418  
FXUS63 KIWX 231940  
AFDIWX  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE NORTHERN INDIANA  
340 PM EDT WED JUL 23 2025  
   
KEY MESSAGES
 
 
- VERY HOT AND HUMID AIRMASS OVERSPREADS THE AREA  
TONIGHT/THURSDAY. HIGHS IN THE LOWER 90S WILL LEAD TO PEAK  
AFTERNOON HEAT INDICES OF 100 TO 105 (HIGH CONFIDENCE)  
THURSDAY WITH A POTENTIAL OF SOME AREAS REACHING 105 TO 110  
(LOWER CONFIDENCE).  
 
- CHANCES FOR SHOWERS AND THUNDERSTORMS BEGIN TO INCREASE LATE THURSDAY  
AFTERNOON INTO EVENING ALONG AND NORTH OF US-6, SPREADING  
SOUTH OVERNIGHT. AN ISOLATED DAMAGING WIND THREAT AND LOCALLY  
HEAVY RAINFALL WOULD BE THE POTENTIAL MAIN THREATS.  
 
- SHOWER STORM CHANCES CONTINUE FRIDAY AND SATURDAY WITH A GOOD  
DEAL OF UNCERTAINTY REMAINING REGARDING COVERAGE/TIMING. EARLY  
INDICATIONS SUGGEST BEST CHANCES EARLY FRIDAY, AND THEN AGAIN  
LATE FRIDAY NIGHT INTO SATURDAY. LOW CONFIDENCE IN ADDITIONAL  
STORM CHANCES LATE WEEKEND/EARLY NEXT WEEK.  
 
- HEAT ADVISORY CRITERIA MAY BE REACHED AGAIN IN THE SUNDAY AND MONDAY  
TIMEFRAME, BUT WILL DEPEND ON CLOUD COVER AND ANY SHOWERS OR  
STORMS.  
 

 
   
DISCUSSION
 
 
ISSUED AT 337 PM EDT WED JUL 23 2025  
 
NO CHANGES WILL BE MADE TO HEAT HEADLINES AT THIS TIME, WITH ONGOING  
HEAT ADVISORY WEST OF US ROUTE 31, AND THE REMAINDER OF THE FORECAST  
ARE JOINING THE ADVISORY FOR THURSDAY.  
 
A VERY STRONG LOW LEVEL THETA-E GRADIENT PERSISTS THIS EVENING. THE  
LEADING EDGE OF THIS GRADIENT IS QUITE EVIDENT ON SURFACE OBS AND  
SATELLITE IMAGERY, WITH ENHANCED CU FIELD ROUGHLY JUST WEST OF A  
SOUTH BEND TO ROCHESTER TO KOKOMO LINE MARKING THE LEADING EDGE OF  
MORE SIGNIFICANT LOW LEVEL MOISTURE. A STRONG INSTABILITY GRADIENT  
ACCOMPANIES THIS MOISTURE AXIS WITH PRIMARY INSTABILITY AXIS AT 19Z  
EXTENDING FROM EAST CENTRAL IL TO THE CHICAGO AREA. MAIN AREA OF LOW  
LEVEL CONFLUENCE SHOULD REMAIN ANCHORED ACROSS EXTREME NW IN AND  
INTO NE ILLINOIS THIS EVENING WHERE A FEW SHOWERS/STORMS COULD  
DEVELOP THROUGH EARLY EVENING AT LEADING EDGE OF THIS INSTABILITY  
GRADIENT. AN EARLIER IN THE DAY MCV WAS AIDING IN DEVELOPMENT OF  
ISOLATED CONVECTION ACROSS NORTHERN ILLINOIS, BUT THIS FEATURE HAS  
DEPARTED TO THE NORTHEAST. WILL KEEP SOME SILENT 10 POPS INTO EARLY  
EVENING ACROSS THE EXTREME WESTERN PORTIONS OF THE FORECAST AREA  
WITH EXPECTATION BEST ISOLATED POTENTIAL WILL REMAIN ACROSS  
NE/CENTRAL ILLINOIS.  
 
