172
FXUS63 KLOT 130829
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
329 AM CDT THU AUG 13 2026
KEY MESSAGES
- AREAS OF DENSE FOG EARLY THIS MORNING. MAINLY WEST AND SOUTH
OF THE CORE OF THE CHICAGO METRO.
- ANOTHER CLUSTER OF THUNDERSTORMS MAY BRING A SEVERE WEATHER
THREAT TO OUR SOUTHERN CWA THIS AFTERNOON.
- STORMY PATTERN PERSISTS INTO THE WEEKEND WITH A THREAT OF
REPEATED ROUNDS OF HEAVY RAINFALL INCREASING THE RISK OF
FLOODING IN SOME AREAS.
DISCUSSION
ISSUED AT 329 AM CDT THU AUG 13 2026
PRIMARY FORECAST CONCERNS ARE DENSE FOG EARLY THIS MORNING,
ANOTHER MCS AND SEVERE WEATHER THREAT FOR OUR FAR SOUTHERN CWA
THIS AFTERNOON, AND THE POTENTIAL FOR A SERIES OF HEAVY RAIN
PRODUCING DISTURBANCES TO INCREASE FLOODING POTENTIAL ACROSS
PARTS OF THE AREA LATER TONIGHT INTO THE WEEKEND.
OUTFLOW FROM WEDNESDSAY'S MCS HAS PUSHED THE EFFECTIVE FRONTAL
BOUNDARY WELL SOUTH OF THE AREA THIS MORNING. WITH MODEST MID-
LEVEL HEIGHT RISES/SUBSIDENCE IN THE WAKE OF YESTERDAY'S
DISTURBANCE, RELATIVELY CLEAR SKIES ATOP A RESIDUAL MOIST
BOUNDARY LAYER HAS LED TO FOG DEVELOPMENT HOWEVER, WITH CALM
WINDS AND MOSTLY 0F TEMP-DEW POINT SPREADS NOTED. COMBINATION OF
CURRENT GOES NIGHTTIME MICROPHYSICS IMAGERY AND SURFACE
OBSERVATIONS DEPICT FOG QUICKLY FILLING IN SOUTH AND WEST OF THE
URBAN CORE OF THE CHICAGO METRO, AND EXPECT THIS TO CONTINUE
WITH SEVERAL MORE HOURS UNTIL SUNRISE. THUS HAVE ISSUED A DENSE
FOG ADVISORY FOR MUCH OF THE AREA THROUGH 9 AM (EXCEPT
MCHENRY/LAKE AND CENTRAL/NORTHERN COOK COUNTIES FOR NOW) WHERE
CURRENT OBS AND MODEL TRENDS SUGGEST WIDESPREAD DENSE FOG IS
LIKELY. VISIBILITIES WILL IMPROVE AS FOG GRADUALLY BURNS OFF A
FEW HOURS AFTER SUNRISE THIS MORNING.
ATTENTION THEN TURNS TO YET ANOTHER CONVECTIVE COMPLEX
CURRENTLY TRACKING ACROSS EASTERN NE/SD, ASSOCIATED WITH THE
NEXT MID-LEVEL (CONVECTIVELY ENHANCED BY THE PRESENCE OF AN MCV)
SHORT WAVE WHICH IS PROGGED TO TRANSIT THE FORECAST AREA LATER
THIS AFTERNOON. CURRENT SUITE OF CAM GUIDANCE IS IN REASONABLE
AGREEMENT IN THIS MCS APPROACHING OUR WESTERN CWA TOWARD MIDDAY,
WITH THE STRONGEST CONVECTIVE ELEMENTS EXPECTED TO TRACK
SOUTHEAST ALONG THE NORTHERN PERIPHERY OF THE LOW-LEVEL
INSTABILITY GRADIENT INTO CENTRAL IL (BRUSHING OUR SOUTHWESTERN
TIER OF CWA COUNTIES) INTO THIS AFTERNOON. WHILE SOME GUIDANCE
HAS INDICATED A GRADUAL WEAKENING OF CONVECTION THIS FAR EAST
LATER TODAY, DIURNAL WARMING SHOULD MAINTAIN INSTABILITY ALONG
IT'S PATH AND THIS MAY MAINTAIN PRIMARILY A DAMAGING WIND THREAT
MAINLY ALONG/SOUTH OF ABOUT A MENDOTA IL TO FOWLER IN LINE
EARLY THIS AFTERNOON IN ADDITION TO THE POTENTIAL FOR HEAVY
DOWNPOURS. SPC'S NEW DAY 1 OUTLOOK HAS PLACED A SLIGHT (LEVEL 2
OF 5) SEVERE RISK ACROSS THIS FAR SOUTH/SOUTHWEST PORTION OF OUR
FORECAST AREA. LOW LEVEL INSTABILITY IS EXPECTED TO DECREASE
MARKEDLY WITH FURTHER NORTHWARD EXTENT, LIMITING ANY SEVERE
WEATHER THREAT FARTHER NORTH.
