206
FXUS63 KLOT 310845
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
345 AM CDT THU JUL 31 2025
KEY MESSAGES
- SCATTERED SHOWERS, INCLUDING A FEW WITH ISOLATED HEAVIER
RAINFALL RATES, WILL GRADUALLY END FROM NORTHWEST TO SOUTHEAST
THROUGH MIDDAY.
- HIGH WAVES AND RIP CURRENTS WILL LEAD TO A HIGH SWIM RISKS AT
ALL LAKE MICHIGAN BEACHES AT LEAST INTO FRIDAY.
- COOLER TEMPERATURES AND MORE COMFORTABLE HUMIDITY LEVELS WILL
LAST THROUGH THE WEEKEND.
DISCUSSION
ISSUED AT 345 AM CDT THU JUL 31 2025
THROUGH FRIDAY:
SOME POTENTIAL FOR HIGHLY LOCALIZED RAINFALL RATES IN EXCESS OF
AN INCH PER HOUR WILL LINGER SOUTH OF I-80/EAST OF I-55 EARLY
THIS MORNING, HOWEVER THE THREAT FOR ANY ORGANIZED FLASH
FLOODING CONTINUES TO DECREASE AND WILL ALLOW THE REMAINDER OF
THE FLOOD WATCH TO EXPIRE HERE AT 4 AM CDT. OTHERWISE, FORECAST
FOCUS IN THE NEAR TERM CONTINUES TO BE ON BREEZY NORTHEAST WINDS
AND RESULTING WAVE ACTION PRODUCING HAZARDOUS SWIM CONDITIONS
AT LAKE MICHIGAN BEACHES. HAZY/SMOKY CONDITIONS DUE TO WILDFIRE
SMOKE, AND THE POTENTIAL FOR A FEW LAKE-EFFECT/ENHANCED
SPRINKLES LATER TONIGHT INTO EARLY FRIDAY ROUND OUT THE SHORT
TERM FORECAST.
COLD FRONT/COMBINED OUTFLOW BOUNDARY HAS SHIFTED SOUTH OF THE
FORECAST AREA INTO CENTRAL IL/IN EARLY THIS MORNING. SCATTERED
SHOWERS PERSIST ROUGHLY SOUTH OF A STERLING TO WAUKEGAN LINE
HOWEVER, IN ASSOCIATION WITH THE ELEVATED BAROCLINIC ZONE AND A
SHEARED, POSITIVE-TILT MID-LEVEL SHORT WAVE WHICH IS PROPAGATING
SLOWLY EAST ACROSS THE AREA. A REGION OF FRONTOGENETIC FORCING
AROUND THE 850 MB FRONTAL ZONE CONTINUES TO BE A FOCUS FOR
HEAVIER SHOWERS GENERALLY SOUTH OF I-80/EAST OF I-55, WHERE
PRECIPITABLE WATER VALUES NEAR 2.00" HAD SUPPORTED SPOTTY
RAINFALL RATES IN EXCESS OF 1- 1.50" PER HOUR EARLIER IN THE
NIGHT, THOUGH RADAR/MRMS TRENDS INDICATE RATES HAVE DECREASED
CONSIDERABLY IN THE PAST COUPLE OF HOURS. BASED ON THIS, WILL
ALLOW THE REMAINDER OF THE FLOOD WATCH TO EXPIRE HERE AT 4 AM.
SHOWERS WILL PERSIST THIS MORNING, BUT WILL GRADUALLY END FROM
NORTHWEST TO SOUTHEAST AS THE FRONT CONTINUES TO SAG TO THE
SOUTH, AND THE MID-LEVEL SHORT WAVE AXIS EXITS TO THE EAST.
BREEZY NORTHEAST WINDS WILL CONTINUE TODAY, AS SURFACE HIGH
PRESSURE CONTINUES TO BUILD ACROSS THE UPPER MIDWEST AND
NORTHERN GREAT LAKES REGION. CONTINUE TO NOTE BUOY AND MARINE
PLATFORMS REPORTING GUSTS IN THE 30-35 KT RANGE, PRODUCING WAVES
IN EXCESS OF 5 FEET ON LAKE MICHIGAN. THIS WILL MAINTAIN
HAZARDOUS SWIM CONDITIONS AT LAKE MICHIGAN BEACHES TODAY INTO
FRIDAY MORNING, BEFORE WINDS/WAVES SUBSIDE. THIS COOLER
NORTHEAST FLOW WILL MAKE FOR COOLER AND LESS HUMID CONDITIONS,
WITH HIGHS RANGING FROM THE LOW-MID 70S ALONG THE LAKE SHORE TO
THE MID-UPPER 70S FARTHER INLAND. SIMILAR CONDITIONS ARE
EXPECTED FRIDAY, THOUGH WITH TEMPS JUST A BIT WARMER INLAND.
