207
FXUS63 KLOT 112341
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
641 PM CDT THU SEP 11 2025
KEY MESSAGES
- AREAS OF FOG MAY DEVELOP NEAR LAKE MICHIGAN AND MOVE INLAND
AFTER MIDNIGHT TONIGHT, PRIMARILY ACROSS FAR NORTHEASTERN
ILLINOIS.
- THERE IS A 20% CHANCE FOR A FEW STORMS IN THE GENERAL REGION
FRIDAY NIGHT THROUGH EARLY SUNDAY MORNING. WIDESPREAD AND
SOAKING RAINFALL IS NOT EXPECTED.
- OTHERWISE, A GENERALLY DRY AND WARM PATTERN WILL PREVAIL
THROUGH THE WEEKEND AND INTO NEXT WEEK.
DISCUSSION
ISSUED AT 223 PM CDT THU SEP 11 2025
OVERVIEW:
A LARGE AREA OF SURFACE HIGH PRESSURE CENTERED OVER
SOUTHEASTERN CANADA CONTINUES TO SERVES AS THE PRIMARY INFLUENCE
ON THE PREVAILING WEATHER PATTERN ACROSS THE GREAT LAKES. LIGHT
EASTERLY (ONSHORE) FLOW, RELATIVELY LOW HUMIDITY LEVELS, AND
MOSTLY SUNNY SKIES ARE HENCE THE NORM, WITH CURRENT TEMPERATURE
READINGS IN THE UPPER 70S TO LOWER 80S AREAWIDE. TAKING A STEP
BACK, WATER VAPOR IMAGERY DEPICTS DEPARTING UPPER-LEVEL
TROUGHING AHEAD OF A BUILDING RIDGE IN THE CENTRAL PLAINS. AN
ELONGATED UPPER-LEVEL TROUGH EXTENDS FROM THE NORTHERN ROCKY
MOUNTAINS TOWARD THE CALIFORNIA COAST. WITH OUR AREA EXPECTED TO
BE IN THE REGION CHARACTERIZED BY INITIALLY POSITIVE AND THEN
NEUTRAL HEIGHT TENDENCIES, GENERALLY QUIET WEATHER WILL BE THE
NORM THROUGH THE WEEKEND AND INTO EARLY NEXT WEEK.
TONIGHT:
WITH LITTLE CHANGE IN THE SURFACE TO UPPER-LEVEL PATTERN
TONIGHT, THE EXPECTATION IS FOR SIMILAR CONDITIONS TO LAST NIGHT
WITH PARTLY CLOUDY SKIES AND OVERNIGHT LOWS IN THE 60S. WE'LL
HAVE TO WATCH FOR AREAS OF LOW CLOUDS AND FOG TO STREAM INLAND
OFF LAKE MICHIGAN AFTER MIDNIGHT AS LIGHT NORTHEASTERLY FLOW
TRANSPORTS THE RELATIVELY MORE MOIST MARINE AIRMASS INLAND. AT
THIS POINT, WILL OFFER PATCHY FOG ACROSS MUCH OF THE AREA WITH
AREAS OF FOG ACROSS FAR NORTHEASTERN ILLINOIS (WHERE MODEL
GUIDANCE IS MOST BULLISH FOR FOG).
TOMORROW:
TOMORROW LOOKS QUITE SIMILAR TO TODAY WITH ONLY A LITTLE
SOUTHEASTWARD JOG IN THE POSITIONING OF THE SURFACE HIGH
PRESSURE SYSTEM AND A MODEST RISE IN UPPER-LEVEL HEIGHTS. HIGHS
SHOULD BE A DEGREE OR TWO WARMER THAN TODAY, A CONSEQUENCE OF
BOTH A SLIGHTLY WARMER START TO MORNING TEMPERATURES (OWING TO
MORE CLOUD COVER TONIGHT) AND THE START OF THE STRENGTHENING OF
A LOW-LEVEL THERMAL RIDGE (850MB TEMPERATURES WARM SOME 2-3C IN
THE NEXT 24 HOURS). SO, WILL ADVERTISE HIGHS IN THE MID TO UPPER
80S ACROSS THE AREA. A WEAK LOW-LEVEL PRESSURE GRADIENT SHOULD
ALLOW FOR A LAKE BREEZE TO MOVE INLAND DURING THE AFTERNOON,
LEADING TO LOCALIZED COOLING ALONG THE SHORE.