CONCERN THEN INCREASES FOR THE POTENTIAL FOR SEVERAL PERIODIC
ROUNDS OF HEAVY RAINFALL AND AN ATTENDANT FLOOD/FLASH FLOOD
THREAT ACROSS PORTIONS OF THE AREA FROM TONIGHT INTO AT LEAST
SATURDAY. GUIDANCE IS IN GOOD AGREEMENT IN STRENGTHENING AN
UPPER LEVEL HIGH PRESSURE RIDGE ACROSS THE GULF COAST NORTHWARD
ACROSS THE LOWER MISSISSIPPI AND OHIO VALLEYS DURING THIS
PERIOD, SETTING UP A PERIOD OF PERSISTENT WEST-SOUTHWEST
WARM/MOIST ADVECTION FLOW AROUND THE NORTHERN EDGE OF THE RIDGE
INTO PARTS OF IA AND NORTHERN IL/IN. MODEL PRECIPITABLE WATER
VALUES REACH 2-2.3" ACROSS THE AREA AS A SERIES OF SHORT WAVES
TOP THE RIDGE TO OUR SOUTH. THERE IS A CONSISTENT SIGNAL FOR
PORTIONS OF THESE AREAS TO POTENTIALLY RECEIVE SEVERAL INCHES
(3+) OF RAIN, WHICH ON TOP OF WET SOILS AND ELEVATED RIVER/CREEK
LEVELS IN SPOTS FROM RECENT HEAVY RAINS SUGGEST AN INCREASING
CONCERN FOR FLOODING. THE FIRST ROUND (ASIDE FROM RAINFALL WITH
THE AFTERNOON MCS) WOULD BE TONIGHT INTO EARLY FRIDAY MORNING,
AS A SHORT WAVE CURRENTLY EMERGING FROM THE HIGH PLAINS OF
WESTERN CO/NE TRANSITS THE REGION. MODEL QPF SUGGEST A NW-SE
ARC OF POTENTIALLY 2-PLUS INCH RAINFALL GENERALLY FROM A BIT
SOUTH OF ROUGHLY A FORRESTON IL TO VALPARAISO IN LINE OVERNIGHT.
WHILE EXACT DETAILS REMAIN A BIT UNCERTAIN, ELECTED TO ISSUE A
FLOOD WATCH FOR THESE AREAS FROM 7 PM TONIGHT THROUGH 10 AM
FRIDAY. DEPENDING ON HOW RAINFALL LAYS OUT, THE WATCH MAY NEED
TO BE EXTENDED (OR ANOTHER ISSUED) INTO THE WEEKEND FOR
ADDITIONAL PERIODS OF HEAVY RAINFALL.
EVENTUALLY, GLOBAL MODELS/ENSEMBLES SHIFT THE CORE OF THE GULF
COAST UPPER RIDGE WESTWARD LATE SATURDAY NIGHT INTO SUNDAY,
WHICH MAY ALLOW THE LINGERING FRONTAL ZONE TO OUR NORTH TO
FINALLY PUSH SOUTH OF THE AREA SOMETIME LATE SUNDAY. RAIN
CHANCES WOULD LIKELY DIMINISH BEHIND THE FRONT WITH MAINLY DRY
WEATHER MONDAY/TUESDAY. IT DOES APPEAR HOWEVER, THAT ANOTHER
FRONT MAY PUSH SOUTHEAST ACROSS THE AREA MID-WEEK WHICH MAY
SUPPORT SOME ADDITIONAL RAIN CHANCES AT THAT TIME.
RATZER
AVIATION /06Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 128 AM CDT THU AUG 13 2026
KEY MESSAGES:
- FOG AND LOW CEILINGS ARE LIKELY TO BE OBSERVED THIS MORNING,
ESPECIALLY AT RFD, DPA, AND GYY.
- A PERIOD OF RAIN IS LIKELY THIS AFTERNOON WITH A 20-25%
CHANCE FOR AN ISOLATED THUNDERSTORM OR TWO TO OCCUR NEAR THE
TERMINALS.
