395  
FXUS63 KLOT 202328  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
628 PM CDT FRI JUN 20 2025  
   
KEY MESSAGES
 
 
- FIRST HEAT WAVE OF THE SUMMER EXPECTED THIS WEEKEND INTO EARLY  
NEXT WEEK WITH MULTIPLE DAYS OF HIGHS WELL INTO THE 90S AND  
PEAK AFTERNOON HEAT INDICES OF AROUND 105F LIKELY.  
 
- EXTREME HEAT WARNING REMAINS IN EFFECT FOR THE CITY OF CHICAGO  
AND COOK CO. WHERE SPECIAL 3-DAY EXTREME HEAT WARNING  
CRITERIA OF PEAK HEAT INDICES OF 100F+ ARE EXPECTED TO BE  
REACHED.  
 
- HEAT ADVISORY REMAINS IN EFFECT FOR THE REST OF THE AREA  
SATURDAY-MONDAY. SOME AREAS MAY NEED TO BE UPGRADED TO AN  
EXCESSIVE HEAT WARNING IF TUESDAY AGAIN HAS ~105F HEAT INDICES.  
 
- SATURDAY-EARLY SATURDAY EVENING WILL BE UNUSUALLY WINDY GIVEN  
THE HEAT, WITH SOUTHWESTERLY GUSTS UP TO 35-45 MPH I-80 AND  
NORTH AND UP TO 30-35 MPH SOUTH OF I-80.  
 

 
   
DISCUSSION
 
 
ISSUED AT 320 PM CDT FRI JUN 20 2025  
 
THROUGH TONIGHT:  
 
SKIES HAVE LARGELY CLEARED OUT ACROSS THE AREA THIS AFTERNOON  
IN THE WAKE OF THE EARLIER RAIN, ALLOWING FOR TEMPERATURES TO  
WARM INTO THE 80S ACROSS THE AREA. THE CORE OF A LOW-LEVEL JET  
AND AN ASSOCIATED PLUME OF RICH LOW-LEVEL THETA-E REMAINS  
POSITIONED JUST TO OUR NORTHWEST AT PRESS TIME AND IS EXPECTED  
TO EXPAND NORTHEASTWARD THIS EVENING. THE NOSE OF THIS LOW-LEVEL  
JET IS WHERE ADDITIONAL SHOWER AND THUNDERSTORM DEVELOPMENT IS  
EXPECTED TO BE FOCUSED LATER TONIGHT, PARTICULARLY AS THE JET  
STARTS TO STRENGTHEN THIS EVENING.  
 
MODEL GUIDANCE IS LARGELY IN AGREEMENT THAT WIDESPREAD  
CONVECTION LIKELY WON'T DEVELOP UNTIL THE MID-LATE EVENING, WHEN  
THE LOW-LEVEL JET'S NOSE IS POINTED INTO CENTRAL/NORTHERN  
WISCONSIN. THE RESULTANT ARC OF CONVECTION WOULD REMAIN  
NORTHEAST OF OUR FORECAST AREA IF THIS WERE INDEED TO BE THE  
CASE, BUT MODELS, IN GENERAL, TEND TO STRUGGLE WITH WHERE AND  
WHEN TO INITIATE CONVECTION IN THESE SUMMERTIME LOW- LEVEL  
JET/ISENTROPIC ASCENT SETUPS ON THE PERIPHERIES OF EML PLUMES.  
HAVEN'T SEEN ANY UPSTREAM AGITATED CUMULUS GROWTH OF NOTE YET  
THAT WOULD BE OF CAUSE FOR CONCERN, BUT OPTED TO ERR ON THE SIDE  
OF CAUTION AND MAINTAIN SOME LOW-END SLIGHT CHANCE POPS FOR OUR  
NORTHEASTERN CWA LATE THIS AFTERNOON/EARLY THIS EVENING IN THE  
ODD EVENT THAT SOMETHING WERE TO GET GOING ALONG THE NOSE OF THE  
LOW-LEVEL JET EARLIER AND/OR FARTHER SOUTH THAN EXPECTED.  
 
