670
FXUS63 KLOT 171158
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
658 AM CDT SUN AUG 17 2025
KEY MESSAGES
- SHOWERS AND STORMS WILL CONTINUE ACROSS THE AREA THIS MORNING
UNTIL AFTER SUNRISE.
- TODAY SHOULD BE A BREAK IN THE HOT AND STORMY PATTERN.
- HEAT, HUMIDITY, AND STORMS WILL RETURN ON MONDAY AND PERHAPS
INTO EARLY TUESDAY.
- A PATTERN CHANGE TOWARD DRIER, COOLER, AND LESS HUMID
CONDITIONS WILL ARRIVE BY MIDWEEK.
DISCUSSION
ISSUED AT 244 AM CDT SUN AUG 17 2025
THROUGH SATURDAY:
A RECENT HAND SURFACE ANALYSIS REVEALED A CONGLOMERATE OUTFLOW
BOUNDARY/COLD POOL AXIS EXTENDING FROM SOUTHWESTERN MINNESOTA
SOUTHEASTWARD ALONG THE WISCONSIN/IOWA BORDER SOUTHWARD THROUGH
CENTRAL ILLINOIS. ISENTROPIC ASCENT AND THE EASTWARD ADVECTION
OF A PLUME OF STEEP MID-LEVEL LAPSE RATES OF AROUND 8 K/KM
FOSTERED BY A WESTERLY 25-30KT 925-850MB LOW-LEVEL JET ATOP THE
RESIDUAL COLD POOL HAS ALLOWED FOR A NEARLY CONTINUOUS BAND OF
CONVECTION FROM SOUTHERN MINNESOTA TO NORTHWESTERN INDIANA THIS
MORNING. SHOWERS AND STORMS ARE POISED TO CONTINUE UNTIL ABOUT
SUNRISE, AFTER WHICH THE LOW- LEVEL JET SHOULD FINALLY START TO
DECAY.
TAKING A STEP BACK, REGIONAL SURFACE OBSERVATIONS REVEAL A WEAK
BACKDOOR FRONT MOVING SOUTHWARD ACROSS WISCONSIN AND NORTHERN
LOWER MICHIGAN ALONG THE SOUTHERN SIDE OF HIGH PRESSURE SYSTEM
DRIFTING SOUTHEASTWARD ACROSS ONTARIO. MOST MODEL GUIDANCE HAS A
GOOD HANDLE ON THE FRONT PROGRESSING SOUTHWARD THROUGHOUT THE
MORNING, WHICH SHOULD CAUSE A NORTHEASTERLY (ONSHORE) WIND SHIFT
AFTER SUNRISE, A BACKDOOR FRONT WILL MOVE SOUTH-SOUTHWESTWARD
FROM LAKE MICHIGAN CAUSING WINDS TO TURN ONSHORE. AS A RESULT,
HAVE TO THINK IT WILL BE TOUGH FOR THE INSTABILITY AXIS
CURRENTLY POSITIONED TO OUR SOUTH AND WEST TO RETREAT
NORTHEASTWARD. WITH THAT SAID, INCREASING LOW-LEVEL CONFLUENCE
ALONG THE EDGE OF THE INSTABILITY AXIS AHEAD OF THE FRONT MAY
ALLOW FOR A FEW STORMS TO DEVELOP BY MID AFTERNOON NEAR AND WEST
OF A LINE FROM DIXON TO CISSNA PARK. OTHERWISE, TODAY WILL A
BREAK IN THE ACTION WITH HIGHS AROUND 80 LAKESIDE TO THE UPPER
80S TO THE SOUTHWEST.
