218
FXUS63 KLOT 301130
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
630 AM CDT SUN AUG 30 2026
KEY MESSAGES
- PERIODIC SHOWER AND STORMS THROUGH THE EARLY PART OF THE WEEK,
THOUGH THERE WILL AMPLE DRY PERIODS.
- HOTTER AND MORE HUMID CONDITIONS BUILD THROUGH MIDWEEK.
DISCUSSION
ISSUED AT 222 AM CDT SUN AUG 30 2026
AS A LOW LEVEL JET (LLJ) INCREASES THIS MORNING, AS SAMPLED
AROUND 40 KNOTS JUST 2000 FT AGL ON THE KLOT VAD, IT IS NO
SURPRISE THAT SHOWER COVERAGE IS GRADUALLY EXPANDING OVER
NORTHERN ILLINOIS. POPS WERE INCREASED TO AROUND 30 TO 40
PERCENT CHANCE TO MATCH THE COVERAGE BEING OBSERVED. THIS
SOUTHWEST TO NORTHEAST ORIENTED LLJ IS EXPECTED TO PIVOT AND
BECOME SLIGHTLY MORE EASTERLY ORIENTED THROUGH THE MORNING AND
BE THE DRIVING FORCE BEHIND AN INCREASE IN SHOWER COVERAGE.
THERE IS MODEST EFFECTIVE SHEAR PRESENT, BUT CAPE VALUES AND
LAPSE RATES ARE NOT PROVIDING CONFIDENCE IN OVERALL COVERAGE OF
THUNDER. THAT BEING SAID, THERE HAVE ALREADY BEEN SMALL CLUSTERS
OF EMBEDDED THUNDERSTORMS DEVELOP EARLY THIS MORNING. MOST
THUNDER IS EXPECTED TO BE ISOLATED, BUT THERE COULD BE AN UPTICK
IN THUNDER COVERAGE RIGHT AROUND AND AFTER DAYBREAK. CURRENT
GUIDANCE IS SUGGESTING THE LLJ WILL GRADUALLY DIMINISH AROUND
MIDDAY ALLOWING THE SHOWERS TO EVENTUALLY ERODE AND SINK
SOUTHWARD AS IT FALLS APART.
AN UPPER LEVEL RIDGE, THE CENTER OF WHICH IS CURRENTLY ANALYZED
OVER OKLAHOMA AND TEXAS, KEEPS NORTHERN ILLINOIS AND NORTHWEST
INDIANA ALONG ITS NORTHERN PERIPHERY THROUGH MIDWEEK. THERE
WILL BE PLENTY OF DRY TIMES, BUT AS EMBEDDED WAVES MOVES THROUGH
THE PATTERN, THERE WILL BE ADDITIONAL PERIODS OF SHOWERS AND
STORMS. FOR EXAMPLE, RAP ANALYSIS IS CURRENTLY SHOWING AN UPPER
LEVEL SHORT WAVE FROM RAPID CITY, SD TO AROUND DODGE CITY, KS
JUST BEHIND ONGOING STORMS IN THE GREAT PLAINS. THERE IS AN
ADDITIONAL SHORT WAVE IN WESTERN WYOMING AND COLORADO. MODELS
ARE SUGGESTING THAT THAT BOTH THESE WAVES ARE EXPECTED TO RIDE
ALONG THE TOP OF THE RIDGE AND PROVIDE ADDITIONAL CHANCES FOR
PRECIPITATION OVERNIGHT TONIGHT (THE PLAINS WAVE) AND AGAIN
OVERNIGHT INTO TUESDAY MORNING. IF THE RIDGE WERE TO STRENGTHEN,
IT COULD FORCE THESE WAVES (AND SUBSEQUENT SHOWER/STORM
CHANCES) FARTHER NORTH. WHETHER OR NOT THEY MAINTAIN THEIR
CURRENT TRAJECTORY OR STRENGTH REMAINS TO BE SEEN. FOR NOW,
CHANCES FOR SHOWERS AND STORMS REMAIN IN THE FORECAST THROUGH
THE MIDDLE OF THE WEEK.
