948
FXUS63 KLOT 011838
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
138 PM CDT WED JUL 1 2026
KEY MESSAGES
- HOT AND HUMID CONDITIONS WILL CONTINUE THROUGH THE HOLIDAY
WEEKEND WITH DAILY PEAK HEAT INDEX VALUES NEAR 100 DEGREES.
- WAVES OF THUNDERSTORMS ARE EXPECTED THURSDAY EVENING THROUGH
THE WEEKEND. WITH THAT SAID, THERE WILL BE MANY DRY HOURS.
DISCUSSION
ISSUED AT 138 PM CDT WED JUL 1 2026
TONIGHT AND THURSDAY:
REGIONAL GOES-19 WATER VAPOR IMAGERY AUGMENTED BY RAP ANALYSIS
DATA IMAGERY DEPICTS A QUINTESSENTIAL SUMMERTIME PATTERN
CHARACTERIZED BY A LARGE UPPER-LEVEL HIGH PRESSURE SYSTEM
(AROUND 596DM) CENTERED OVER THE SOUTHEASTERN UNITED STATES.
SUCH A "HEAT DOME" CONTINUES TO PROVIDE HOT AND HUMID CONDITIONS
IN THE GENERAL REGION WITH HEAT INDICES AROUND 100 DEGREES IN
THE LOCAL AREA AT PRESS TIME. MEANWHILE, A RECENT HAND SURFACE
ANALYSIS REVEALED AN OUTFLOW-REINFORCED FRONTAL BOUNDARY
STRETCHING FROM CENTRAL LOWER MICHIGAN THROUGH CENTRAL WISCONSIN
AND THROUGH NORTHWESTERN IOWA. THE FRONTAL BOUNDARY CONTINUES
TO SERVE AS A HIGHWAY FOR EPISODIC CLUSTERS OF SHOWERS AND
STORMS IN THE CENTRAL AND UPPER GREAT LAKES REGION, INCLUDING A
SUPERCELL THAT RECENTLY PRODUCED A 3 INCH HAILSTONE NEAR FOND DU
LAC, WISCONSIN. THIS IS ALL TO SAY, THE WEATHER OUTSIDE IS
DEFINITELY LIVING UP TO THE STANDARDS OF JULY.
HAVE BEEN WATCHING AN AREA OF CLUMPING CUMULUS CLOUDS ACROSS
FAR NORTHEASTERN IOWA EARLY THIS AFTERNOON FOR SIGNS OF
INITIATION INTO THUNDERSTORMS AS THEY APPROACH THE
ILLINOIS/WISCONSIN STATE LINE. WITH AN ABSENCE OF LARGE-SCALE
FORCING AS WELL AS AROUND 100 J/KG OF MLCIN NOTED ON THE 18Z DVN
RAOB, DO NOT CURRENTLY EXPECT THUNDERSTORMS TO EMERGE OUT OF
THE CLUMPING CUMULUS CLOUDS. HOWEVER, IF THE OFF CHANCE THEY DO
TAKE OFF, THEY WOULD BE PRONE TO IMPACTING PORTIONS OF NORTHERN
WINNEBAGO, BOONE, AND PERHAPS MCHENRY COUNTIES LATER THIS
AFTERNOON/EVENING WITH A LOCALIZED THREAT FOR DAMAGING WINDS,
DAMAGING HAIL, AND FLASH FLOODING.
WITH THAT SAID, ANOTHER ROUND OF THUNDERSTORMS IS EXPECTED
ALONG THE STALLED FRONTAL ZONE ACROSS NORTHERN IOWA AND SOUTHERN
MINNESOTA ALONG THE NOSE OF A MODEST SOUTHWESTERLY 850MB LOW-
LEVEL JET. A SOMEWHAT UNEVEN OVERLAP OF MOISTURE-LADEN
INSTABILITY BUT MARGINAL DEEP-LAYER SHEAR WILL ENCOURAGE
THUNDERSTORMS TO EVOLVE INTO LOOSELY ORGANIZED OUTFLOW-DRIVEN
CLUSTERS THAT TEND TO TRACK INTO CENTRAL WISCONSIN THROUGH THE
NIGHT. OUR AREA WILL HENCE BE DRY, WARM, AND MUGGY TONIGHT WITH
OVERNIGHT LOWS GENERALLY IN THE MID TO LOCALLY UPPER 70S.