OTHERWISE, SOUTHERLY WINDS WILL PERSIST THROUGH THE NIGHT WITH  
OCCASIONAL GUSTS NEAR AND IN EXCESS OF 20 MPH. THESE WINDS WILL  
HELP KEEP TEMPERATURES PROPPED UP OVERNIGHT WHILE ALSO ADVECTING  
HIGHER DEW POINTS INTO THE AREA. LOW TEMPERATURES TONIGHT WILL  
THUS LIKELY REMAIN ABOVE 70F ACROSS OUR FORECAST AREA, SETTING  
THE STAGE FOR THE BEGINNING OF OUR UPCOMING HEAT WAVE.  
 
OGOREK  
 
SATURDAY THROUGH FRIDAY:  
 
OVERVIEW OF THE HEAT WAVE THROUGH TUESDAY:  
 
SATURDAY: BLAST OF HEAT AND OPPRESSIVE HUMIDITY DRIVEN BY STRONG  
SOUTHWEST WINDS GUSTING TO ~40 MPH, STRONGEST NORTH OF I-80. HEAT  
INDICES AROUND 105F.  
 
SUNDAY-MONDAY: BOTH BREEZY BUT NOT AS WINDY, TEMPS A DEGREE OR SO  
WARMER EACH DAY AND DEW POINTS/HUMIDITY NOT QUITE AS OPPRESSIVELY  
HIGH, ESPECIALLY ON MONDAY. TEMPS MAY NEAR 100F IN SPOTS ON MONDAY.  
 
TUESDAY: POTENTIAL FOR ANOTHER DAY OF NEAR TO A BIT ABOVE 105F  
HEAT INDICES BUT HIGHEST CONFIDENCE SOUTH OF I-80 DUE TO HIGHER  
STORM CHANCES AND WINDS POSSIBLY TURNING ONSHORE NEAR THE LAKE.  
 
METEOROLOGICAL SUMMARY:  
 
AS THE POWERHOUSE 500 MB RIDGE AMPLIFIES NORTHWARD FROM THE MID  
SOUTH ON SATURDAY, STRONG FOR JUNE MID 990S SURFACE LOW PRESSURE  
OVER THE NORTHERN PLAINS AND HIGH PRESSURE TO OUR SOUTHEAST WILL  
SET UP A TIGHT PRESSURE GRADIENT (AND STRONG SOUTHWESTERLY WINDS  
ALOFT FOR THIS TIME OF YEAR). CONDITIONS WILL BE AKIN TO A "BLAST  
FURNACE" OR PERHAPS A BLAST OF STEAM IS THE BETTER VERBIAGE. MODEL  
FORECAST BACKWARDS TRAJECTORIES OF THE AIR MASS OVERHEAD  
TOMORROW AFTERNOON SUGGESTS OUR SOURCE REGION WILL BE ARKANSAS  
AND FAR SOUTHERN MISSOURI, WHERE IT'S BEEN EXCEPTIONALLY WET  
RECENTLY, AND DEW POINTS ARE IN THE MID TO LOCALLY UPPER 70S.  
 
ANALOGOUS (BUT IN OPPOSITE DIRECTION) TO A DIRECT ARCTIC  
DISCHARGE IN THE WINTER FROM A PV LOBE, THE UNUSUALLY STRONG  
SOUTHWESTERLY FLOW UP TO 850 MB WILL RESULT IN CLOSE TO DIRECT  
TRANSPORT OF THE AIR MASS WELL TO THE SOUTH TODAY. WITH THIS IN  
MIND, AND AIDED BY MOIST GROUND FROM RECENT RAIN LOCALLY,  
EXPECTING DEW POINTS TO MIX OUT LESS THAN THEY TYPICALLY WOULD  
ON A WINDY DAY. TEMPS MAY NOT QUITE MAX OUT VS. CLIMO OF PROGGED  
925 MB TEMPS (MID-UPPER 20S C) BECAUSE OF THE VERY HIGH DEW  
POINTS, BUT LOW-MID 90S WILL BE ENOUGH FOR HEAT INDICES AROUND  
105F AND PERHAPS LOCALLY NEAR 110F. DEW POINTS WILL PROBABLY MIX  
OUT SLIGHTLY MORE IN AND JUST OUTSIDE CHICAGO AND POINTS NORTH,  
RESULTING IN TEMPS UP IN THE 96-97F RANGE THERE.  
 