- A CORRIDOR OF WIDESPREAD SHOWERS AND STORMS WILL SET UP IN THE
REGION THURSDAY NIGHT INTO FRIDAY MORNING, THOUGH MOST OF
THIS ACTIVITY IS FAVORED TO REMAIN SOUTH OF THE TERMINALS.
LIGHT/CALM WINDS AND MOSTLY CLEAR SKIES HAVE ALLOWED FOR FOG TO
START DEVELOPING IN THE AREA EARLY THIS MORNING. INCREASING
CIRRUS COVERAGE OVERNIGHT INTRODUCES UNCERTAINTY TO HOW DENSE
THIS FOG MIGHT GET, BUT IFR OR LOWER VISIBILITIES APPEAR TO BE A
DISTINCT POSSIBILITY, PRIMARILY AT RFD, DPA, AND GYY. LOW
STRATUS COVERAGE MAY ALSO INCREASE OVER THE NEXT FEW HOURS AND
ALSO RESULT IN MVFR OR LOWER CEILINGS BEING OBSERVED AT THE
TERMINALS FOR SOME TIME THIS MORNING.
THE NEXT DISTURBANCE IN THIS VERY ACTIVE WEATHER PATTERN WILL
ARRIVE IN NORTHERN ILLINOIS THIS AFTERNOON. IT APPEARS THAT THE
STRONGER THUNDERSTORM ACTIVITY ASSOCIATED WITH THIS DISTURBANCE
WILL TRACK SAFELY SOUTH OF THE TERMINALS, BUT A PERIOD OF RAIN
IS NEVERTHELESS STILL LIKELY TO OCCUR AS THIS DISTURBANCE PASSES
THROUGH. THERE IS ABOUT A 20-25% CHANCE FOR AN ISOLATED
THUNDERSTORM OR TWO TO DEVELOP ON THE BACK SIDE OF THIS RAIN
SHIELD AND AFFECT THE TERMINALS. THIS IS STILL TOO LOW OF A
CHANCE TO WARRANT A FORMAL TSRA OR VCTS MENTION IN THE TAFS, BUT
IT REMAINS POSSIBLE THAT SUCH A MENTION WILL EVENTUALLY BE
NEEDED AS FORECAST AND OBSERVATIONAL TRENDS BECOME MORE APPARENT.
IN THE WAKE OF THIS DISTURBANCE, SHOWERS AND THUNDERSTORMS ARE
LIKELY TO FIRE ALONG THE NORTHERN PERIPHERY OF AN ELEVATED
INSTABILITY PLUME AND PERSIST INTO FRIDAY. SHOWER AND STORM
COVERAGE ACROSS THIS CORRIDOR IS LIKELY TO BE WIDESPREAD AND
FEATURE TRAINING CONVECTIVE ELEMENTS. WHILE THIS AXIS OF
CONVECTIVE ACTIVITY IS FAVORED TO LARGELY REMAIN FOCUSED SOUTH
OF THE TERMINALS, SOME OF THE ASSOCIATED RAIN MAY EVENTUALLY
LIFT FAR ENOUGH NORTHWARD TO AFFECT THE TERMINALS. IF THIS AXIS
ENDS UP SETTING UP FARTHER NORTH THAN PRESENTLY EXPECTED, THEN
IT IS POSSIBLE THAT THE TERMINALS WILL SEE AN EXTENDED PERIOD OF
STORMS AND TORRENTIAL RAINFALL THURSDAY NIGHT INTO FRIDAY
MORNING, BUT THIS IS NOT THE MOST LIKELY OUTCOME AT THIS TIME.
OGOREK
LOT WATCHES/WARNINGS/ADVISORIES
IL...DENSE FOG ADVISORY UNTIL 9 AM CDT THIS MORNING FOR ILZ003-
ILZ004-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-
ILZ108.
FLOOD WATCH FROM THIS EVENING THROUGH FRIDAY MORNING FOR
ILZ008-ILZ010-ILZ011-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
ILZ033-ILZ039-ILZ106-ILZ107-ILZ108.
IN...DENSE FOG ADVISORY UNTIL 9 AM CDT /10 AM EDT/ THIS MORNING FOR
INZ001-INZ002-INZ010-INZ011-INZ019.
FLOOD WATCH FROM THIS EVENING THROUGH FRIDAY MORNING FOR
INZ010-INZ011-INZ019.
LM...NONE.
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