A COMPLEX MID/UPPER LEVEL FLOW PATTERN ACROSS CENTRAL CONUS WILL  
ALLOW A SERIES OF WAVES TO EJECT EASTWARD ACROSS THE GREAT LAKES  
REGION INTO THURSDAY ASSOCIATED WITH BOTH NORTHERN STREAM, AND  
ADDITIONAL CUT-OFF SHORT WAVES IN SOUTHERN STREAM. APPROACH OF THESE  
SHORT WAVES AND CONTINUED DEPARTURE OF MID ATLANTIC ANTICYCLONE  
WILL ALLOW FOR SLIGHT VEERING/STRENGTHENING OF LOW LEVEL  
SOUTHWEST FLOW ON THURSDAY. THIS WILL BE A FAVORABLE TRAJECTORY  
TO ADVECT THE MUCH HIGHER THETA-E AIR UPSTREAM INTO THE LOCAL  
AREA FOR THURSDAY. DEW POINTS IN THE MID 70S TO AS HIGH AS 80 IN  
A FEW SPOTS STILL APPEAR LIKELY WITH LOW LEVEL THERMAL PROGS  
SUPPORTING HIGHS INTO LOWER 90S ACROSS MUCH OF THE AREA. AS  
PREVIOUS FORECAST MENTIONED, SOME HIGH CLOUD DEBRIS COULD SPREAD  
ACROSS THE AREA FROM UPSTREAM CONVECTION WHICH WOULD HAVE  
IMPACT ON TEMPS/HEAT INDICES. CONFIDENCE REMAINS HIGH IN  
ADVISORY LEVEL HEAT INDICES. SOME ISOLATED WARNING AMOUNTS STILL  
APPEAR TO BE A GOOD POSSIBILITY, ESPECIALLY ACROSS SOUTH  
CENTRAL LOCATIONS WHICH WILL BE IN CORE OF LOW LEVEL MOIST  
TONGUE AND FOR AREAS THAT MIGHT BE MORE IMMUNE FROM ANY HIGH  
CLOUD DEBRIS. CONFIDENCE IS NOT HIGH ENOUGH AT THIS TIME TO GO  
WITH A WARNING, AND WILL CONTINUE WITH HEAT ADVISORY HEADLINES  
AND A MENTION OF ISOLATED HIGHER HEAT INDICES.  
 
THE SERIES OF AFOREMENTIONED WAVES SHOULD DAMPEN RIDGE ENOUGH TO  
ALLOW FOR INCREASED SHOWER/STORM CHANCES ALONG SOUTHWARD SAGGING  
CONVERGENCE ZONE EARLY THURSDAY EVENING. HOWEVER, GOOD DEAL OF  
UNCERTAINTY REMAINS IN TERMS OF STORM CHANCES LATE THU  
AFTERNOON/EVENING DUE TO QUESTIONS WITH FORCING MECHANISMS. THE  
HIGHLY SHEARED NATURE OF THE WAVES WILL RESULT IN LIMITED SOUTHWARD  
PUSH TO FRONTAL ZONE WITH CONVERGENCE NOT EXPECTED TO BE  
PARTICULARLY DEEP IN NATURE. WILL MAINTAIN LIKELY POPS BY THURSDAY  
EVENING GIVEN MODERATE TO STRONG INSTABILITY PROFILES AND AT LEAST  
AXIS OF SHALLOW LOW LEVEL CONVERGENCE SINKING SOUTH. SHEAR  
PROFILES WILL BE LIMITED SO WOULD EXPECT OUTFLOW DOMINANCE TO  
OCCUR FAIRLY QUICKLY WHICH COULD LIMIT ANY ISOLD SEVERE WIND  
THREAT TO JUST A FEW HOURS AFTER INITIATION. SOME BETTER  
COVERAGE SHOWERS/EMBEDDED STORMS COULD EVOLVE AGAIN LATER  
THURSDAY NIGHT INTO FRIDAY AM IF UPSTREAM CONVECTIVELY ENHANCED  
VORT MAX IS ABLE TO MATERIALIZE.  
 
FRIDAY MAY FEATURE A LULL IN THE STRONGER MID/UPPER FORCING BUT WITH  
WASHING OUT LOW LEVEL FRONT IN THE AREA, SCATTERED CONVECTION STILL  
APPEARS A GOOD POSSIBILITY. HEAT SHOULD BE TEMPERED SOMEWHAT BY  
GREATER CLOUD COVER/PRECIP ON FRIDAY WITH HEAT INDICES  
REMAINING BELOW ADVISORY CRITERIA.  
 
