542
FXUS63 KLOT 301738
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1238 PM 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 /18Z TAFS THROUGH 00Z TUESDAY/
ISSUED AT 1238 PM CDT SUN AUG 30 2026
FORECAST CONCERNS INCLUDE...
CHANCE OF THUNDERSTORMS THIS AFTERNOON AND MONDAY MORNING.
GUSTY SOUTH/SOUTHWEST WINDS EARLY THIS AFTERNOON.
SCATTERED THUNDERSTORMS ARE CURRENTLY MOVING ALONG THE IL/WI
STATE LINE IN RESPONSE TO AN UPPER LEVEL WAVE APPROACHING THE
AREA. ADDITIONAL SHOWERS AND ISOLATED THUNDERSTORMS WILL BE
POSSIBLE ACROSS NORTHERN IL AND THE TERMINALS THROUGH LATE THIS
AFTERNOON, THOUGH THUNDER COVERAGE REMAINS UNCERTAIN. GIVEN THE
LATEST TRENDS, PROB THUNDER MENTION SEEMS REASONABLE FOR THE
CHICAGO TERMINALS WITH TEMPO MENTION AT RFD. THE OVERALL TREND
IN GUIDANCE IS FOR THIS ACTIVITY TO SLOWLY WEAKEN BY LATE
AFTERNOON/EARLY THIS EVENING, BUT LINGERING SHOWERS MAY PERSIST
INTO THIS EVENING.
THERE WILL BE TWO MORE TIME PERIODS WITH POTENTIAL ISOLATED
THUNDERSTORMS, THE FIRST AROUND/AFTER DAYBREAK MONDAY MORNING
AND THEN MONDAY AFTERNOON. CONFIDENCE FOR BOTH TIME PERIODS
REMAINS VERY LOW AND OPTED TO MAKE NO CHANGES TO THE FORECAST,
KEEPING CURRENT PROB THUNDER MENTION FOR MONDAY MORNING AND
KEEPING THE FORECAST DRY FOR MONDAY AFTERNOON.
SOUTH/SOUTHWEST WINDS WILL GUST INTO THE LOWER/MID 20KT RANGE
FOR A FEW MORE HOURS, THEN GUSTS ARE EXPECTED TO DIMINISH BY
MID AFTERNOON. WIND DIRECTIONS ARE EXPECTED TO BECOME SOUTH/
SOUTHEAST THIS EVENING, THEN TURN BACK TO SOUTH/SOUTHWEST
OVERNIGHT AND CONTINUE ON MONDAY. CMS
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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