354
FXUS63 KLOT 171110
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
610 AM CDT FRI OCT 17 2025
KEY MESSAGES
- PERIODS OF SHOWERS AND STORMS ARE EXPECTED STARTING LATE
TONIGHT AND LASTING THROUGH SUNDAY MORNING AS A FALL STORM
SYSTEM MOVES THROUGH THE AREA.
- PARTS OF THE AREA MAY PICK UP MUCH-NEEDED SOAKING RAINFALL
OVER THE WEEKEND. THE HIGHEST RAINFALL TOTALS (1-2+ INCHES)
ARE CURRENTLY FAVORED TO OCCUR NEAR AND EAST OF INTERSTATE 55.
- A PERIOD OF GUSTY NORTHWEST WINDS (GUSTS OF 30 TO 35 MPH) IS
EXPECTED ON SUNDAY. ADDITIONAL PERIODS OF BREEZY WINDS ARE
POSSIBLE ON MONDAY AND TUESDAY AS ANOTHER FALL STORM SYSTEM
MOVES THROUGH THE REGION.
- TEMPERATURES WILL BE ON A SEE-SAW TODAY THROUGH THE MIDDLE OF
NEXT WEEK AS THE SERIES OF FALL STORMS MOVES THROUGH THE
REGION.
DISCUSSION
ISSUED AT 357 AM CDT FRI OCT 17 2025
THROUGH SUNDAY:
BREEZY RETURN FLOW ON THE BACKSIDE OF A DEPARTING HIGH PRESSURE
SYSTEM WILL YIELD AN UNSEASONABLY MILD DAY ACROSS THE AREA TODAY
WITH HIGH TEMPERATURES RANGING FROM THE MID 70S TO THE LOW 80S,
WARMEST ACROSS OUR WESTERN COUNTIES AND COOLEST IN OUR EASTERN
COUNTIES. WEAK ISENTROPIC UPGLIDE MAY END UP GENERATING A FEW
SPRINKLES OR LIGHT RAIN SHOWERS IN FAR NORTHERN ILLINOIS THIS
MORNING (PARTICULARLY NEAR THE WISCONSIN STATE LINE), BUT
OTHERWISE, DRY CONDITIONS SHOULD PREVAIL THROUGH THE DAYTIME
HOURS.
THINGS WILL GET BUSIER WEATHER-WISE GOING INTO THE WEEKEND,
HOWEVER, AS A HANDFUL OF UPPER-LEVEL DISTURBANCES ENTER THE
REGION AND INTERACT WITH ONE ANOTHER. INITIALLY, A COMPACT,
CLOSED-OFF UPPER LOW PRESENTLY OVER THE DAKOTAS WILL DRIFT OFF
TOWARDS WESTERN ONTARIO TODAY WHILE AN ASSOCIATED SURFACE LOW
OCCLUDES AND FOLLOWS SUIT. THE ATTENDANT COLD FRONT/BAROCLINIC
ZONE EXTENDING TO THE SOUTH OF THE LOW WILL GRADUALLY SHUFFLE ITS
WAY EASTWARD AND WILL LIKELY REACH NORTHWESTERN ILLINOIS AT SOME
POINT TONIGHT. A STRENGTHENING LOW-LEVEL JET WILL LIKELY
INVIGORATE CONVECTION ALONG AND AHEAD OF THE FRONT AS ISENTROPIC
ASCENT/WARM AIR ADVECTION RAMPS UP COMMENSURATELY, AND THIS
CONVECTIVE CORRIDOR WILL SLIDE ACROSS OUR FORECAST AREA FROM WEST
TO EAST BETWEEN ABOUT MIDNIGHT AND MID-MORNING. WHILE MID-LEVEL
LAPSE RATES WILL BE SOMEWHAT LACKLUSTER, THEY SHOULD BE STEEP
ENOUGH TO PUSH EQUILIBRIUM LEVELS PAST -20C HEIGHTS AND POSSIBLY
SUPPORT A FEW NON-SEVERE THUNDERSTORMS WITHIN THE BROADER
CORRIDOR OF CONVECTIVE SHOWER ACTIVITY.
