112
FXUS63 KIWX 222232
AFDIWX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
632 PM EDT TUE JUL 22 2025
KEY MESSAGES
- HEAT AND HUMIDITY TO BUILD THROUGH THURSDAY. HIGHEST HEAT
INDICES ON WEDNESDAY OF AROUND 100 ARE EXPECTED FOR AREAS
GENERALLY WEST OF US ROUTE 31. HEAT INDICES OF 100 TO 105 ARE
EXPECTED TO BECOME MORE WIDESPREAD ACROSS THE AREA ON
THURSDAY.
- MAINLY DRY CONDITIONS PERSIST INTO THURSDAY AFTERNOON, BUT
PERIODIC CHANCES OF SHOWERS AND STORMS ENTER THE PICTURE
AGAIN THURSDAY NIGHT INTO THE THE EARLY PARTS OF NEXT WEEK.
- MORE CLOUDS AND PRECIP CHANCES LIKELY TO BRING SOME RELIEF
FROM HEAT IMPACTS FRIDAY INTO THE WEEKEND, BUT HEAT MAY
REBUILD IN THE MONDAY/TUESDAY TIMEFRAME.
DISCUSSION
ISSUED AT 320 PM EDT TUE JUL 22 2025
BUILDING HEAT AND HUMIDITY REMAINS THE PRIMARY CONCERN FOR THIS
FORECAST, WITH THURSDAY LOOKING LIKE THE DAY OF POTENTIAL
GREATEST HEAT IMPACTS. AT THIS TIME, WILL MAINTAIN HEAT ADVISORY
HEADLINE FOR WESTERN TIER OF COUNTIES FOR WEDNESDAY. NO
HEADLINES WILL BE ISSUED AT THIS TIME FOR THURSDAY, BUT
HEADLINES WILL LIKELY BE NEEDED.
LARGE SCALE FLOW AMPLIFICATION IS ONGOING THIS AFTERNOON WITH SOME
INCREASE IN AMPLITUDE OF CENTRAL CONUS UPPER LEVEL RIDGING INTO THIS
EVENING. THIS AMPLIFYING PATTERN HAS PROVIDED A TEMPORARY PUSH OF
COOL/DRY AIR ADVECTION ACROSS THE EASTERN GREAT LAKES, WITH THE MOST
NOTICEABLE IMPACT TO LOW LEVEL DEW POINTS. THIS DRIER LOW LEVEL
AIR, LIGHT WINDS, AND MAINLY CLEAR SKIES SHOULD SUPPORT SOME
DECENT RADIATIONAL COOLING CONDITIONS TONIGHT, PARTICULARLY
EAST OF I-69 WHERE DRIEST AIR IS EXPECTED TO RESIDE. THIS COULD
PROMOTE SOME MINS INTO THE UPPER 50S ACROSS THESE LOCATIONS
TONIGHT.
THE BROAD GREAT LAKES ANTICYCLONE SINKS SOUTHEASTWARD TOWARD THE MID
ATLANTIC REGION ON WEDNESDAY WHICH WILL ALLOW LOW LEVEL WINDS TO
VEER MORE SOUTHERLY. THESE TRAJECTORIES WILL BE MORE FAVORABLE
FOR LOW LEVEL MOISTURE TRANSPORT, WITH CURRENT INDICATIONS
STILL SUGGESTING BEST SFC DEW POINT RISES ALONG/WEST OF US 31.
FORECAST MAX TEMPS/MAX HEAT INDICES ON WEDNESDAY STILL SUPPORT
THE PREVIOUSLY ISSUED HEAT ADVISORY HEADLINE ACROSS WESTERN
PORTIONS OF THE AREA (PEAK AFTERNOON HEAT INDICES AROUND 100).
BY LATE WEDNESDAY INTO THURSDAY, A MORE ACTIVE NORTHERN STREAM FLOW
WILL ACT TO DAMPEN THE UPPER RIDGE A BIT, WHICH WILL ALLOW FOR
CONTINUED VEERING OF LOW LEVEL FLOW THAT WILL RESULT IN EASTWARD
ADVECTION OF ANOMALOUS LOW LEVEL MOISTURE ACROSS THE CORN BELT
REGION. THIS WESTERLY FLOW WILL ALSO ACT TO ADVECT WARMER LOW LEVEL
TEMPERATURES EASTWARD FROM THE MID MS VALLEY. CONFIDENCE STILL
REMAINS ON THE HIGHER SIDE IN REACHING AT LEAST HEAT ADVISORY
CRITERIA THURSDAY GIVEN THIS SYNOPTIC SETUP, ESPECIALLY
CONSIDERING BULK OF CONVECTIVE ACTIVITY SHOULD HOLD OFF UNTIL
LATER THURSDAY NIGHT. FORECASTED HIGHS IN THE LOW-MID 90S AND
DEW POINTS IN THE LOW-MID 70S STILL SUPPORT PREVIOUSLY
FORECASTED PEAK HEAT INDICES IN THE 100-105 RANGE WITH SOME
LOCALLY HIGHER VALUES POSSIBLE. CONFIDENCE IN HEAT INDICES
REMAINS HIGHER FOR THE 100 TO 105 DEGREE, BUT DROPS OFF QUITE A
BIT FOR VALUES WARMER THAN 105. FOR THIS REASON, UPON
COLLABORATION WITH SURROUNDING WFOS, WILL HOLD OFF ON HEAT
ADVISORY HEADLINE AT THIS TIME GIVEN THIS IS A 4TH PERIOD
CONCERN.
