL
ALSO BE A THREAT FOR AT LEAST LOCALIZED FLASH FLOODING WITH
ANY FOCUSED CORRIDORS OF THUNDERSTORMS THROUGH THE WEEKEND.
DISCUSSION
ISSUED AT 315 PM CDT THU JUL 2 2026
THROUGH EARLY THIS EVENING:
THE OUTFLOW BOUNDARY THAT SERVED AS AN IMPETUS FOR ISOLATED
CONVECTION (SHOWERS AND A FEW THUNDERSTORMS) NEAR AND NORTH OF
I-80 HAS MOSTLY WASHED OUT, TAKING AWAY THE ENHANCED LOWER LEVEL
CONVERGENCE. AS SUCH, GIVEN CAPPING, AND MID-LEVEL DRY AIR AND
SUBSIDENCE IN PLACE FOR MOST OF THE AREA, THE THREAT FOR
ADDITIONAL CONVECTION WILL BE MINIMAL THROUGH SUNSET. A MINOR
EXCEPTION WILL BE INTO PORTIONS OF CENTRAL ILLINOIS, AS WIDELY
SCATTERED SHOWERS AND STORMS DEVELOPED NEAR AND SOUTH OF A PIA
TO DNV LINE. RAP ANALYSIS AND NEAR TERM FORECASTS INDICATE A
PRETTY SHARP MID-LEVEL MOISTURE GRADIENT INTO OUR CENTRAL IL
COUNTIES, SO ANY THUNDERSTORMS THAT DO MANAGE TO REACH INTO FAR
SOUTHERN AREAS MAY TEND TO DISSIPATE WITH FARTHER NORTHEASTWARD
EXTENT.
TONIGHT-EARLY FRIDAY:
STRONG TO SEVERE THUNDERSTORMS ONGOING AS OF THIS WRITING ACROSS
THE UPPER MS VALLEY ARE TIED TO A LOW AMPLITUDE SHORT-WAVE
EMANATING FROM THE CENTRAL PLAINS, INTERACTING WITH A MUCH LESS
CAPPED, MORE STRONGLY UNSTABLE ENVIRONMENT IN THAT AREA. WHILE
THE STRONGEST LARGE SCALE FORCING WILL GENERALLY REMAIN NORTH OF
OUR AREA TONIGHT, EXPECTATION IS FOR A COMBINATION OF EROSION
OF CAPPING AND POSSIBLY CONGEALING OUTFLOWS FROM STORMS JUST TO
OUR TO NORTH AND NORTHWEST TO YIELD INCREASING THUNDERSTORM
COVERAGE WITH TIME.
THE EXACT EXTENT OF CONVECTIVE COVERAGE IS STILL A BIT UNCLEAR,
SO OPTED FOR SCATTERED WORDING IN THE GRIDS NEAR/NORTH OF I-80.
THE THREAT FOR STORMS SHOULD START EARLIEST NEAR THE IL/WI
STATE LINE (MAINLY AFTER 8 OR 9PM) AND THEN BUILD SOUTH LATER IN
THE EVENING AND EARLY OVERNIGHT. WITH A LARGE INSTABILITY
RESERVOIR, HIGH PWATS (CONDUCIVE TO PRECIP LOADING AND WET
MICROBURSTS), DCAPE STILL UPWARDS OF 1K J/KG, AND MODEST DEEP
LAYER SHEAR, THE MAIN THUNDERSTORM THREAT WILL BE DAMAGING WIND
GUSTS. IT'S CERTAINLY PLAUSIBLE THAT SOME OF OUR FAR NORTHERN IL
COUNTIES WILL NEED TO BE INCLUDED IN A CONVECTIVE WATCH THIS
EVENING.
IF MORE ORGANIZED CLUSTERS/LINES OF STORMS ARE ABLE TO DEVELOP
AND SEND A COHERENT OUTFLOW BOUNDARY FARTHER SOUTH PRIOR TO
EXHAUST OF AVAILABLE MUCAPE, WE'LL ALSO NEED TO WATCH FOR
CORRIDORS OF TRAINING CONVECTION AND AN ASSOCIATED FLASH
FLOODING THREAT, MOST FAVORED ACROSS FAR NORTHERN ILLINOIS.
SHOWER AND THUNDERSTORM COVERAGE IS THEN EXPECTED TO WANE AFTER
4 AM OR SO AND TRANSLATE TO A PRIMARILY DRY START TO FRIDAY.
HOWEVER, WE DID HOLD ONTO SLIGHT CHANCE POPS FROM 7-10 AM TO
ACCOUNT FOR ROGUE LINGERING CONVECTION.
HEAT AND HUMIDITY ON FRIDAY:
SIMILAR TO TODAY, LINGERING OUTFLOW BOUNDARIES, ANY ASSOCIATED
WIND SHIFTS, AND OUTFLOW MODIFIED PROFILES ARE QUITE PLAUSIBLE.
