641
FXUS63 KLOT 110753
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
253 AM CDT TUE AUG 11 2026
KEY MESSAGES
- SCATTERED THUNDERSTORMS WILL AFFECT FAR NORTHERN IL/NORTHWEST
IN EARLY THIS MORNING, WITH ISOLATED INSTANCES OF LARGE HAIL
AND DAMAGING WIND AS WELL AS LOCALIZED FLOODING ARE POSSIBLE.
- POSSIBLE LINE/COMPLEX OF SEVERE THUNDERSTORMS TO AFFECT THE
AREA BY MID/LATE MORNING INTO EARLY THIS AFTERNOON. SEVERE
WIND GUSTS TO 80 MPH POSSIBLE, ALONG WITH HEAVY RAIN AND
LOCALIZED FLOODING.
- PATTERN REMAINS ACTIVE WITH ADDITIONAL THUNDERSTORM CHANCES
THROUGH THE END OF THE WEEK/WEEKEND.
DISCUSSION
ISSUED AT 253 AM CDT TUE AUG 11 2026
FORECAST REMAINS COMPLEX TODAY, WITH ONGOING STORMS EARLY THIS
MORNING AND THE POTENTIAL EVOLUTION OF A MORE SIGNIFICANT
WIDESPREAD SEVERE WEATHER/DAMAGING WIND THREAT LATER THIS
MORNING AND EARLY AFTERNOON.
EARLY MORNING SURFACE ANALYSIS DEPICTS A RATHER DIFFUSE WARM
FRONTAL ZONE STRETCHING FROM NORTH-CENTRAL IA EAST-SOUTHEAST
ACROSS NORTHERN IL AND NORTHWEST IN, ROUGHLY FROM NORTH OF THE
QUAD CITIES TO LOT AND VPZ. KLOT VAD WIND PROFILE AND SPC/RAP
MESOANALYSIS INDICATES A ~25 KT WEST-SOUTHWESTERLY LOW LEVEL JET
IMPINGING ON THE FRONTAL ZONE, WITH ASSOCIATED ISENTROPIC
ASCENT ATOP THE SHALLOW FRONTAL SLOPE RESULTING IN ISOLATED
THUNDERSTORMS FROM NEAR ARX OVER TO THE LAKE MICHIGAN SHORE NEAR
MICHIGAN CITY IN AT THIS HOUR. LOW-LEVEL JET FORCING MAY
INCREASE A BIT DURING THE PREDAWN HOURS AS A SUBTLE MID-LEVEL
SHORT WAVE TRACKS ACROSS THE AREA. THERE HAS BEEN A SIGNAL IN
CAM GUIDANCE FOR POTENTIAL ADDITIONAL DEVELOPMENT ALONG THE
FRONT PRIOR TO SUNRISE, AND WE CONTINUE TO MONITOR FOR THAT.
MUCAPE NEAR 3000 J/KG AND ~40 KTS OF EFFECTIVE SHEAR PER SPC
MESOANALYSIS WOULD SUPPORT STORM ORGANIZATION AND AN ASSOCIATED
SEVERE (PRIMARILY WIND/HAIL) THREAT AS EVIDENCED BY THE
SCATTERED SUPERCELL STORMS CURRENTLY OVER LM/IWX'S CWA.
ATTENTION THEN TURNS TO A DEVELOPING AREA OF THUNDERSTORMS OVER
FAR SOUTHEAST SOUTH DAKOTA, WHICH MAY EVOLVE INTO A MORE
WIDESPREAD AND POTENTIALLY SIGNIFICANT SEVERE WEATHER THREAT
ACROSS OUR FORECAST AREA LATER THIS MORNING AND EARLY AFTERNOON.