FRIDAY NIGHT INTO SATURDAY:
BY FRIDAY NIGHT, THE CENTRAL AXIS OF THE BUILDING UPPER-LEVEL
RIDGE WILL STILL BE POSITIONED FROM MINNESOTA THROUGH MISSOURI
REGION. AS A RESULT, THE GREAT LAKES WILL BE POSITIONED BENEATH
NORTHWESTERLY FLOW ALOFT AND ON THE EASTERN FRINGE OF A MODEST
EML PLUME, THE COMBINATION OF WHICH CAN SUPPORT EPISODES OF
CONVECTION TIED TO PASSING UPPER-LEVEL SHORTWAVES. BOTH ENSEMBLE
AND DETERMINISTIC GUIDANCE CONTINUES TO ADVERTISE ONE SUCH WAVE
MOVING THROUGH THE GENERAL REGION IN THE FRIDAY NIGHT THROUGH
SATURDAY NIGHT TIMEFRAME, BUT DIFFER ON EXACT LOCATION,
STRENGTH, AND ARRIVAL TIMING. WHILE FORECAST SOUNDINGS ACROSS
THE AREA DEPICT SOMEWHAT MEAGER LOW- TO MID- LEVEL MOISTURE
(WHICH MAY PREVENT THE ACTIVATION OF THE INSTABILITY PLUME),
FELT IT WOULD BE UNWISE TO REMOVE THE INHERITED BROAD-BRUSHED
LOW-END CHANCES (15-20%) FOR THUNDERSTORMS STARTING FRIDAY
NIGHT LASTING ALL THE WAY THROUGH EARLY SUNDAY MORNING.
OUTSIDE THE LOW CHANCES FOR THUNDERSTORMS, SATURDAY LOOKS LIKE
ANOTHER WARM DAY WITH A CONTINUED STRENGTHENING OF THE LOW-
LEVEL THERMAL RIDGING SUPPORTING HIGHS IN THE UPPER 80S TO
AROUND 90. SOUTH TO SOUTHWESTERLY FLOW OF AROUND 10 MPH SHOULD
LIMIT THE INLAND PENETRATION OF THE LAKE BREEZE, THOUGH
SHORELINE LOCATIONS SHOULD STILL ACHIEVE SOME 5 TO 10 DEGREES
OF COOLING BY MID AFTERNOON.
SUNDAY AND BEYOND:
EARLY NEXT WEEK, THE UPPER-LEVEL PATTERN ACROSS THE WESTERN US
WILL UNDERGO AN INTERESTING EVOLUTION AS A SERIES OF INTERACTING
SHORT-WAVELENGTH PACIFIC SHORTWAVES CAUSE THE BACKWARD
PROPAGATION OF MEAN TROUGHING TOWARD THE STRAITS OF ALASKA. AT
THE SAME TIME, PROLONGED MID-LEVEL WARM AIR ADVECTION ACROSS THE
CENTRAL US AHEAD OF INCREASINGLY-PINCHED OFF WESTERN US-
TROUGHING WILL REINFORCE UPPER-LEVEL RIDGING INTO CENTRAL
CANADA. THE RESULTING PROCESSES WILL CAUSE THE UPPER-LEVEL
RIDGE AND ASSOCIATED POOL OF UNSEASONABLY WARM AIR TO
EFFECTIVELY STAGNATE OVER THE GREAT LAKES SUNDAY AND INTO EARLY
NEXT WEEK AS REMNANT TROUGHING GETS CUT-OFF ACROSS THE WEST.