LATE FRIDAY NIGHT INTO SATURDAY COULD BE THE PERIOD TO WATCH FOR A  
HIGHER POTENTIAL OF GREATER COVERAGE SHOWERS/STORMS AND HEAVY RAIN  
POTENTIAL. BY SATURDAY, A DISTURBANCE CURRENTLY LIFTING NORTH ACROSS  
S NEVADA IS EXPECTED TO TOP THE RIDGE WITH PERHAPS THE STRONGEST  
LOW/MID LEVEL WESTERLY WIND PROFILES ACROSS THE SOUTHERN GREAT  
LAKES OF THIS FORECAST PERIOD. A RENEWED LOW LEVEL THETA-E  
ADVECTIVE SURGE MAY PRECEDE THIS SHORT WAVE. GUIDANCE DOES  
SUGGEST WEAKENING PROPAGATION VECTORS IN THIS STRENGTHENING  
WEST-SOUTHWEST FLOW REGIME. WARM CLOUD DEPTHS IN EXCESS OF 12K  
FEET AND POCKET OF 2+ INCH PWATS COUPLED WITH THESE WEAK  
PROPAGATION VECTORS COULD ALLOW FOR SOME HEAVY RAINFALL ACROSS  
THE AREA ON SATURDAY. SOME ISOLATED SEVERE POTENTIAL MAY ALSO  
EXIST, PARTICULARLY DOWNSTREAM OF ANY CONVECTIVELY ENHANCED VORT  
WHERE SOME DESTABILIZATION CAN TAKE PLACE. OVERALL CONFIDENCE  
IN SATURDAY SCENARIO REMAINS LOW HOWEVER, AS MUCH OF THIS THREAT  
DEPENDS ON HOW THE UPSTREAM WAVE IS CONVECTIVELY MODIFIED WHICH  
IS DIFFICULT TO DISCERN AT THIS FORECAST DISTANCE.  
 
SOME INDICATIONS THAT MID/UPPER RIDGING TRIES TO REBUILD IN WAKE OF  
SATURDAY WAVE FOR SUNDAY LEADING TO LOWER CONFIDENCE IN ISOLATED  
CONVECTION SUN/MON. BUT ALSO INCREASED POSSIBILITY OF SOME AREAS  
HAVING MORE HEAT ISSUES TO DEAL WITH. MEDIUM RANGE GUIDANCE DOES  
SUGGEST SOME ADDITIONAL CHANCES OF SHOWERS AND STORMS WOULD BE  
POSSIBLE TOWARD END OF PERIOD WITH NW FLOW WAVES POSSIBLE.  
CONFIDENCE IS ALSO INCREASING BASED ON MEDIUM RANGE GUIDANCE  
CONSENSUS IN LESS HUMID CONDITIONS DEVELOPING BY THE MIDDLE OF  
NEXT WEEK.  
 

 
   
AVIATION /18Z TAFS THROUGH 18Z THURSDAY/
 
 
ISSUED AT 117 PM EDT WED JUL 23 2025  
 
VFR CONDITIONS THROUGH AT LEAST THE NEXT 24 HOURS WITH SOUTH TO  
SOUTHWESTERLY WINDS. A STRONG RIDGE OF HIGH PRESSURE WILL KEEP  
SKIES MOSTLY CLEAR, ALTHOUGH CIRRUS MAY START TO INCREASE ON  
THURSDAY MORNING AHEAD OF AN APPROACHING SHORTWAVE. SOUTHWEST  
WINDS WILL ALSO INCREASE ON THURSDAY AND COULD BECOME GUSTY IN  
THE AFTERNOON. THE NEXT CHANCE FOR SHOWERS AND THUNDERSTORMS IS  
AFTER 18Z THURSDAY.  
 

 
   
IWX WATCHES/WARNINGS/ADVISORIES
 
 
IN...HEAT ADVISORY FROM 11 AM TO 8 PM EDT THURSDAY FOR INZ005>009-  
017-018-023>027-032>034-116-216.  
HEAT ADVISORY UNTIL 8 PM EDT /7 PM CDT/ THURSDAY FOR  
INZ012>015-020-022-103-104-203-204.  
OH...HEAT ADVISORY FROM 11 AM TO 8 PM EDT THURSDAY FOR OHZ001-002-  
004-005-015-016-024-025.  
MI...HEAT ADVISORY FROM 11 AM TO 8 PM EDT THURSDAY FOR MIZ079>081.  
HEAT ADVISORY UNTIL 8 PM EDT THURSDAY FOR MIZ078-177-277.  
MARINE...NONE.  
 

 
 

 
 
DISCUSSION...MARSILI  
AVIATION...COBB  
 
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