THERE MAY BE A RELATIVE LULL IN SHOWER ACTIVITY DURING THE LATE
MORNING AS THE AFOREMENTIONED LOW-LEVEL JET WEAKENS AND THE
INCREASINGLY ILL-DEFINED FRONTAL ZONE STALLS OUT SOMEWHERE OVER
THE SOUTHEASTERN HALF OF OUR CWA. HOWEVER, ANOTHER UPTICK IN
CONVECTIVE COVERAGE ALONG THE FRONTAL ZONE APPEARS LIKELY
TOMORROW AFTERNOON AS SURFACE TEMPERATURES WARM INTO THE MID 70S
AND A MORE PRONOUNCED UPPER-LEVEL TROUGH SWOOPS INTO THE
MIDWEST FROM THE NORTHERN PLAINS, PROVIDING A BOOST TO LARGE-
SCALE FORCING WHILE ALSO MARKEDLY STEEPENING MID-LEVEL LAPSE
RATES. INCREASING DEEP-LAYER SHEAR COULD ALLOW FOR SOME DEGREE
OF ORGANIZATION TO OCCUR WITH THE THUNDERSTORMS THAT DEVELOP IN
OUR SOUTHEASTERN CWA TOMORROW AFTERNOON, LEADING TO SOME
POTENTIAL FOR STRONG TO MARGINALLY SEVERE DOWNBURST WINDS AND
SMALL HAIL WITH THE STRONGEST STORMS.
FORCING FOR ASCENT WILL CONTINUE TO STRENGTHEN GOING INTO
TOMORROW EVENING AS THE AFOREMENTIONED POTENT NORTHERN STREAM
UPPER-LEVEL TROUGH ATTEMPTS TO PHASE WITH A WEAKER SOUTHERN
STREAM SHORTWAVE TROUGH WELL TO OUR SOUTH. A BROAD PRECIPITATION
SHIELD WILL LIKELY BLOSSOM TO OUR SOUTHWEST IN RESPONSE AND
LIFT NORTHEASTWARD AS THE MAIN TROUGH STARTS TO ACQUIRE A
NEGATIVE TILT AND THE NASCENT STAGES OF SURFACE CYCLOGENESIS
BEGIN TO TAKE SHAPE. VARIANCE ACROSS THE LATEST SUITE OF MODEL
GUIDANCE REMAINS QUITE LARGE FOR BEING 36-48 HOURS OUT OWING TO
UNCERTAINTY OVER HOW THE PHASING PROCESS BETWEEN THE TWO UPPER-
LEVEL DISTURBANCES WILL PLAY OUT, BUT THE MOST LIKELY SOLUTION
AT THIS TIME INVOLVES THE MAIN PRECIPITATION SHIELD STAYING
FOCUSED OVER THE SOUTHEASTERN HALF OF OUR FORECAST AREA SATURDAY
NIGHT INTO SUNDAY MORNING WHILE A SECONDARY BAND OF STRONGLY-
FORCED PRECIPITATION FORMS ALONG A REINFORCING COLD FRONT
ASSOCIATED WITH THE NORTHERN STREAM TROUGH AND SWEEPS ACROSS THE
REMAINDER OF OUR CWA IN TANDEM WITH THE FRONT.
IF THE PHASING PLAYS OUT SOMEWHAT QUICKLY AND CLEANLY, THEN
STRONG DEFORMATION WITHIN THE MAIN PRECIPITATION SHIELD COULD
LAY OUT A STRIPE OF 2+" RAINFALL TOTALS SOMEWHERE. IN SUCH A
SCENARIO, THERE IS A LOWER-END CHANCE THAT SOME MINOR HYDROLOGIC
CONCERNS ARISE WITHIN THE CORRIDOR OF WHERE HEAVIER RAINFALL
OCCURS (PARTICULARLY IF THIS HEAVIER RAINFALL ENDS UP OCCURRING
OVER THE MORE URBANIZED PORTIONS OF THE CHICAGO METRO). HOWEVER,
THE ANTECEDENT DRYNESS AND THE RELATIVELY LONG DURATION OF THE
RAINFALL SHOULD OTHERWISE PRECLUDE A MORE APPRECIABLE THREAT FOR
FLOODING FROM MATERIALIZING.