TAKEN TOGETHER, CONFIDENCE IS RELATIVELY HIGH THAT TEMPERATURE
SUNDAY THROUGH AT LEAST TUESDAY WILL BE ABOVE AVERAGE WITH HIGHS
IN THE LOWER 90S AND OVERNIGHT LOWS IN THE MID 60S. WEAK COLUMN
WINDS (AT OR BELOW 20KT FROM THE SURFACE TO ABOUT 20KFT) WILL
PROVE LITTLE RESISTANCE TO DAILY LAKE BREEZES, LEADING TO
LOCALLY COOLER LOCATIONS ALONG THE SHORE. SOMEWHAT STAGNANT LOW-
LEVEL MOISTURE TRAPPED IN THE REGION AND MINIMAL CAPPING MAY
SUPPORT DAILY BOUTS OF ISOLATED SHOWERS AND STORMS IN THE
GENERAL REGION, AS WELL (THOUGH WILL NOTE BLENDED NBM GUIDANCE
CAME IN COMPLETELY DRY). IN ALL, THE WEATHER EARLY NEXT WEEK
LOOKS MORE REMINISCENT OF THE DOG DAYS OF SUMMER RATHER THAN MID
SEPTEMBER.
WITH THE MOST PROMINENT UPPER-LEVEL JET POSITIONED SO FAR NORTH
INTO CANADA AND A CONTINUED TENDENCY OF MEAN TROUGHING TO
RETROGRADE WESTWARD ACROSS THE CENTRAL PACIFIC, THE PATTERN NEXT
WEEK ACROSS THE CENTRAL UNITED STATES MAY TAKE QUITE SOME TIME
TO BREAK. WITH THAT SAID, ENSEMBLE GUIDANCE ADVERTISES A VERY
GRADUAL SLIDE IN TEMPERATURES AND A RETURN OF PRECIPITATION
CHANCES TOWARD THE END OF THE WEEK (SEPTEMBER 19 TO 21
TIMEFRAME), PRESUMABLY AS THE REMNANTS OF THE CUT-OFF WESTERN US
TROUGHING FINALLY APPROACH THE REGION.
BORCHARDT
AVIATION /00Z TAFS THROUGH 06Z SATURDAY/
ISSUED AT 641 PM CDT THU SEP 11 2025
THE MAIN CONCERN TONIGHT INTO EARLY FRIDAY MORNING IS THE
POTENTIAL FOR LOW STRATUS AND/OR FOG DEVELOPMENT. AT THIS TIME,
THE MAIN SIGNAL FOR MORE WIDESPREAD IFR/LIFR CONDITIONS REMAINS
ACROSS PARTS OF FAR NORTHERN ILLINOIS AND ESPECIALLY INTO
SOUTHERN WISCONSIN WHERE NEAR-SURFACE WINDS ARE FORECAST TO
REMAIN THE LIGHTEST. IN COORDINATION WITH THE CWSU, INTRODUCED
TEMPO MENTIONS FOR VFR BR AT ORD AND MDW GIVEN LATEST MODEL
TRENDS, AND CAN'T RULE OUT THE NEED TO INTRODUCE LOWER CIGS
AND/OR VSBYS IN FUTURE TAFS. NO MAJOR CHANGES ARE NEEDED TO THE
INHERITED BR/MIFG MENTIONS AT DPA AND GYY.
AT RFD, THE SIGNAL FOR AT LEAST SCT-BKN IFR/LIFR CIGS APPEARS
TO BE INCREASING, BUT THE PRESENCE OF 15 KTS OF FLOW
IMMEDIATELY ABOVE THE SURFACE CASTS UNCERTAINTY ON THE CIG/VSBY
FORECAST THERE. FOR THIS SET OF TAFS, ELECTED TO INTRODUCE A
HIGH-MVFR MENTION, AND WILL CLOSELY MONITOR MODEL AND UPSTREAM
OBSERVATION TRENDS TONIGHT.
ANY LOW STRATUS/BR WILL LIFT AND SCATTER QUICKLY FRIDAY MORNING
WITH LIGHT SE WINDS RETURNING. A LAKE BREEZE WILL SHIFT WINDS TO
THE ESE/E AT THE LAKE-ADJACENT TAFS DURING THE AFTERNOON.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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