THE STRONG SOUTHWEST WINDS ARE A KEY FORECAST MESSAGE GIVEN  
INCREASED CONFIDENCE IN GUSTS INTO THE 35-45 MPH RANGE I-80 AND  
NORTH AND UP TO 30-35 MPH SOUTH OF I-80. LIGHTWEIGHT STRUCTURES AT  
OUTDOOR EVENTS (FOR SHADING OR OTHERWISE) MAY BE COMPROMISED BY  
WINDS THIS STRONG. EXPECT SOUTHWEST WINDS TO ONLY SLOWLY SUBSIDE  
SATURDAY NIGHT, PAIRING WITH THE HIGH (LOW-MID 70S) DEW POINT  
TEMPERATURES FOR AN EXCEPTIONALLY WARM NIGHT, ESPECIALLY IN  
CHICAGO (AROUND 80F). THE VERY WARM AND MUGGY OVERNIGHT  
CONDITIONS, PARTICULARLY IN URBAN AREAS, WILL MAKE IT TOUGHER TO  
COOL OFF AND RECOVER FROM THE DAY'S HEAT, ESPECIALLY FOR  
VULNERABLE INDIVIDUALS AND/OR THOSE WHO SPENT PROLONGED TIME  
OUTDOORS DURING THE MOST OPPRESSIVE CONDITIONS.  
 
SUNDAY AND MONDAY WILL BOTH BE SIMILAR THERMALLY AT THE 925 MB  
LEVEL, SO THE KEY DRIVERS TO TEMPS AND HEAT INDICES WILL BE THE  
EXTENT TO WHICH DEW POINTS MIX OUT EACH AFTERNOON AT PEAK HEATING.  
THE PRESSURE GRADIENT WILL REMAIN TIGHT AS THE DEEP SURFACE LOW  
MENTIONED EARLIER TRACKS NORTH OF THE GREAT LAKES, THOUGH IT WON'T  
BE QUITE AS WINDY, WITH GUSTS INSTEAD PEAKING IN THE 30-35 MPH  
RANGE IN THE AFTERNOON. GIVEN STRONG EVAPORATION FROM THE MOIST  
GROUND EACH DAY AND SOME AIR MASS RECYCLING, SUNDAY APPEARS LIKELY  
TO SEE AFTERNOON DEW POINTS IN THE LOWER TO LOCALLY MID 70S, WHICH  
SHOULD ENABLE TEMPS A DEGREE OR SO WARMER. AS SUCH, PEAK HEAT  
INDICES SHOULD BE VERY SIMILAR TO THOSE OF TOMORROW (SATURDAY).  
 
MONDAY IS THE DAY WHERE GUIDANCE AGREEMENT REMAINS SOLID IN DEW  
POINTS MIXING OUT INTO THE 60S, ESPECIALLY I-80 AND NORTH. WITH  
925 MB TEMPS NEAR +27C, ASSUMING MOISTURE MIX OUT DOES OCCUR, THIS  
COULD SET THE STAGE FOR TEMPS NEARING 100F IN/NEAR CHICAGO AND  
ROCKFORD (SIMILAR SETUP TO JUNE 21, 2022 WHICH HAD A HIGH OF 100F  
AT RFD AND MDW). HEAT INDICES WILL LIKELY BE MORE SO IN THE  
100-105F RANGE, BUT ACCOUNTING FOR CUMULATIVE EFFECTS EASILY  
SUPPORTIVE OF MAINTAINING THE GOING HEAT ADVISORY (AND EXTREME  
HEAT WARNING FOR CHICAGO/COOK COUNTY).  
 