LASTLY, POST-FRONTAL NORTHWESTERLY WINDS COULD BECOME A BIT
STRONG ON SUNDAY WITH GUSTS NEARING 40 MPH AT TIMES NOT OUT OF
THE QUESTION IN A SCENARIO FEATURING A CLEANER PHASING OF THE
AFOREMENTIONED WAVES AND ASSOCIATED ASSOCIATED STRONGER SURFACE
CYCLOGENESIS. COLD AIR ADVECTION OFF OF THESE WINDS WILL RESULT IN
COOLER DAYTIME TEMPERATURES ON SUNDAY WITH MOST LOCATIONS IN OUR
FORECAST AREA CURRENTLY LOOKING LIKE THEY WON'T BREAK 60 DEGREES.
THE RAIN SHOULD COME TO END DURING THE DAYTIME ON SUNDAY AS HIGH
PRESSURE STARTS TO BUILD BACK INTO THE REGION.
OGOREK
SUNDAY NIGHT THROUGH THURSDAY:
THE UPPER-LEVEL PATTERN WILL REMAIN ENERGETIC IN THE WAKE OF
THE WEEKEND STORM SYSTEM WITH A SECONDARY UPPER-LEVEL TROUGH AND
ASSOCIATED SURFACE LOW POISED TO DIVE SOUTHEASTWARD INTO THE
UPPER MIDWEST IN THE MONDAY TO TUESDAY TIMEFRAME. OVER THE PAST
FEW MODEL CYCLES, ENSEMBLE GUIDANCE HAS TRENDED DEEPER/STRONGER
WITH THE SECONDARY TROUGH, WITH A GENERAL IDEA OF RAPID
SECONDARY LOW DEVELOPMENT TAKING PLACE SOMEWHERE IN THE UPPER
GREAT LAKES.
EVEN WITH THE USUAL VARIANCE IN THE EVOLUTION OF THE LOW
DEPICTED AMONG GUIDANCE, A SIGNAL IS GROWING FOR A PERIOD OF
BREEZY SOUTHWEST WINDS ON MONDAY (WAA AHEAD OF THE SYSTEM)
TRANSITIONING TO BREEZY WEST TO NORTHWEST WINDS ON TUESDAY (CAA
BEHIND THE SYSTEM). IN SCENARIOS WHERE THE SURFACE LOW IS STRONG
(SUB 1000MB) AND MOVES/WIGGLES CLOSE TO OUR AREA, IT IS EASY TO
ENVISION PEAK GUSTS IN EITHER REGIME TAGGING 40 TO 45 MPH, AS
HINTED BY 40KT OF FLOW AT THE TOP OF THE MIXED LAYER IN
DETERMINISTIC GFS AND ECMWF FORECAST SOUNDINGS. THIS'LL BE
SOMETHING TO KEEP AN EYE ON AS WE GET CLOSER. UNLIKE THE STORM
SYSTEM COMING THROUGH THIS WEEKEND, CHANCES FOR MEANINGFUL
RAINFALL WITH THE SECONDARY SYSTEM WILL BE RELEGATED TO A
QUICKILY-MOVING BAND OF SHOWERS ALONG THE SOUTHEASTWARD-SURGING
COLD FRONT MONDAY EVENING AND PERHAPS A FEW SHOWERS LINGERING
BEHIND AS THE MID-LEVEL COLD POOL ASSOCIATED WITH THE TROUGH
SLIDE OVERHEAD.
AFTER A RELATIVELY WARM MONDAY WITH HIGHS IN THE MID 60S,
TEMPERATURES BEHIND THE FRONT ON TUESDAY WILL BE NOTICEABLY
COOLER AND IN THE MID TO UPPER 50S. WE'LL HAVE TO WATCH FOR ANY
EMBEDDED SHORTWAVES TO MOVE OVER THE AREA ALONG THE BACKSIDE OF
THE UPPER-LEVEL TROUGH TO SUPPORT A DIURNAL FLARE IN SCATTERED
SHOWERS, AS IS TYPICAL THIS TIME OF YEAR. AS THE SYSTEM PULLS
AWAY TUESDAY NIGHT, TEMPERATURES WILL BE POISED TO FALL INTO THE
UPPER 30S TO LOWER 40S AWAY FROM THE LAKE. IN SCENARIOS WHERE
THE BACKSIDE OF THE CYCLONIC SHEAR AXIS PASSES OVERHEAD
(ENCOURAGING CLOUDS TO CLEAR) TUESDAY NIGHT, TEMPERATURES
OVERNIGHT AND INTO WEDNESDAY MORNING MAY MAKE A RUN FOR THE
FREEZING MARK.
FORECAST CONFIDENCE LOWERS FROM THE MIDDLE OF THE WEEK ONWARD.