SUFFICIENTLY HIGH THUNDERSTORM COVERAGE TONIGHT AND SAGGING
OUTFLOWS TONIGHT COULD SOMEWHAT DELAY AIR MASS RECOVERY WITH
NORTHWARD EXTENT (AND VICE VERSA). ALL IN ALL, THE LOWER
THUNDERSTORM COVERAGE EXPECTED TONIGHT SOUTH OF I-80 SHOULD
ENABLE AN EARLIER JUMP ON DIURNAL WARMING, PLUS HIGHER DEW
POINTS (SOLIDLY MID TO LOCALLY UPPER 70S) ARE FORECAST.
THEREFORE, CONFIDENCE IS HIGHEST IN PEAK HEAT INDICES
APPROACHING OR LOCALLY EXCEEDING 105F FOR LOCATIONS NEAR AND
SOUTH OF THE KANKAKEE RIVER. WITH FRIDAY BEING DAY 5 OF THE
CURRENT HEAT WAVE, FELT THE PATH OF LEAST REGRET WAS TO EXTEND
THE EXTREME HEAT WARNING UNTIL 7 PM CDT FRIDAY FOR THE
AFOREMENTIONED LOCALES. DEPENDING ON CONVECTIVE EVOLUTION THIS
EVENING INTO EARLY FRIDAY, WE MAY NEED TO CONSIDER CHANGES TO
HEAT HEADLINES (THE WARNING FOR THE REST OF THE AREA EXPIRES AT
MIDNIGHT CDT TONIGHT).
CASTRO
FRIDAY AFTERNOON-EVENING CONVECTION:
A MORE POTENT WAVE CURRENTLY CROSSING NORTHERN ARIZONA WILL
BECOME ANOTHER FOCUS FOR CONVECTION OVER OUR AREA ON FRIDAY.
THIS WAVE WILL CROSS NORTHERN IL AND SOUTHERN WISCONSIN DURING
THE AFTERNOON, SETTING THE STAGE FOR A POTENTIAL LINE OR
CLUSTERS OF THUNDERSTORMS IN THE AREA FRIDAY AFTERNOON INTO THE
EARLY EVENING TRAVERSING NORTHERN ILLINOIS INTO NORTHWEST
INDIANA. INEVITABLY, EFFECTS OF PRIOR CONVECTION (OR LACK
THEREOF IN SPOTS) CAN BE EXPECTED TO AUGMENT THE LOCATION AND
TIMING OF CONVECTIVE INITIATION, LOWERING CONFIDENCE IN
SPECIFICS. WITH THIS AFTERNOON'S FORECAST ISSUANCE, WE'RE
MESSAGING A FAVORED TIMING OF ABOUT 1 PM TO 7 PM CDT FOR HIGHEST
THUNDERSTORM COVERAGE ON FRIDAY (EARLIEST WEST, LATEST EAST). MODEST
DEEP LAYER SHEAR AND AN INHERENT HIGH DCAPE RESERVOIR WITH THE
WARM/HUMID AIRMASS SUPPORT THE RISK FOR DAMAGING WINDS WITH ANY
LINEAR SEGMENT OR MULTICELL CLUSTERS.
CONCEPTUALLY, SUBSIDENCE IN THE WAKE OF THE SHORT-WAVE AND
ASSOCIATED CONVECTION SHOULD RESULT IN A STEADY DOWNWARD TREND
IN THUNDERSTORM COVERAGE THE REMAINDER OF FRIDAY EVENING, WHICH
*MAY* BODE WELL FOR LATE EVENING COMMUNITY FIREWORKS DISPLAYS.
IT'S CERTAINLY TOO EARLY FOR AN ALL CLEAR AREA-WIDE IN THAT
PERIOD (MID-LATE FRIDAY EVENING), THOUGH THERE HAS BEEN A
SOMEWHAT CONSISTENT SIGNAL (INCLUDING IN PRODUCTS SUCH AS THE
HREF CALIBRATED THUNDER PROBABILITIES) IN LOWER CONVECTIVE
COVERAGE TO BE ABLE TO NOTE THIS IN OUR GRAPHICAL MESSAGING.
CASTRO/KLUBER
FRIDAY NIGHT ONWARD:
A MID-LEVEL IMPULSE EXITING THE NORTHERN ROCKIES LATE ON FRIDAY
WILL PROVIDE RENEWED CONVECTIVE EPISODES IN THE REGION AS THE
MID-LEVEL PATTERN BECOMES ZONAL. THE TRACK OF THIS CONVECTION
WILL LIKELY FOCUS ALONG AN EFFECTIVE COLD FRONT PRODUCED BY THE
COMBINED EFFECTS OF CONVECTION OVER THE NEXT 48 HOURS. SO WHILE
CONVECTION SEEMS LIKELY IN THE REGION ON SATURDAY/INDEPENDENCE
DAY, THE LOCATION OF CONVECTION COULD RANGE FROM SOUTHERN
WISCONSIN TO SOUTHERN ILLINOIS. FURTHERMORE, WITH FORECAST
GUIDANCE CONSISTENTLY DEPICTING A SLOWING OR STALLING MID-LEVEL
WAVE OVER THE WESTERN GREAT LAKES LATE SATURDAY INTO EARLY NEXT
WEEK, TIMING OUT PRECIP CHANCES BECOMES EVEN MORE DIFFICULT IF
NOT IMPOSSIBLE.