LATEST GUIDANCE DOES SUGGEST THE POTENTIAL FOR SOME LAKE-EFFECT
(ENHANCED) CLOUD COVER AND PERHAPS A FEW SPRINKLES OR LIGHT
SHOWERS ACROSS NORTHEAST IL FROM LATER THIS EVENING INTO FRIDAY
MORNING, AS ANOTHER MID-LEVEL WAVE DIGS SOUTHEAST ACROSS THE
REGION. FAIRLY COOL TEMPS ALOFT (-11/12C AT 500 MB) AND WARM
LAKE TEMPS MAY PRODUCE JUST ENOUGH LOW-LEVEL INSTABILITY TO
PRODUCE A STRATOCU LAYER DEEP ENOUGH FOR A FEW SPRINKLES, AS
DEPICTED BY SOME RUNS OF THE ARW/RAP/HRRR. NOT EXPECTING MUCH
IMPACT FROM THESE (ESPECIALLY IF MOISTURE ENDS UP SHALLOWER THAN
PROGGED), BUT HAVE INCLUDED A MENTION OF ISOLATED SPRINKLES FOR
NORTHEAST IL DURING THIS PERIOD.
LASTLY, THE NORTH-NORTHEAST FLOW BEHIND THE COLD FRONT HAS
SPREAD CANADIAN WILDFIRE SMOKE BACK INTO THE GREAT LAKES REGION,
AS SEEN IN WIDESPREAD SURFACE VISIBILITIES AS LOW AS 3-5 MILES
ACROSS PARTS OF WI/MI YESTERDAY. HRRR NEAR-SURFACE SMOKE
CONCENTRATION SUGGEST THIS WILL ROTATE SOUTH-SOUTHWESTWARD
ACROSS THE FORECAST AREA TODAY INTO FRIDAY, THUS HAVE MAINTAINED
A PATCHY SMOKE MENTION IN THE FORECAST. ALSO MAINTAINED A
MINIMUM 25 PERCENT SKY COVER TO ACCOUNT FOR HAZY
CONDITIONS.
RATZER
FRIDAY NIGHT THROUGH WEDNESDAY:
THE PREVIOUSLY ADVERTISED BROAD SURFACE HIGH WILL STILL BE
RESIDING OVER THE GREAT LAKES THROUGH THE WEEKEND WHICH WILL
KEEP TRANQUIL WEATHER CONDITIONS AROUND. WHILE WINDS WILL STILL
BE SOMEWHAT ONSHORE, THERE LIGHTER SPEEDS AND MORE SOUTHERLY
COMPONENT SHOULD ALLOW TEMPERATURES TO BE SLIGHTLY WARMER THAN
FRIDAY. THEREFORE, EXPECT HIGH TEMPERATURES TO BE IN THE UPPER
70S TO LOWER 80S WITH OVERNIGHT LOWS IN THE MID-50S TO AROUND
60.
HEADING INTO NEXT WEEK, THE DRY WEATHER DOES LOOK TO LINGER
THROUGH AT LEAST MONDAY BEFORE THE UPPER PATTERN SHIFTS BACK TO
A MORE ACTIVE REGIME TOWARDS THE MIDDLE OF THE WEEK. THIS
PATTERN SHIFT WILL BE CHARACTERIZED BY A BUILDING UPPER RIDGE IN
THE WESTERN CONUS NOISING INTO THE CENTRAL PLAINS ALLOWING
SEVERAL SHORTWAVES TO TRAVERSE THE NORTHERN PERIPHERY AND PIVOT
INTO THE GREAT LAKES RESULTING IN PERIODIC SHOWER AND
THUNDERSTORM CHANCES. THAT SAID, THERE IS STILL A FAIR AMOUNT OF
UNCERTAINTY AS TO EXACTLY WHEN AND WHERE EACH SHORTWAVE WILL
TRACK AND THE COVERAGE OF SHOWERS/STORMS ASSOCIATED WITH EACH
WAVE. AT THIS POINT THE BROAD 20% RAIN CHANCES TUESDAY ONWARD
OFFERED FROM THE NBM SEEMS REASONABLE GIVEN THE UNCERTAINTY, BUT
SUSPECT FURTHER FORECAST REFINEMENT MAY RESULT IN MORE DRY
PERIODS. REGARDLESS OF THE SHOWER/STORM COVERAGE, TEMPERATURES
AND HUMIDITY LEVELS ARE EXPECTED TO RETURN TO MORE TYPICAL
AUGUST READINGS WITH HIGHS IN THE MID-80S AND DEW POINTS IN THE
60S TO AROUND 70 MONDAY ONWARD.