WITH THAT SAID, ENSEMBLE MODEL GUIDANCE IS STARTING TO EXHIBIT A
SIGNAL FOR YET ANOTHER SYSTEM APPROACHING THE GENERAL AREA
TOWARD THE END OF THE WORKWEEK.
BORCHARDT
AVIATION /12Z TAFS THROUGH 18Z SATURDAY/
ISSUED AT 610 AM CDT FRI OCT 17 2025
A SURFACE HIGH PRESSURE SYSTEM WILL SLOWLY DEPART THE GREAT
LAKES REGION TODAY AS A SURFACE LOW OVER THE DAKOTAS LIFTS
NORTHEASTWARD INTO SOUTHERN CANADA. A TRAILING SURFACE FRONT IN
THE UPPER MIDWEST WILL CAUSE SURFACE TROUGHING TO ENCROACH
EASTWARD INTO THE GREAT LAKES, FORCING THE LOW-LEVEL PRESSURE
GRADIENT TO BOTH TIGHTEN AND REORIENT FROM SOUTHWEST TO
NORTHEAST. AS A RESULT, THE EXPECTATION IS FOR SURFACE WINDS TO
BECOME DECIDEDLY SOUTHWESTERLY AND INCREASE IN MAGNITUDE WITH
GUSTS OF 20-25KT WITHIN A FEW HOURS AFTER SUNRISE. A PARCHED
AIRMASS SHOULD RELEGATE CLOUDS TO THE MID TO UPPER-LEVELS,
LARGELY ABOVE 10KFT.
TONIGHT, THE SURFACE FRONT WILL SHIFT EASTWARD TOWARD THE
MISSISSIPPI RIVER IN TANDEM WITH THE EASTWARD SHOVE OF AN
UPPER-LEVEL SHEAR AXIS. MOST GUIDANCE DEPICTS THE EXCITATION OF
THE NOCTURNAL LOW-LEVEL JET AHEAD OF BOTH FEATURES, WITH
925-850MB FLOW PROGGED TO INCREASE TO 35 TO LOCALLY 40KT BY 06Z.
WITH THE DRY LOW-LEVEL AIRMASS FACILITATING EFFICIENT SURFACE
DECOUPLING, DO HAVE SOME CONCERN FOR MARGINAL LLWS THRESHOLDS
BEING MET FOR A FEW HOURS DURING THE OVERNIGHT HOURS. HOWEVER,
SUSPECT THE TIGHTENING LOW-LEVEL PRESSURE GRADIENT MAY ALLOW
FOR SURFACE WINDS TO REMAIN JUST ELEVATED ENOUGH (10-13KT OR SO)
TO LIMIT CRITERIA FROM BEING MET, SO WILL CONTINUE TO WITHHOLD
A FORMAL MENTION IN THE OUTGOING TAFS.
TOWARD THE END OF THE TAF PERIOD, INCREASING ISENTROPIC ASCENT
IMMEDIATELY AHEAD OF THE SLOWING SURFACE FRONT SHOULD ALLOW FOR
WIDELY SCATTERED HIGH-BASED SHOWERS TO DEVELOP NEAR THE
MISSISSIPPI RIVER AND SPREAD EASTWARD TOWARD/OVER THE TERMINALS
(LARGELY AFTER 06Z). A QUICK LOOK AT FORECAST SOUNDINGS SHOW
SKINNY AND LARGELY LOW-CENTROID INSTABILITY PROFILES (MOSTLY UNDER
THE MINUS 20C LEVEL), SUGGESTING COVERAGE OF THUNDER MAY BE MORE
ISOLATED THAN WIDESPREAD (CALL IT A 20 TO 30% CHANCE AT ANY
GIVEN TERMINAL). SO, WILL GO AHEAD AND ADVERTISE PREVAILING
SHOWERS STARTING JUST BEFORE DAYBREAK.
FINALLY, THERE IS A MODEST SIGNAL THAT CIGS MAY ATTEMPT TO
BUILD DOWNWARD INTO MVFR TOWARD THE VERY END OF THE 30-HOUR TAF
WINDOW AT ORD/MDW AS THE FRONTAL ZONE WASHES OUT OVER THE AREA.
FOR NOW, WILL TUCK IN A SCT018 IN FAVOR OF EVALUATING MODEL
TRENDS TO INFORM LATER TAF PACKAGES.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page