AS THE AFOREMENTIONED WAVE EVENTUALLY DRIFTS EASTWARD, DRIER
AND SOMEWHAT COOLER CONDITIONS ARE EXPECTED BY THE MIDDLE OF
NEXT WEEK.
KLUBER
AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1227 AM CDT FRI JUL 3 2026
FORECAST CONCERNS FOR THE TERMINALS...
- LINE OF THUNDERSTORMS TO IMPACT THE CHICAGO AREA TERMINALS
THROUGH THE EARLY OVERNIGHT HOURS. A FEW STORMS MAY PRODUCE
GUSTY WINDS IN EXCESS OF 35 KTS AND HEAVY DOWNPOURS RESULTING
IN IFR VISIBILITY.
- ANOTHER PERIOD OF SHOWERS AND THUNDERSTORMS ARE POSSIBLE
FRIDAY AFTERNOON ESPECIALLY NEAR AND SOUTH OF A VYS TO VPZ
LINE.
A LINE OF THUNDERSTORMS CONTINUES TO TRAIN ACROSS PORTIONS OF
NORTHEAST IL AND IS EXPECTED TO PERSIST THROUGH THE EARLY
OVERNIGHT HOURS. BASED ON THE ORIENTATION OF THE LINE, THE MAIN
TERMINALS TO BE IMPACTED BY THESE STORMS ARE ORD, DPA, AND MDW
BUT CANNOT RULE OUT A FEW STORMS NEARING GYY AFTER 07-08Z. WHILE
MOST OF THESE STORMS HAVE BEEN MAINLY HEAVY RAIN PRODUCERS
(RESULTING IN IFR VISIBILITIES), A FEW HAVE PRODUCED GUSTY WINDS
IN EXCESS 35 KTS. AFTER 08-09Z THE LINE OF STORMS SHOULD BEGIN
TO WEAKEN AS INSTABILITY GETS USED UP, BUT CONFIDENCE IS LOW ON
THE EXACT TIME THESE SHOWERS/STORMS WILL SUBSIDE. ADDITIONALLY,
THERE IS ANOTHER CLUSTER OF SHOWERS AND STORMS IN NORTHERN IA
THAT COULD MOVE INTO NORTHWEST IL TOWARDS THE PREDAWN HOURS.
GIVEN THAT INSTABILITY LOCALLY SHOULD BE USED UP BY THAT TIME
SUSPECT THE IA ACTIVITY SHOULD WEAKEN PRIOR TO REACHING RFD, BUT
HAVE MAINTAINED A VCSH MENTION THERE OVERNIGHT DUE TO LOWER
CONFIDENCE.
OUTSIDE OF THE SHOWER/STORMS TONIGHT, WEST-SOUTHWEST WINDS
AROUND 10-12 KTS CAN BE EXPECTED OVERNIGHT AND THROUGH THE DAY
ON FRIDAY WITH OTHERWISE VFR CONDITIONS. HOWEVER, THERE IS
ANOTHER CHANCE FOR SHOWERS AND THUNDERSTORMS FRIDAY AFTERNOON
AND EVENING. LATEST GUIDANCE TRENDS INDICATE THAT THE FAVORED
ZONE FOR SHOWER/STORMS ON FRIDAY WILL BE NEAR AND SOUTH OF THE
OUTFLOW BOUNDARY FROM TONIGHT'S STORMS. WHILE THE EXACT LOCATION
OF THIS BOUNDARY IS LOW CONFIDENCE, THE GENERAL CONSENSUS IS
THAT THE BOUNDARY SHOULD BE NEAR A VYS TO VPZ LINE. FOR NOW HAVE
OPTED TO MAINTAIN THE PROB30S AT ALL TAF SITES FOR THIS
POTENTIAL BUT SUSPECT THE SOME SITES COULD MISS OUT ON MOST OF
THE THUNDERSTORM ACTIVITY. THAT SAID, WHERE STORMS DO DEVELOP ON
FRIDAY EXPECT A THREAT FOR GUSTY WINDS AND HEAVY DOWNPOURS TO
ACCOMPANY THEM.
FINALLY, THERE IS ALSO A SIGNAL FOR ANOTHER CLUSTER OF SHOWERS
AND STORMS TO MOVE INTO THE AREA LATE FRIDAY NIGHT INTO SATURDAY
MORNING. GIVEN THE UNCERTAINTY IN HOW THE AFTERNOON STORMS WILL
IMPACT THE ENVIRONMENT AND THE LATE ARRIVAL TIME (AFTER THE
CONCLUSION OF THE 30-HOUR TAFS) HAVE LEFT DRY CONDITIONS TO
CONCLUDE THE TAFS FOR NOW.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...EXTREME HEAT WARNING UNTIL 7 PM CDT THIS EVENING FOR ILZ023-
ILZ032-ILZ033-ILZ039.
IN...EXTREME HEAT WARNING UNTIL 7 PM CDT /8 PM EDT/ THIS EVENING
FOR INZ010-INZ011-INZ019.
LM...NONE.
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