LOOKING AHEAD TO MONDAY NIGHT AND BEYOND, VARIANCE INCREASES  
ENOUGH IN THE GUIDANCE TO LOWER CONFIDENCE. THE BIG QUESTION IS  
THE TIMING OF THE RIDGE FLATTENING OUT ENOUGH FOR CONVECTIVE  
ACTIVITY FAR TO OUR NORTH THROUGH THE WEEKEND TO DRAW CLOSER.  
CONVECTIVE INFLUENCE ON THE SURFACE PATTERN CONCEPTUALLY SHOULD  
RESULT IN A WEAK SURFACE BOUNDARY SAGGING SOUTH AND THE WEAKER  
PRESSURE GRADIENT OPENING UP THE POSSIBILITY FOR LAKE INFLUENCED  
ONSHORE WINDS BY TUESDAY. IN THE MOST RECENT 12Z CYCLE, THE  
ECMWF/EPS SUITE POINTS STRONGLY TOWARD TUESDAY BEING ONE MORE DAY  
OF EXTREME HEAT. EVEN IF CONVECTION AND COOLER TEMPS, ESPECIALLY  
NEAR THE LAKE, DO COME INTO PLAY ON TUESDAY, CAPPING MAY HOLD  
FARTHER SOUTH. THIS IS IMPORTANT BECAUSE THEN THIS WOULD BE A  
FOURTH CONSECUTIVE DAY OF NEAR/AROUND 105F HEAT INDICES, WHICH MAY  
NECESSITATE ISSUANCE OF AN EXTREME HEAT WARNING FOR PARTS OF OUR  
AREA (DUE TO CUMULATIVE EFFECTS OF THE HEAT).  
 
BY TUESDAY NIGHT INTO WEDNESDAY THROUGH FRIDAY, THE RIDGE SHOULD  
HAVE FLATTENED ENOUGH BY THEN TO INCREASE CONFIDENCE IN OCCASIONAL  
ROUNDS OF CONVECTION AND ALSO LAKE INFLUENCE ON TEMPS NEAR THE  
LAKE. THE AIR MASS WILL REMAIN VERY WARM TO HOT AND HUMID ASIDE  
FROM LAKE COOLING AND RAIN BRINGING BRIEF RELIEF FROM THE HEAT. IT  
WILL BE A PSEUDO "RING OF FIRE" TYPE PATTERN, THOUGH AT THIS  
JUNCTURE, MID-LEVEL FLOW MAY BE JUST MARGINAL ENOUGH TO PREVENT  
A HIGHER END SEVERE THREAT, THOUGH NOT ISOLATED STRONG/SEVERE  
STORMS. IN ADDITION, THE HIGH PWATS AND LARGE INSTABILITY  
RESERVOIR SHOULD ALSO BE FAVORABLE FOR POTENTIAL EPISODES OF  
FLASH FLOODING MID TO LATE NEXT WEEK.  
 
CASTRO  
 

 
   
AVIATION /00Z TAFS THROUGH 06Z SATURDAY/
 
 
ISSUED AT 628 PM CDT FRI JUN 20 2025  
 
ONLY FORECAST CONCERN THIS PERIOD ARE THE WINDS.  
 
SOUTHERLY WINDS WILL GUST BETWEEN 15-20KT TONIGHT, THOUGH SOME  
AREAS MAY LOSE GUSTS THIS EVENING. WIND DIRECTIONS MAY ALSO  
SHIFT MORE TO THE SOUTH/SOUTHEAST THIS EVENING THEN TURN BACK TO  
THE SOUTH/SOUTHWEST OVERNIGHT. LOW LEVEL WINDS WILL STEADILY  
INCREASE THIS EVENING WITH LOW LEVEL WIND SHEAR EXPECTED THROUGH  
DAYBREAK SATURDAY MORNING. SURFACE WINDS/GUSTS WILL QUICKLY  
INCREASE THROUGH MID MORNING WITH GUSTS INTO THE 30KT RANGE AND  
THESE GUSTS WILL INCREASE INTO THE MID 30KT RANGE FOR THE  
MID/LATE AFTERNOON ON SATURDAY. GUSTS WILL DIMINISH INTO THE  
LOWER 20KT RANGE WITH SUNSET SATURDAY EVENING WITH LOW LEVEL  
WIND SHEAR AGAIN EXPECTED SATURDAY NIGHT. CMS  
 

 
   
MARINE
 
 
ISSUED AT 403 PM CDT FRI JUN 20 2025  
 
A GALE WARNING HAS BEEN ISSUED FOR THE ILLINOIS NEARSHORE WATERS  
AND THE INDIANA NEARSHORE WATERS FROM CALUMET HARBOR TO BURNS  
HARBOR FROM 9 AM TO 10 PM SATURDAY.  
 
