400
FXUS63 KLOT 151127
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
627 AM CDT THU OCT 15 2026
KEY MESSAGES
- A FALL STORM SYSTEM WILL MOVE THROUGH THE AREA TODAY AND
FRIDAY BRINGING PERIODS OF RAIN SHOWERS AND GUSTY
NORTHWESTERLY WINDS (THE LATTER PRIMARILY ON FRIDAY).
- FROSTY AND NEAR-FREEZING CONDITIONS ARE FAVORED AWAY FROM LAKE
MICHIGAN FRIDAY NIGHT INTO SATURDAY AND AGAIN SATURDAY NIGHT.
- AFTER THE BRIEF PERIOD OF CHILLY WEATHER, THERE WILL BE A
WARMING TREND THIS WEEKEND INTO EARLY NEXT WEEK.
DISCUSSION
ISSUED AT 308 AM CDT THU OCT 15 2026
THOUGH TONIGHT:
PER RECENT GOES-19 WATER VAPOR IMAGERY, AN UPPER-LEVEL TROUGH
IS LIFTING NORTHEASTWARD ACROSS THE ROCKY MOUNTAINS TOWARD THE
CENTRAL PLAINS AHEAD OF A VIGOROUS NORTHERN-STREAM TROUGH
DIGGING SOUTHEASTWARD INTO THE DAKOTAS. PRESSURE FALLS ON THE
ORDER OF 1 TO 2 MB/HR HAVE COMMENCED IN CENTRAL OKLAHOMA (LIKELY
AUGMENTED BY CONVECTION), WHICH WILL SPREAD NORTHEASTWARD WITH
TIME AS THE LEAD UPPER-LEVEL TROUGH LIFTS ATOP A SOUTHWEST- TO
NORTHEASTWARD-ORIENTED LOW-LEVEL BAROCLINIC ZONE STRETCHING
INTO THE GREAT LAKES. GENERALLY SPEAKING, THE FORECAST FOR THE
NEXT 24 TO 48 HOURS REMAINS ON TRACK WITH THE EXPECTATION FOR
THE FIRST FALL STORM OF THE SEASON TO MOVE THROUGH THE GREAT
LAKES REGION. AS THIS OCCURS, A MYRIAD OF WEATHER CONDITIONS
WILL OCCUR IN OUR LOCAL AREA.
AFTER DAYBREAK, SHOWERS AND AREAS OF DRIZZLE (ALREADY DEVELOPING
AT PRESS TIME) WILL GRADUALLY EXPAND ACROSS NORTHERN ILLINOIS
AND NORTHWESTERN INDIANA. AS OUR AREA WILL BE SOMEWHAT REMOVED
FROM THE STRONGEST FORCING (WARM CONVEYER BELT FOCUSED JUST
SOUTH OF OUR AREA, AND INCREASING FRONTOGENETICAL FORCING
FOCUSED TO OUR NORTH), AM NOT EXPECTING HEFTY PRECIPITATION
TOTALS THROUGH THE FIRST HALF OF THE DAY. COVERAGE OF SHOWERS
AND DRIZZLE SHOULD MAXIMIZE THIS AFTERNOON AS THE SURFACE LOW
MOVES DIRECTLY OVER OUR AREA, THOUGH AGAIN COVERAGE MAY BE MORE
SCATTERED THAN WIDESPREAD OWING TO SOMEWHAT DISJOINTED FORCING.
NEVERTHELESS, SEASONABLY HIGH PWATS (NEARING 2 INCHES) WILL
SUPPORT TORRENTIAL DOWNPOURS WHERE SHOWERS ARE MOST PERSISTENT
THIS AFTERNOON.