BY THURSDAY NIGHT, CHANCES OF SHOWERS AND THUNDERSTORMS ARE EXPECTED
TO INCREASE AS SERIES OF NORTHERN STREAM SHORT WAVES TRACK ACROSS
THE NORTHERN GREAT LAKES, AND A CUT-OFF UPPER LOW ACROSS SOUTHWEST
CONUS EJECTS NORTHEAST INTO MID MS VALLEY. STILL SOME UNCERTAINTY
PERSISTS IN HOW FAR SOUTH ASSOCIATED FRONTAL BOUNDARY WILL SAG
THURSDAY NIGHT INTO FRIDAY ALONG WITH UNCERTAINTY IN THE STRENGTH OF
UPSTREAM CONVECTIVELY ENHANCED SHORT WAVE. DEPTH OF FRONTAL
CONVERGENCE MAY BE FAIRLY SHALLOW AS FAR SOUTH AS LOCAL AREA WHICH
COULD LIMIT OVERALL COVERAGE.
IT STILL APPEARS AS THOUGH HEAT CONCERNS SHOULD BE MITIGATED SOME BY
FRIDAY AND SATURDAY AS LOW LEVEL BOUNDARY WASHES OUT ACROSS THE
SOUTHERN GREAT LAKES WITH LIKELIHOOD OF GREATER CLOUD COVER AND
SCATTERED SHOWERS/STORMS. SEVERE THREAT STILL LOOKS ON THE LOW
SIDE AS DEEP LAYER SHEAR SHOULD STILL BE LIMITED. A POSSIBLE
EXCEPTION TO THIS COULD BE ANY LOCALIZED AREAS OF ENHANCED SHEAR
ASSOCIATED WITH MCV-TYPE FEATURES WHICH COULD BE SUPPORTED IN
THIS PATTERN. GIVEN WEAK STEERING FLOW AND HIGH PWAT AIRMASS
STILL IN PLACE, SOME LOCALLY HEAVY RAINFALL IS POSSIBLE.
FORECAST CONFIDENCE WANES HEADING INTO THE SUNDAY THROUGH TUESDAY
PERIOD AS SOME ADDITIONAL TEMPORARY SUPPRESSION OF THE UPPER RIDGE
ACROSS THE GREAT LAKES/OHIO VALLEY IS STILL POSSIBLE DUE TO GREATER
POTENTIAL OF NW FLOW WAVES INTERACTING WITH INSTABILITY GRADIENT
ACROSS THE REGION. IT IS POSSIBLE NEAR HEAT ADVISORY CRITERIA COULD
EMERGE EARLY NEXT WEEK FOR A TIME, BUT A VERY LOW CONFIDENCE AT THIS
FORECAST DISTANCE. BROADBRUSH POPS MAINTAINED THROUGH MOST OF THIS
FORECAST PERIOD WITH DIFFICULTY RESOLVING ANY GREATER 12 HOUR POP
PERIODS DUE TO LOW PREDICTABILITY IN SMALLER SCALE FEATURES.
AVIATION /00Z TAFS THROUGH 00Z THURSDAY/
ISSUED AT 629 PM EDT TUE JUL 22 2025
EXPANSIVE SURFACE HIGH PRESSURE CENTERED OVER IN THE
NORTHEASTERN US AND SOUTHEASTERN CANADA CONTINUES TO KEEP DRY
AND QUIET WEATHER AROUND LOCALLY. EXPECT VFR CEILINGS TO PERSIST
WITH ONLY A FEW HIGH CLOUDS OVERNIGHT. EAST/SOUTHEAST WINDS
WILL DIMINSIH AFTER SUNSET DOWN TO 5 KTS OVERNIGHT INTO TOMORROW
MORNING.
IWX WATCHES/WARNINGS/ADVISORIES
IN...HEAT ADVISORY FROM 2 PM EDT /1 PM CDT/ TO 8 PM EDT /7 PM CDT/
WEDNESDAY FOR INZ012>015-020-022-103-104-203-204.
OH...NONE.
MI...HEAT ADVISORY FROM 2 PM TO 8 PM EDT WEDNESDAY FOR MIZ078-177-
277.
MARINE...NONE.
DISCUSSION...MARSILI
AVIATION...JOHNSON
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