YACK
AVIATION /06Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 100 AM CDT THU JUL 31 2025
FORECAST CONCERNS FOR THE TERMINALS...
- LINGERING SHOWERS OVERNIGHT MAINLY SOUTH OF A VYS TO VPZ LINE.
- MVFR CEILINGS TO PERSIST THROUGH DAYBREAK, MAINLY AT THE
CHICAGO AREA TERMINALS, WITH POCKETS OF IFR POSSIBLE AT TIMES
ESPECIALLY WITH ANY RAIN.
- BREEZY NORTHEAST WINDS TO PERSIST THROUGH THURSDAY AFTERNOON.
A MID-LEVEL CIRCULATION AND ITS ASSOCIATED FRONTAL ZONE
CONTINUE TO DEVELOP SHOWERS ACROSS PORTIONS OF NORTHEAST IL AND
NORTHWEST IN. WHILE THESE SHOWERS ARE EXPECTED TO GRADUALLY
PIVOT SOUTHWARD OVERNIGHT, SOME REDEVELOPMENT AS FAR NORTH AS
I-80 (ROUGHLY A VYS TO VPZ LINE) IS POSSIBLE WHICH COULD PROLONG
SHOWERS AT GYY. GIVEN THAT THIS REDEVELOPMENT IS LOWER
CONFIDENCE HAVE OPTED TO MAINTAIN A DRY FORECAST AT GYY (AND
EVERYWHERE ELSE) AFTER 08Z WHEN THE CURRENT SHOWERS MOVE OUT.
THAT SAID, THE LINGERING HUMIDITY WILL MAKE ANY SHOWERS VERY
EFFICIENT RAIN PRODUCERS WHICH COULD RESULT IN INSTANCES OF
FLOODING IN ADDITION TO REDUCED VISIBILITIES. REGARDLESS,
SHOWERS ARE EXPECTED TO CONCLUDE ACROSS THE AREA BY MID-MORNING
AREAWIDE.
AS SHOWERS TAPER OVERNIGHT, THE MAIN CONCERN WILL TURN TO THE
MVFR CEILINGS WHICH ARE EXPECTED TO PERSIST THROUGH DAYBREAK
THURSDAY MORNING BEFORE SCATTERING OUT. THOUGH, RECENT SATELLITE
TRENDS DO SHOW THAT THE BULK OF THE MVFR MAY REMAIN MOSTLY OVER
THE CHICAGO AREA TERMINALS AND POINTS SOUTH. FURTHERMORE, SOME
IFR CEILINGS ARE STILL BE SEEN IN PORTIONS OF THE AREA BUT THESE
LOOK TO BE MAINLY TIED TO THE AFOREMENTIONED SHOWERS. SO EXPECT
ANY IFR TO REMAIN MORE SPOTTY AND TO ERODE AS SHOWERS TAPER.
OTHERWISE, EXPECT BREEZY NORTHEAST WINDS TO PREVAIL THROUGH
THURSDAY AFTERNOON WITH GUSTS IN THE 20-25 KT RANGE BEFORE
SUBSIDING TO AROUND 10 KTS THURSDAY EVENING AND NIGHT.
FINALLY, THERE IS A LOW CHANCE (<20%) THAT SOME ISOLATED SHOWERS
REDEVELOP THURSDAY NIGHT ALONG A LAND BREEZE BOUNDARY. WHILE THE
IS A FAIR AMOUNT OF LAKE INDUCED INSTABILITY, UNCERTAINTY
REMAINS IF SUFFICIENT SATURATION WILL OCCUR TO ACTUALLY GENERATE
SHOWERS. THAT SAID, IF SHOWERS DEVELOP THEY WOULD MAINLY IMPACT
ORD, MDW, AND GYY AND SHOULD TAPER BY 12Z FRIDAY. GIVEN THE
UNCERTAINTY HAVE DECIDED TO MAINTAIN DRY TAFS FOR NOW BUT DID
INTRODUCE A SCT050 MENTION TO TEASE THIS POTENTIAL.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR ILZ006-
ILZ103-ILZ104.
FLOOD WATCH UNTIL 4 AM CDT EARLY THIS MORNING FOR ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-
ILZ108.
IN...BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR INZ001-
INZ002.
FLOOD WATCH UNTIL 4 AM CDT /5 AM EDT/ EARLY THIS MORNING FOR
INZ001-INZ002-INZ010-INZ011-INZ019.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CDT FRIDAY FOR THE IL AND IN
NEARSHORE WATERS.
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