IF CONVECTIVE COVERAGE IS RELATIVELY HIGH THIS EVENING AND
OVERNIGHT ACROSS WISCONSIN, AMALGAMATION OF REPEATED COLD POOLS
SHOULD LEAD TO A SOUTHWARD-DRIFTING REGION OF OUTFLOW THAT WOULD
BE PRONE TO SPILLING INTO FAR NORTHERN ILLINOIS OR PERHAPS
SOUTHWESTWARD OFF LAKE MICHIGAN TOWARD DAYBREAK. SUCH AN
EVOLUTION IS DEPICTED BY BOTH THE NAM3/HRDPS, WHICH TYPICALLY
PERFORM WELL IN THESE SORTS OF REGIMES. EVEN IF OUTFLOW DOES
REACH OUR AREA TOMORROW MORNING, CURRENT THINKING IS THAT IT
WOULD HAVE MINIMAL IMPACT ON HOT AND HUMID CONDITIONS WITH HIGHS
EXPECTED TO SOAR BACK INTO THE UPPER 80S TO LOWER 90S WITH PEAK
HEAT INDICES OF 100 TO 105F. MOST DAYLIGHT HOURS LOOK DRY.
THURSDAY EVENING INTO EARLY FRIDAY MORNING:
BY TOMORROW EVENING, THE UPPER-LEVEL HEAT DOME WILL BEGIN TO
"FLATTEN" AS THE FRONTAL BOUNDARY STALLS FROM CENTRAL IOWA
TOWARD THE ILLINOIS AND WISCONSIN BORDER. CAM-BASED GUIDANCE IS
FAIRLY CONSISTENT IN SHOWING AN UPPER-LEVEL SHORTWAVE RIDING THE
TOP OF THE HEAT DOME INTO THE LOWER GREAT LAKES DURING THE
EVENING HOURS LEADING TO THE DEVELOPMENT OF STORMS ALONG THE
FRONTAL BOUNDARY. WITH FORECAST SOUNDINGS DEPICTING MINIMAL
CAPPING BY EARLY AFTERNOON, SUPPOSE A THUNDERSTORM OR TWO MAY
DEVELOP AS EARLY AS 4 PM. HOWEVER, COVERAGE SHOULD INCREASE
MARKEDLY WITH THE SHORTWAVE ITSELF FAVORING AFTER 7 OR 8 PM.
WITH THE TRAJECTORY OF THE SHORTWAVE FAVORING SOUTHERN
WISCONSIN, STORMS SHOULD REMAIN LARGELY NORTH OF INTERSTATE 88.
HOWEVER, IF THERE IS A HIGH COVERAGE OF STORMS, CAN CERTAINLY
ENVISION THE SOUTHWARD SPILL OF OUTFLOW-DRIVEN STORMS TOWARD OR
EVEN FURTHER SOUTH OF I-80 BY DAYBREAK FRIDAY. ANY STORM MAY
PRODUCE LOCALLY DAMAGING WINDS OR FLASH FLOODING TOMORROW
EVENING AND NIGHT, AGAIN FAVORING AREAS NORTH OF I-88.
FRIDAY THROUGH THE WEEKEND:
ENSEMBLE MODEL GUIDANCE REMAINS IN AGREEMENT THAT THE UPPER-
LEVEL HEAT DOME WILL BREAK DOWN FRIDAY AND TRANSITION TOWARD
SOMEWHAT WEAK BUT QUASI-ZONAL UPPER-LEVEL FLOW THROUGH THE
WEEKEND. WITH 850MB TEMPERATURES REMAINING SOMEWHAT STAGNANT AND
NEAR +20C, WILL GO WITH A PERSISTENCE FORECAST FOR BOTH HEAT
AND HUMIDITY LEVELS WITH DAILY PEAK HEAT INDEX VALUES NEAR 100
DEGREES. SUBTLE UPPER-LEVEL SHORTWAVES EMBEDDED WITHIN THE
QUASI-ZONAL FLOW (IF NOT JUST AFTERNOON HEATING OF THE BOUNDARY
LAYER) SHOULD SUPPORT EPISODES OF THUNDERSTORMS IN THE AREA. IN
THIS KIND OF PATTERN, ANY STORM CAN BE PRONE TO PRODUCING
DAMAGING WINDS AND FLASH FLOODING, THOUGH MORESO ON A LOCALIZED
BASIS THAN WIDESPREAD. WITH ALL OF THAT SAID, MANY HOURS SHOULD
BE DRY.
WE ARE VERY COGNIZANT OF THE LARGE NUMBER OF OUTDOOR EVENTS AND
ACTIVITIES SCHEDULED FOR THE UPCOMING HOLIDAY WEEKEND WITH
HEIGHTENED ATTENTION ON THE FORECAST FOR THE TIMING AND COVERAGE
OF THUNDERSTORMS. THIS TYPE OF PATTERN IS ONE WHERE CONFIDENCE
IN TIMING AND LOCATION OF STORMS COMES MUCH MORE INTO FOCUS
ABOUT 12 TO 24 HOURS BEFOREHAND. SO, STAY TUNED TO THE FORECAST.