WE'RE EXPECTING STRONG SOUTH-SOUTHWEST WINDS TO DEVELOP BY THE  
LATE MORNING DUE TO A TIGHT PRESSURE GRADIENT ACROSS THE REGION.  
HOT TEMPERATURES AND DEEP MIXING ON LAND BUT THE STABILIZING  
INFLUENCE OF THE STILL COOL LAKE WATERS WILL RESULT IN THE  
STRONGEST WINDS LIKELY BEING CONFINED TO THE FIRST COUPLE MILES  
OR SO OF THE NEARSHORE.  
 
CONFIDENCE IS HIGHEST IN GALE FORCE SPEEDS/FREQUENT GALE FORCE  
GUSTS TO AROUND 35 KT IN THE GALE WARNING AREA, WITH A BIT LOWER  
CONFIDENCE FROM BURNS HARBOR TO MICHIGAN CITY. WE MAY NEED TO  
CONSIDER EXPANDING THE WARNING IF CONFIDENCE IN FREQUENT GALES  
INCREASES FOR THIS NEARSHORE ZONE.  
 
IF HAZARDOUS WINDS FOR SMALL BOATS DON'T PERSIST THROUGH  
SATURDAY NIGHT, THEY'RE EXPECTED TO REDEVELOP ON SUNDAY, WITH A  
SIMILAR PROCESS REPEATED INTO MONDAY.  
 
CASTRO  
 

 
   
CLIMATE
 
 
ISSUED AT 114 PM CDT THU JUN 19 2025  
 
TEMPERATURES THIS WEEKEND INTO EARLY NEXT WEEK COULD POTENTIALLY  
THREATEN RECORD HIGHS AND RECORD WARM LOWS FOR CHICAGO AND ROCKFORD.  
HERE ARE THE CURRENT RECORDS:  
 
CHICAGO HIGH WARM LOW  
SATURDAY 6/21 101 (1988) 74 (1923)  
SUNDAY 6/22 97 (1988) 76 (1923)  
MONDAY 6/23 97 (1930) 79 (1923)  
 
ROCKFORD HIGH WARM LOW  
SATURDAY 6/21 100 (2022) 71 (1995)  
SUNDAY 6/22 97 (1923) 73 (1908)  
MONDAY 6/23 97 (1923) 74 (1908)  
 
NWS CHICAGO  
 

 
   
LOT WATCHES/WARNINGS/ADVISORIES
 
 
IL...HEAT ADVISORY FROM 10 AM SATURDAY TO MIDNIGHT CDT MONDAY NIGHT  
FOR ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-  
ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-  
ILZ039-ILZ106-ILZ107-ILZ108.  
 
EXTREME HEAT WARNING FROM 10 AM SATURDAY TO MIDNIGHT CDT  
MONDAY NIGHT FOR ILZ103-ILZ104-ILZ105.  
 
IN...HEAT ADVISORY FROM 10 AM CDT /11 AM EDT/ SATURDAY TO MIDNIGHT  
CDT /1 AM EDT/ MONDAY NIGHT FOR INZ001-INZ002-INZ010-  
INZ011-INZ019.  
 
LM...GALE WARNING FROM 9 AM TO 10 PM CDT SATURDAY FOR WINTHROP  
HARBOR IL TO BURNS HARBOR IN.  
 
SMALL CRAFT ADVISORY FROM 9 AM TO 10 PM CDT SATURDAY FOR BURNS  
HARBOR TO MICHIGAN CITY IN.  
 

 
 

 
 
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