598
FXUS63 KLOT 311732
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1232 PM 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 /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1232 PM CDT THU JUL 31 2025
WILDFIRE SMOKE REACHING THE SURFACE WILL CONTINUE LIMITING
VISIBILITY TO THE 2 TO 4 MILE RANGE THROUGH THE AFTERNOON AND
EVENING. PER EXPERIMENTAL HRRR SMOKE MODEL OUTPUT AND UPSTREAM
OBSERVATIONS ACROSS MICHIGAN, DO EXPECT GRADUAL IMPROVEMENT IN
VISIBILITIES AFTER 00Z. OUTSIDE OF SMOKE, BREEZY NORTHEAST
WINDS WILL CONTINUE THROUGH THE EVENING.
TONIGHT, AN INVERTED SURFACE TROUGH WILL SWING OVER LAKE
MICHIGAN CAUSING WINDS TO TURN EASTERLY. LAKE INSTABILITY
(WATER/850MB TEMPERATURE DIFFERENTIALS PUSHING 12-14C) WILL
ENCOURAGE THE DEVELOPMENT OF A STRATOCUMULUS DECK OVERNIGHT,
FAVORING THE 03 TO 12Z WINDOW OR SO AT ORD/MDW/DPA/GYY. IN
SPITE OF MARGINAL MOISTURE IN THE LAKE CONVECTIVE LAYER,
SHORELINE CONVERGENCE SHOULD BE ENOUGH TO SUPPORT AT LEAST
ISOLATED SHOWERS DURING THE OVERNIGHT HOURS. (WILL NOTE A FEW
SHOWERS NEAR SAGINAW BAY IN MICHIGAN IN THE UPSTREAM AIRMASS AS
A SANITY CHECK). CHANCES FOR SHOWERS IMPACTING ANY TERMINAL
SEEMS LOWER THAN 30%, SO WITHHOLD FORMAL MENTION FOR NOW. WITH
THAT SAID, CAN EASILY ENVISION A NEED FOR AT LEAST VCSH TO BE
INTRODUCED AT ORD/DPA/MDW AT SOME POINT FOR THE OVERNIGHT HOURS.
FORECAST SOUNDINGS DEPICT AN EROSION OF MOISTURE IN THE LAKE
CONVECTIVE LAYER AFTER DAYBREAK, SUGGESTING THAT LAKE
STRATOCUMULUS CLOUDS WILL SIMILARLY ERODE BY MID-MORNING. FOR
NOW, WILL MAINTAIN SCT025 IN FAVOR OF WATCHING TRENDS OVERNIGHT
(AS WATER/850MB TEMPERATURE DIFFERENTIALS WILL REMAIN FAVORABLE
FOR AT LEAST A FEW CLOUDS). WINDS TOMORROW WILL TURN
NORTHEASTERLY AND REMAIN SO THROUGH THE REMAINDER OF THE TAF.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...AIR QUALITY ALERT UNTIL MIDNIGHT CDT TONIGHT FOR ILZ003-ILZ004-
ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR ILZ006-
ILZ103-ILZ104.
IN...AIR QUALITY ALERT UNTIL MIDNIGHT CDT TONIGHT FOR INZ001-INZ002-
INZ010-INZ011.
BEACH HAZARDS STATEMENT THROUGH FRIDAY AFTERNOON FOR INZ001-
INZ002.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CDT FRIDAY FOR THE IL AND IN
NEARSHORE WATERS.
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