ON MONDAY, SURFACE FLOW WILL GRADUALLY VEER BACK SOUTH TO
SOUTHWESTERLY AHEAD OF A DECAYING SHORTWAVE APPROACHING THE
LOWER GREAT LAKES. AS A RESULT, THE OCEAN OF HIGH DEW POINT AIR
TO OUR SOUTHWEST WILL SLOSH BACK NORTHEASTWARD. THE SPEED AT
WHICH WINDS TURN SOUTHWESTERLY WILL BE KEY TO HOW FAST THE
MOISTURE AXIS RETREATS NORTHWARD, THOUGH CURRENT INDICATES ARE
THAT HOT AND HUMID CONDITIONS (PEAK HEAT INDEX VALUES NEAR 100)
WILL RETURN ON MONDAY.
ATTENTION THEN TURNS TO A SUBTLE UPPER-LEVEL SHORTWAVE
CURRENTLY LIFTING NORTHEASTWARD INTO CENTRAL WYOMING ON THE
"UPWARD" BRANCH OF THE STAGNANT UPPER-LEVEL RIDGING PARKED OVER
THE CENTRAL US. MOST GUIDANCE SUGGESTS CONVECTION THIS EVENING
WILL ENHANCE THE WAVE IN SOME SHAPE OR FASHION, WITH MORE
AGGRESSIVE GUIDANCE SUCH AS THE HRRR/NAM3 DEPICTING THE
DEVELOPMENT OF A STOUT 30-40KT 700 WIND MAX AS THE WAVE
APPROACHES THE LOWER GREAT LAKES ON MONDAY. DEPENDING ON THE
EXACT EVOLUTION AND TIMING OF THE WAVE, WOULD EXPECT ANOTHER
ROUND OF STORMS IN THE GENERAL AREA MONDAY AFTERNOON AND
EVENING, INCLUDING A THREAT FOR SEVERE WEATHER IF THE MORE
AGGRESSIVE SCENARIOS PAN OUT (E.G., AVAILABILITY OF WIND SHEAR).
WE'LL DIG INTO THAT MORE IN LATER FORECASTS.
SHOWERS AND STORMS MAY LINGER INTO TUESDAY, DEPENDING ON THE
EXACT SPEED OF THE SHORTWAVE ON MONDAY. THEREAFTER, ENSEMBLE
MODEL GUIDANCE IS ADVERTISING A PATTERN CHANGE FROM MIDWEEK
ONWARD TOWARD COOLER, DRIER, AND LESS HUMID CONDITIONS.
BORCHARDT
AVIATION /12Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 658 AM CDT SUN AUG 17 2025
TS/SHRA HAVE ENDED OR WILL END SHORTLY FOR THE CHICAGO
TERMINALS. A SURFACE BOUNDARY OVER THE NORTHERN CHICAGO METRO
WILL CROSS ORD/MDW/DPA BY 13Z WITH A N/NE WIND SHIFT. DEVELOPING
IFR AND LOW-END MVFR STRATUS BEHIND THE BOUNDARY IS EXPECTED TO
AFFECT THE TERMINALS THROUGH THE MORNING HOURS WHILE SLOWLY
LIFTING. THERE MAY BE HOLES IN THE CEILINGS, ESPECIALLY BY LATE
MORNING, BEFORE THE STRATUS LIFTS AND SCATTERS INTO HIGH-END
MVFR CUMULUS DURING THE AFTERNOON. VFR CONDITIONS ARE THEN
EXPECTED TONIGHT INTO MONDAY, THOUGH MVFR BR AND/OR MIFG CANNOT
BE RULED OUT FOR RFD/DPA/GYY LATE TONIGHT THROUGH SUNRISE
MONDAY. OTHERWISE, WINDS WILL SETTLE NE AT AROUND 10 KNOTS BY
THIS AFTERNOON THEN VEER SE UP TO 10 KNOTS OVERNIGHT AND MONDAY
MORNING.
KLUBER
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT FROM 1 PM CDT THIS AFTERNOON THROUGH
MONDAY MORNING FOR ILZ006-ILZ103-ILZ104.
IN...BEACH HAZARDS STATEMENT FROM 1 PM CDT THIS AFTERNOON THROUGH
MONDAY MORNING FOR INZ001-INZ002.
LM...SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 10 AM CDT
MONDAY FOR THE IL NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
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