AM SEEING QUITE A BIT OF VARIANCE IN THE EXACT TRACK OF THE
SURFACE LOW IN HIGH-RES GUIDANCE EARLY THIS MORNING, SO WILL
HEDGE TOWARD GLOBAL GUIDANCE (WHICH OFTEN DOES NOT SUFFER
IMPACTS OF CONVECTIVE EFFECTS). SUCH A HEDGE FAVORS THE TRACK OF
THE LOW PASSING DIRECTLY FROM LASALLE TO CHICAGO THIS EVENING
WHILE DEEPENING A MB OR TWO PER HOUR. WITH THIS IN MIND, WILL
HAVE TO WATCH FOR A NARROW WINDOW FOR MOISTURE-DRIVEN SURFACE-
BASED INSTABILITY TO SNEAK ALONG AND EAST OF I-55 THIS EVENING
(5 TO 9 PM) TO SUPPORT LOW-TOPPED CONVECTION ALONG OR JUST AHEAD
OF THE SYNOPTIC COLD FRONT. THE OVERLAP OF STRONGLY VEERING
LOW-LEVEL WIND PROFILES WITHIN THE LOW-CENTROID AND HIGH
MOISTURE-DRIVEN CONVECTIVE LAYER DOES RAISE CONCERN FOR LOW-
TOPPED SUPERCELL STRUCTURES, THOUGH WOULD HAVE TO THINK THEY
WOULD BE HARMLESS GIVEN LIMITED OPPORTUNITY FOR BREAKS IN CLOUD
COVER TO STEEPEN LOW-LEVEL LAPSE RATES (EFFECTIVELY LIMITING THE
OPPORTUNITY FOR TRULY SURFACE-BASED CONVECTION). IN THE OFF
CHANCE THAT THE SURFACE LOW MOVES EVEN FURTHER NORTH THAN
EXPECTED AND THERE IS PARTIAL SUNSHINE IN THE NARROW WARM SECTOR
TO STEEPEN LOW-LEVEL LAPSE RATES, CANNOT DISCOUNT A THREAT FOR
A BRIEF TORNADO. AT THIS POINT, WE ASSESS THE CHANCE OF SUCH AN
OUTCOME TO BE VERY LOW (LESS THAN 5%), A SENTIMENT THAT IS
SHARED BY SPC (WE DID CHECK IN WITH THEM THIS MORNING). SO, WILL
CONTINUE TO WITHHOLD MENTIONING A THREAT FOR SEVERE WEATHER
TODAY IN THE HWO AND GRAPHICAL PRODUCTS AND ALLOW FOR
OBSERVATIONS THIS AFTERNOON GUIDE ADJUSTMENTS IN STRATEGY, IF
NEEDED.
IN THE IMMEDIATE WAKE OF THE COLD FRONT THIS EVENING, INITIAL
PRESSURE RISES ON THE ORDER OF 2 TO 3 MB/HR WILL DRIVE A PERIOD
OF WESTERLY WINDS GUSTING 30 TO 35 MPH. ROBUST COLD AIR
ADVECTION WILL SEND TEMPERATURES FALLING INTO THE UPPER 40S TO
LOWER 50S OVERNIGHT WITH WIND CHILLS BY DAYBREAK FRIDAY SOLIDLY
IN THE LOW TO MID 40S. THE OVERNIGHT PERIOD IS FAVORED TO BE DRY
FOR MOST AREAS, THOUGH CAN ENVISION A FEW SHOWERS CONTINUING
NEAR AND SOUTHEAST OF I-57 NEAR AND JUST BEHIND THE COLD FRONT.
FRIDAY:
ON FRIDAY, THE NORTHERN-STREAM TROUGH WILL WHIP THROUGH THE
GREAT LAKES ANCHORED BY A SEASONABLY COOL 500MB TEMPERATURES
NEAR -25C. A LOW-LEVEL TROWEL-LIKE STRUCTURE WITHIN THE
CYCLONIC SHEAR AXIS OF THE TROUGH WILL SUPPORT A DIURNAL FLARE
OF SHOWERS WITH PEAK COVERAGE DURING THE AFTERNOON HOURS ACROSS
THE AREA. PRESSURE RISES WILL ACTUALLY BE GREATEST DURING THE
AFTERNOON (+10MB/6HR) AS A 1040MB SURFACE HIGH BUILDS INTO THE
PLAINS. AS A RESULT, THE EXPECTATION IS FOR A PERIOD OF STRONG
NORTH TO NORTHWESTERLY WINDS FRIDAY AFTERNOON, PEAKING IN THE 35
TO 40 MPH RANGE. LOCALLY HIGHER GUSTS OF 45 TO 55 MPH CANNOT BE
RULED OUT IN THE TALLEST SHOWERS. EXTENSIVE STRATUS AND
CONTINUED COLD AIR ADVECTION WILL LIMIT HIGHS TO THE LOWER 50S,
THOUGH WOULD NOT BE SURPRISED IF TEMPERATURES REMAIN IN THE
UPPER 40S AS HINTED BY MORE AGGRESSIVE DETERMINISTIC GUIDANCE.