A NOTE ABOUT HEAT HEADLINES:
YESTERDAY WAS DEFINITELY THE MOST OPPRESSIVE DAY OF THE ONGOING
HEAT WAVE. WHILE FORECASTED HEAT INDEX AND HEATRISK VALUES THIS
AFTERNOON THROUGH THE UPCOMING HOLIDAY WEEKEND OUTSIDE OF COOK
COUNTY ARE FORECASTED TO FALL WELL SHORT OF LOCAL EXTREME HEAT
WARNING CRITERIA (FOUR OR MORE CONSECUTIVE DAYS WITH PEAK HEAT
INDEX >105F AND/OR WIDESPREAD HEATRISK LEVEL OF 4 OUT OF 4),
WILL MAINTAIN THE PRODUCT ACROSS THE ENTIRE AREA FOR THE SAKE OF
A CONSISTENT MESSAGE WITH NEIGHBORING FORECAST OFFICES.
IN TERMS OF ANY EXTENSION IN HEAT HEADLINES BEYOND THURSDAY, AM
NOT SURE WE WILL MEET LOCAL CRITERIA EVEN FOR A HEAT ADVISORY
GIVEN FORECAST HEAT INDICES ARE EXPECTED TO REMAIN BELOW 105F.
WITH THAT SAID, THERE IS SOMETHING TO BE SAID ABOUT THE
CUMULATIVE EFFECT OF THE HEAT (AS FRIDAY WOULD BE DAY 5 OF HEAT
INDEX VALUES NEAR OR ABOVE 100F), SO FUTURE SHIFTS MAY NEED TO
CONSIDER A HEAT ADVISORY ANYWAY FOR FRIDAY OR SATURDAY
REGARDLESS BARRING, OF COURSE, THE COOLING EFFECTS OF
THUNDERSTORMS.
BORCHARDT
AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 1232 PM CDT WED JUL 1 2026
MAIN CONCERN:
- POTENTIAL FOR WEAKENING SHRA/TS NEARBY TO DRIVE AN OUTFLOW
ASSOCIATED WIND SHIFT ACROSS THE AREA EARLY THURSDAY MORNING
VFR CONDITIONS ARE EXPECTED THROUGH THE PERIOD, WITH THE SMALL
POSSIBLE EXCEPTION OF ANY PRECIP RELATED VSBY EFFECTS.
SOUTHWESTERLY WIND GUSTS ARE EXPECTED TO INCREASE INTO THE
25-30 KT RANGE (AT LEAST OCCASIONALLY) LATE THIS AFTERNOON INTO
EARLY THIS EVENING, THEN LIKELY EASE WITH OR JUST AFTER SUNSET.
THERE MAY BE SPORADIC GUSTS NEAR 20 KT, ESPECIALLY AT ORD AND
MDW.
THE AREA TO WATCH LATE TONIGHT IS FROM FAR NE IOWA ACROSS TO
CENTRAL AND SOUTHERN WISCONSIN. EXACT COVERAGE OF TS IN THAT
AREA WILL LIKELY CONTRIBUTE TO POTENTIAL FOR AN OUTFLOW BOUNDARY
TO PUSH ACROSS THE AREA AND RESULT IN A TEMPORARY WIND SHIFT (NW
TO NE TO SE BEFORE RETURNING TO SW). ALSO CAN'T RULE OUT SHRA
AND A FEW ISOLATED TS MAKING IT INTO NORTHERN IL (~20% CHANCE).
DUE TO UNCERTAINTY, CONFIDENCE WAS NOT HIGH ENOUGH FOR ANY
FORMAL MENTION OF A WIND SHIFT AND PRECIP IN THE TAFS. THE NEXT
DECENT CHANCE FOR TS AT THE TAF SITES IS LOOKING TO BE THURSDAY
EVENING, BEYOND THE CURRENT ORD/MDW 30-HOUR TAF PERIOD.
CASTRO
CLIMATE
ISSUED AT 1246 AM CDT WED JUL 1 2026
.DAILY RECORDS FOR THE UPCOMING WEEK........
CHICAGO (KORD)
RECORD HIGH RECORD WARM MINIMUM
JULY 1 103 (1956) 78 (1931)
JULY 2 99 (1970) 79 (1911)
JULY 3 102 (1949) 78 (1911)
JULY 4 102 (2012) 80 (1921)
ROCKFORD (KRFD)
RECORD HIGH RECORD WARM MINIMUM
JULY 1 101 (1931) 76 (1970)
JULY 2 99 (1910) 74 (1970)
JULY 3 99 (1925) 75 (2012)
JULY 4 102 (2012) 77 (1977)
NWS CHICAGO
LOT WATCHES/WARNINGS/ADVISORIES
IL...EXTREME HEAT WARNING UNTIL MIDNIGHT CDT THURSDAY NIGHT FOR
ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-
ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-
ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...EXTREME HEAT WARNING UNTIL MIDNIGHT CDT /1 AM EDT/ THURSDAY
NIGHT FOR INZ001-INZ002-INZ010-INZ011-INZ019.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
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