AS THE RIDGE REMAINS OVER THE CONUS THROUGH MIDWEEK, SO DOES
SOUTHWESTERLY FLOW AT THE MID-LEVELS. THIS WIND DIRECTION WILL
AID IN ADVECTING IN A WARMER AIR MASS TO THE WESTERN GREAT LAKES
WITH PROJECTED 850 MB TEMPERATURES OVER 20C, POTENTIALLY PUSHING
25C. WINDS AT THE SURFACE WILL ALSO REMAIN OUT OF THE SOUTH,
GRADUALLY INCREASING DEW POINTS THROUGH THE WEEK BACK INTO THE
70S. HOTTER TEMPERATURES IN THE UPPER 80S AND LOWER 90S ARE
EXPECTED. THIS COMBINATION OF HOT TEMPERATURES AND HUMID
CONDITIONS ARE EXPECTED TO INCREASE HEAT INDICES INTO THE MID TO
UPPER 90S BY MIDWEEK, WITH THE SOUTHWESTERN PORTION OF THE
FORECAST AREA WITH A CHANCE OF EXCEEDING 100. CHANCES FOR
SHOWER/STORMS THROUGH THE WEAK LOWERS CONFIDENCE IN SPECIFIC
TEMPERATURES VALUES; THEREFORE, THERE WAS NO CONSIDERATION AT
THIS TIME FOR ANY HEAT HAZARD PRODUCTS (ADVISORY/WATCH).
MODELS ARE SUGGESTING THAT THE RIDGE COULD BREAK DOWN BY THE END
OF THE WORK WEEK. THAT WILL HELP THE HOTTEST OF TEMPERATURES TO
COOL OFF A BIT, BUT IT WILL RETURN THE UPPER LEVEL FLOW PATTERN
TO THE NORTHWEST AND THUS BRING WITH IT ADDITIONAL CHANCES FOR
SHOWERS/STORMS OVER THE WEEKEND.
DK
AVIATION /12Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 630 AM CDT SUN AUG 30 2026
SHOWERS AND THUNDERSTORMS CONTINUE TO EXPAND ACROSS THE AREA
THIS MORNING WITHIN A REGION OF BROAD ISENTROPIC ASCENT BEING
FORCED BY A SOUTHWESTERLY 30-40KT LOW-LEVEL JET. THE PEAK IN
COVERAGE OVER THE TERMINALS SHOULD BE FROM ROUGHLY 12-16Z WITH A
DOWNWARD TREND IN COVERAGE THEREAFTER AS THE LOW-LEVEL JET
WEAKENS. WINDS THIS MORNING SHOULD REMAIN SOUTHWESTERLY WITH AN
INCREASE IN GUSTS TOWARD 20-25KT ALONG THE BACK EDGE OF THE
SHOWERS.
AM CURRENTLY ANTICIPATING A RELATIVE MINIMA IN COVERAGE OF
ACTIVITY THROUGH EARLY AFTERNOON BEFORE SHOWERS ATTEMPT TO
REDEVELOP BY 19-20Z AS A HYBRID GRAVITY WAVE/MCV CURRENTLY
MOVING INTO IOWA LIFTS TOWARD THE ILLINOIS AND WISCONSIN BORDER.
DO THINK THERE IS A VERY REALISTIC CHANCE THAT A FEW
THUNDERSTORMS DEVELOP THIS AFTERNOON WITH THE MCV GIVEN LITTLE
TO NO CHANGE IN THE INTEGRITY OF THE LAPSE RATE PLUME CURRENTLY
SUPPORTING THUNDER ACROSS THE AREA, BUT WILL ADVERTISE -SHRA
FROM EARLY AFTERNOON THROUGH THE EVENING FOR NOW OWING TO
SOMEWHAT LOW CONFIDENCE AND TO AVOID HAVING CONTINUOUS THUNDER
IN THE TAF. SOUTHWESTERLY WINDS MAY SWING SOUTHEASTERLY FOR A
TIME THIS EVENING (23-04Z OR SO) ON THE BACKSIDE OF THE GRAVITY
WAVE BEFORE SETTLING BACK ON A SOUTH-SOUTHWEST DIRECTION BEFORE
MIDNIGHT.
TOWARD THE END OF THE TAF PERIOD, ANOTHER LOW-LEVEL JET WILL
DEVELOP OVER THE MIDDLE MISSISSIPPI RIVER VALLEY AND TERMINATE
OVER THE LOWER GREAT LAKES. WITH LITTLE CHANGE IN THE OVERALL
LOW- TO MID-LEVEL MOISTURE AND LAPSE RATES PROFILES, SUSPECT WE
MAY HAVE A REPEAT PERFORMANCE OF THIS MORNING WITH SCATTERED
SHOWERS AND THUNDERSTORMS IN THE GENERAL REGION BY DAYBREAK.
SO, WILL INTRODUCE PROB30 GROUPS FOR -TSRA AT RFD/ORD/MDW AND
CONSIDER INCLUSION AT DPA/GYY IN THE NEXT TAF SET.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 PM CDT THIS AFTERNOON FOR THE IL
NEARSHORE WATERS.
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