AS THE UPPER-LEVEL TROUGH WHIPS OVERHEAD, TUMBLING TEMPERATURES
ATOP THE WARM WATERS OF LAKE MICHIGAN WILL SET THE STAGE FOR
THE BLOSSOMING OF LAKE EFFECT RAIN SHOWERS. WITH INVERSION
HEIGHTS PROGGED TO CLIMB TO NEARLY 20KFT AND ABOVE THE
MINUS 20C LEVEL, CANNOT RULE OUT A FEW LIGHTNING STRIKES AND
GRAUPEL (SOFT HAIL) IN THE DEEPEST LAKE EFFECT CONVECTIVE CORES.
THE NORTHERLY WIND DIRECTION WILL POINT THE STREAM OF LAKE
EFFECT SHOWERS INTO NORTHWESTERN INDIANA DURING THE AFTERNOON,
THOUGH ANY MESOLOWS MAY TEND TO PUSH SHOWERS INTO COOK COUNTY,
AS WELL. WIND GUSTS ALONG THE IMMEDIATE SHORELINE MAY ALSO
ROUTINELY POKE ABOVE 45 MPH (STRONGER NEAR ANY MESOLOWS). WILL
BRIEF THE DAY SHIFT TO CONSIDER HOISTING A WIND ADVISORY FOR
LAKE AND PORTER COUNTIES IN NORTHWESTERN INDIANA CENTERED ON THE
FRIDAY AFTERNOON PERIOD (TYPICALLY LIKE TO WAIT UNTIL WITHIN 24
HOURS OF AN EVENT TO PULL THE TRIGGER ON A HEADLINE).
TAKEN ALTOGETHER, FRIDAY IS SHAPING UP TO BE A RATHER RAW AND
CHILLY DAY OUT THERE. THE TREE CANOPY WILL LIKELY LOSE A LOT OF
COLORFUL LEAVES.
FRIDAY NIGHT INTO SATURDAY:
THE PATTERN WILL REMAIN FAIRLY PROGRESSIVE AS THE SURFACE LOW
PULLS AWAY FROM OUR AREA AND THE SURFACE HIGH PRESSURE SYSTEM
SLIDES TOWARD THE MIDDLE MISSISSIPPI RIVER VALLEY. AM NOT
CONVINCED WINDS WILL CRASH QUICKLY FRIDAY NIGHT OWING TO A
CONTINUED PRESSURE GRADIENT IN OUR LOCAL AREA AHEAD OF THE
APPROACHING SURFACE HIGH PRESSURE SYSTEM UNTIL LITERALLY
DAYBREAK. EVEN SO, CLEARING SKIES (BY WAY OF TUMBLING INVERSION
HEIGHTS) AND CONTINUED COLD AIR ADVECTION WILL STILL PUSH
TEMPERATURES INTO THE LOW TO MID 30S OVERNIGHT. THE EXCEPTION
WILL BE ACROSS NORTHWESTERN INDIANA WHERE LAKE EFFECT STRATUS
WILL LIKELY CONTINUE THROUGH THE NIGHT. THIS PERIOD CONTINUES TO
BE A FOCUS FOR POTENTIAL FROST/FREEZE HEADLINES PRIMARILY
ACROSS PORTIONS OF ILLINOIS.