CAM GUIDANCE HAS BEEN CONSISTENT IN THIS CLUSTER OF STORMS
EVENTUALLY GROWING UPSCALE INTO A FORWARD-PROPAGATING MCS ACROSS
NORTHWEST IA/SOUTHWEST MN OVER THE NEXT FEW HOURS AND
EVENTUALLY TRACKING EAST-SOUTHEAST ALONG THE CORRIDOR STRONG
INSTABILITY ALONG THE AFOREMENTIONED FRONTAL ZONE INTO NORTHERN
IL TOWARD MID-LATE MORNING. THE COMBINATION OF UNSEASONABLY
STRONG WESTERLY MID-LEVEL FLOW AND RESULTING DEEP SHEAR(50+ KTS
AT 700 MB), STEEP MID-LEVEL LAPSE RATES (NEAR 8 C/KM) MUCAPE
VALUES IN EXCESS OF 3500 J/KG, THE POTENTIAL FOR SIGNIFICANT
SEVERE WEATHER APPEARS RATHER HIGH. SEVERAL CAMS ARE DEPICTING
POCKETS SURFACE WIND GUSTS OF 80+ MPH WITH THIS MCS ACROSS
PORTIONS OF THE FORECAST AREA. A TORNADO THREAT WOULD ALSO
LIKELY EXIST GIVEN SUFFICIENT LOW-LEVEL SHEAR AND THE PROXIMITY
OF THE PREVIOUSLY MENTIONED SURFACE FRONTAL BOUNDARY. WHILE THE
EXACT EVOLUTION OF THIS MCS REMAINS UNCERTAIN, SPC'S DAY 1
ENHANCED (LEVEL 3 OF 5) FOR OUT FORECAST AREA APPEARS WARRANTED.
CURRENTLY TIMING OF THIS THREAT LOOKS TO BE FROM PERHAPS 9 AM
IN OUR NORTHWEST CWA THROUGH AROUND 3 PM IN THE FAR EAST. THERE
MAY ALSO BE SOME CONVECTIVE DEVELOPMENT ALONG THE FRONT AHEAD OF
THE MCS, IF A STRONG MCV DEVELOPS PRIOR TO REACHING THE CWA
INCREASES WARM-ADVECTION ASCENT AHEAD OF THE MAIN CLUSTER AS
INDICATED BY SOME OF THE CAMS.
AS IF THE SIGNIFICANT SEVERE/WIDESPREAD DAMAGING WIND THREAT
WASN'T ENOUGH, WE'LL ALSO NEED TO MONITOR THE LOCATION OF HEAVY
RAINFALL AND FLOODING POTENTIAL - PARTICULARLY FOR THE URBANIZED
CHICAGO METRO AREA AS WELL AS LOCATIONS WHICH SAW 3-5"+ OF
RAINFALL SUNDAY AND EVENING. HAD CONTEMPLATED ISSUING A FLOOD
WATCH, THOUGH WITH CURRENT PREDAWN CONVECTION SLOW TO FILL-IN
(SO FAR) AND SOME UNCERTAINTY REMAINING WITH THE EVOLUTION OF
THE MCS LATER THIS MORNING, ELECTED TO HOLD OFF AT THIS TIME
PENDING SOME FURTHER CLARITY ON LOCATION OF HIGHER RAINFALL
IMPACTS.
IN THE WAKE OF TODAY'S EXPECTED MCS, A LULL IN CONVECTIVE
ACTIVITY IS EXPECTED FROM LATER THIS AFTERNOON INTO TONIGHT,
THOUGH THIS WILL LIKELY DEPEND ON THE SIZE AND MAGNITUDE OF THE
REMNANT COLD POOL AND HOW FAR SOUTH THE EFFECTIVE FRONTAL ZONE
ENDS UP ALONG IT'S SOUTHERN PERIPHERY. THE OVERALL PATTERN SOME
UNCERTAINTY IS HOW FAR SOUTH AN AREA OF SURFACE HIGH PRESSURE
BUILDS ACROSS THE WESTERN LAKES IN THE FRIDAY/SATURDAY PERIOD.
THIS WILL HAVE SIGNIFICANT IMPACTS ON THUNDERSTORM TRENDS/TIMING
FOR THE LOCAL AREA.
RATZER
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 136 AM CDT TUE AUG 11 2026
KEY MESSAGES:
- SCATTERED SHOWERS AND PERHAPS A FEW STORMS MAY OCCUR NEAR THE
TERMINALS THROUGH EARLY THIS MORNING.
- AN ORGANIZED COMPLEX OF THUNDERSTORMS IS LIKELY TO IMPACT THE
TERMINALS BETWEEN MID-MORNING AND MID-AFTERNOON TODAY. SEVERE
WIND GUSTS IN EXCESS OF 50 KTS MAY OCCUR WITH THESE STORMS.