WHILE SATURDAY WILL FEATURE FULL SUNSHINE AND WEAK WINDS (AS
THE SURFACE HIGH SLIDES DIRECTLY OVERHEAD), THE VERY CHILLY
START TO THE DAY WILL SERVE AS A LOW STARTING POINT FOR
TEMPERATURES LEADING TO AFTERNOON HIGHS GENERALLY IN THE 50S.
SATURDAY NIGHT WILL BE CHILLY ONCE AGAIN, THOUGH THE COLDEST
TEMPERATURES WILL FAVOR AREAS EAST OF I-55 WHERE WINDS WILL BE
WEAKEST ALONG THE BACKSIDE OF THE SURFACE HIGH. CAN ENVISION A
NEED FOR FROST HEADLINES AGAIN WHERE TEMPERATURES ARE COLDEST.
SOUTHWESTERLY RETURN FLOW WILL BECOME ESTABLISHED BY SUNDAY
ALONG THE BACKSIDE OF THE SURFACE HIGH PRESSURE SYSTEM MARKING
THE START OF A WARMING TREND INTO NEXT WEEK.
BORCHARDT
AVIATION /12Z TAFS THROUGH 18Z FRIDAY/
ISSUED AT 627 AM CDT THU OCT 15 2026
FORECAST CONCERNS FOR THE TERMINALS...
- INCREASING RAIN CHANCES THIS MORNING WITH SHOWERS EXPECTED TO
PERSIST THROUGH THIS EVENING.
- CEILINGS GRADUALLY LOWERING TO IFR THROUGH THE MORNING. A
PERIOD OF LIFR CONDITIONS IS EXPECTED AS THE SURFACE LOW
PASSES OVERHEAD LATE THIS AFTERNOON THROUGH EARLY EVENING.
- GUSTY NORTHWEST WINDS DEVELOPING AFTER MIDNIGHT WITH PEAK
GUSTS AROUND 20-25 KTS, BUT LOCALLY HIGHER GUSTS ARE POSSIBLE.
A STORM SYSTEM CONTINUES TO DEVELOP IN THE SOUTHERN PLAINS THIS
MORNING AND WILL BE MOVING INTO NORTHERN IL AND NORTHWEST IN
LATER TODAY. AHEAD OF THE DEVELOPING SYSTEM, WIDESPREAD SHOWERS
HAVE DEVELOPED ACROSS IA AND MO AND THESE SHOWERS WILL SPREAD
INTO THE TERMINALS AFTER 15Z (13Z AT RFD) THIS MORNING AND
PERSIST OFF AND ON THROUGH THIS EVENING. AS RAIN BEGINS,
CEILINGS ARE EXPECTED TO GRADUALLY LOWER TO IFR AROUND MIDDAY
WITH VISIBILITIES GENERALLY REMAINING IN THE MVFR (3-5 MILE)
RANGE. HOWEVER, A PERIOD OF LIFR CONDITIONS IS EXPECTED AS THE
SURFACE LOW PASSES DIRECTLY OVERHEAD AND LEADS TO THE
DEVELOPMENT OF SOME DRIZZLE LATER THIS AFTERNOON INTO THE EARLY
EVENING HOURS. GIVEN THAT THE LIFR CONDITIONS SHOULD BE TIED TO
THE CORE OF THE SURFACE LOW (AS NOTED IN UPSTREAM OBSERVATIONS)
HAVE OPTED TO HANDLE THESE CONDITIONS WITH TEMPO GROUPS IN THE
TAFS CENTERED AROUND WHEN THE CENTER OF THE LOW WILL BE OVER
EACH TERMINAL. AS THE SURFACE LOW DEPARTS, THE SYSTEMS COLD
FRONT WILL SWING THROUGH AND SHOULD ALLOW CEILINGS TO SLOWLY
BEGIN TO IMPROVE THOUGH SOME IFR CEILINGS MAY LINGER UNTIL RAIN
FULLY SUBSIDES AFTER MIDNIGHT. ONCE THE RAIN TAPERS, CEILINGS
WILL RISE TO MVFR AND REMAIN AS SUCH THROUGH THE REST OF THE TAF
PERIOD.
AS FOR THE WINDS, EASTERLY WINDS AROUND 10 KTS WILL PREVAIL
THROUGH THE MORNING BEFORE DIRECTIONS TURN MORE SOUTH-SOUTHEAST
THIS AFTERNOON BENEATH THE SURFACE LOW. IN FACT, WINDS MAY EVEN
BECOME LIGHT AND VARIABLE FOR A SHORT TIME AS THE LOW PASSES
DIRECTLY OVER THE TERMINALS. WINDS WILL THEN TURN WEST-SOUTHWEST
BEHIND THE LOW BEFORE SETTLING INTO A NORTHWEST DIRECTION BEHIND
THE SYSTEM'S COLD FRONT. SPEEDS WILL ALSO QUICKLY INCREASE
BEHIND THE FRONT WITH GUSTS PEAKING IN THE 20-25 KT RANGE
TONIGHT INTO FRIDAY MORNING. HOWEVER, IF DEEPER MIXING IS
ACHIEVED THERE IS A CHANCE FOR SOME LOCALLY HIGHER GUSTS AROUND
30 KTS.
FINALLY, AN ADDITIONAL PERIOD OF SCATTERED SHOWERS IS FORECAST
FOR FRIDAY AS THE UPPER TROUGH DIVES THROUGH THE GREAT LAKES.
RIGHT NOW IT APPEARS THAT MOST OF THESE SHOWERS SHOULD HOLD OFF
UNTIL AFTER THE END OF THE 30-HOUR TAF SO HAVE MAINTAINED DRY
CONDITIONS AFTER MIDNIGHT TONIGHT. THAT SAID, A FORMAL MENTION
FOR THESE SHOWERS WILL LIKELY BE NEEDED IN FUTURE TAFS.
YACK
MARINE
ISSUED AT 308 AM CDT THU OCT 15 2026
OVER THE NEXT 24 TO 48 HOURS, THE FIRST FALL STORM SYSTEM OF
THE SEASON WILL MOVE THROUGH THE GREAT LAKES. AS THE LOW LIFTS
FROM NEAR KANSAS CITY TO CHICAGO TODAY, EAST-SOUTHEASTERLY WINDS
WILL INCREASE ACROSS SOUTHERN LAKE MICHIGAN LEADING TO WAVES
GENERALLY OF 3 TO 5 FEET ALONG THE ILLINOIS SHORELINE. WESTERLY
WINDS WILL INCREASE IN MAGNITUDE AFTER SUNSET ON THE IMMEDIATE
BACKSIDE OF THE SYSTEM AND BEHIND A COLD FRONT WITH FREQUENT
GUSTS OF 25 TO 30 KT AND A FEW GALE FORCE GUSTS TO 35 KT.
FRIDAY AFTERNOON, A SECONDARY COLD FRONT WILL SURGE DOWN LAKE
MICHIGAN LEADING TO A CHANGE IN WIND DIRECTION TO NORTHERLY AND
RAPID INCREASE IN WIND SPEEDS. SUSTAINED WINDS OF 30 TO 35 KT
WITH FREQUENT GUSTS TO 40 KT ARE EXPECTED, WITH LOCALLY HIGHER
GUSTS IN EXCESS OF 45 KT POSSIBLE ON THE WEST SIDE OF LAKE
EFFECT RAIN SHOWERS. THE STRONGEST WIND GUSTS ARE EXPECTED
FRIDAY AFTERNOON AND EVENING BEFORE GRADUALLY TAPERING AFTER
MIDNIGHT EARLY SATURDAY MORNING. A GALE WARNING IS IN EFFECT
FROM 1 PM TO 1 AM FRIDAY NIGHT INTO SATURDAY MORNING.
FINALLY, STRONG LAKE INDUCED INSTABILITY MAY SUPPORT
WATERSPOUTS NEAR LAKE EFFECT SHOWERS FRIDAY AFTERNOON AND
EVENING.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 1 PM CDT FRIDAY FOR THE IL AND IN
NEARSHORE WATERS.
GALE WARNING FROM 1 PM FRIDAY TO 1 AM CDT SATURDAY FOR THE IL
AND IN NEARSHORE WATERS.
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