- SUB-VFR CEILINGS AND VISIBILITIES WILL BE OBSERVED AT TIMES
THROUGH THIS MORNING.
ISENTROPIC LIFT OVER A SURFACE BOUNDARY MAY CONTINUE TO GENERATE
SHOWERS AND POSSIBLY A FEW ADDITIONAL STORMS THROUGH EARLY THIS
MORNING. MUCH OF THIS ACTIVITY HAS FOCUSED NORTH AND EAST OF THE
TERMINALS THUS FAR, AND CONFIDENCE IS LOW THAT ANY NEW CELLS
THAT DEVELOP WILL AFFECT THE TERMINALS, BUT (AT LEAST FOR NOW)
OPTED TO MAINTAIN THE INHERITED PROB30 GROUPS FOR TSRA FOR LATER
TONIGHT/THIS MORNING AS A PRECAUTION.
THE TERMINALS WILL, HOWEVER, LIKELY BE AFFECTED BY A
THUNDERSTORM COMPLEX BETWEEN THE MID-MORNING AND MID-AFTERNOON
TODAY. CONFIDENCE IN THE PRECISE ARRIVAL TIME OF THESE STORMS
REMAINS SOMEWHAT LOW AT THIS TIME AS THE COMPLEX HAS YET TO
REALLY MATERIALIZE (AS OF THIS WRITING), AND MODEL GUIDANCE HAS
A POOR HANDLE ON WHAT'S GOING ON UPSTREAM OF OUR AREA, SO MADE
NO CHANGES TO THE INHERITED TEMPO GROUP TIMINGS. IF THIS COMPLEX
MATURES QUICKLY ENOUGH INTO AN ORGANIZED LINE OR BOW AND TAKES
A FAVORABLE TRACK ACROSS THE TERMINALS LIKE MANY HIGH-RESOLUTION
MODEL SOLUTIONS ARE SUGGESTING, THEN SEVERE WIND GUSTS IN
EXCESS (AND POTENTIALLY WELL IN EXCESS) OF 50 KTS MAY BE
OBSERVED AS THESE STORMS ROLL THROUGH.
LASTLY, THE PERSISTING ABUNDANCE OF LOW-LEVEL MOISTURE HAS
ALLOWED FOR MVFR STRATUS AND FOG/MIST TO DEVELOP ONCE AGAIN
TONIGHT. THE REDUCED VISIBILITIES COULD DIP BELOW MVFR AS THEY
PRIMARILY REMAIN AT RFD, GYY, AND DPA, BUT THE MVFR CEILINGS
WILL LIKELY CONTINUE TO AFFECT ALL OF OUR TAF SITES AT TIMES
THROUGH THIS MORNING. ANOTHER EPISODE OF FOG AND LOW CEILINGS
APPEARS POSSIBLE EARLY WEDNESDAY MORNING, AND THAT IS SOMETHING
THAT WILL NEED TO BE ASSESSED MORE CLOSELY FOR FORTHCOMING TAF
ISSUANCES.
OGOREK
MARINE
ISSUED AT 253 AM CDT TUE AUG 11 2026
A POTENTIAL EXISTS FOR A SEVERE THUNDERSTORM COMPLEX TO PRODUCE
WIDESPREAD DAMAGING WINDS (IN EXCESS OF 65 KNOTS) ACROSS FAR
SOUTHERN LAKE MICHIGAN LATER THIS MORNING/MIDDAY. IN ADDITION TO
THE WIND THREAT ITSELF, THE LINE OF STORMS ARE EXPECTED TO
PROPAGATE IN AN EAST-SOUTHEAST DIRECTION, WHICH COULD
POTENTIALLY LEAD TO A SEICHE (WATER IS PUSHED AHEAD OF THE
STORMS, THEN SLOSHES BACK AND FORTH FOR SEVERAL HOURS AFTER THE
STORMS PASS) ON THE SOUTHERN PORTION OF THE LAKE. IF THIS WERE
TO OCCUR, IT COULD RESULT IN FLUCTUATING LAKE LEVELS ALONG THE
ILLINOIS AND INDIANA SHORES FOR A TIME FROM LATER MORNING INTO
THIS AFTERNOON.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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