286
FXUS63 KLOT 211919
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
219 PM CDT THU AUG 21 2025
KEY MESSAGES
- AREAS OF FOG (DENSE IN SPOTS) WILL DEVELOP OUTSIDE OF CHICAGO
LATE TONIGHT AND LINGER INTO FRIDAY MORNING.
- LITTLE TO NO IMPACTFUL WEATHER IS EXPECTED THROUGH THE
UPCOMING WEEKEND AND EARLY TO MID NEXT WEEK.
- AN EARLY FALL TEMPERATURE PREVIEW REMAINS IN STORE SUNDAY-
WEDNESDAY.
DISCUSSION
ISSUED AT 219 PM CDT THU AUG 21 2025
THROUGH FRIDAY:
THE MAIN FORECAST CONCERN IN THE NEAR TERM CONTINUES TO CENTER
AROUND THE THREAT FOR FOG DEVELOPMENT LATE TONIGHT INTO EARLY
FRIDAY MORNING.
LIGHT WINDS AND MAINLY CLEAR SKIES ARE EXPECTED TONIGHT AS A
SURFACE HIGH BECOMES CENTERED OVERHEAD. THIS WILL FOSTER
EFFICIENT RADIATIONAL COOLING AND RESULT IN A RATHER COOL NIGHT
AS TEMPERATURES BOTTOM OUT IN THE MID 50S TO AROUND 60 OUTSIDE
OF THE CHICAGO URBAN HEAT ISLAND. THIS IS PROBLEMATIC, AS
AFTERNOON DEWPOINTS REMAIN IN THE LOW TO MID 60S. WITH LITTLE
CHANGE IN THESE VALUES EXPECTED THROUGH THE REMAINDER OF THE
AFTERNOON, IT APPEARS THAT MOST AREAS OUTSIDE OF CHICAGO WILL
FALL SEVERAL DEGREES BELOW THEIR CROSS OVER TEMPERATURES LATE
TONIGHT. ACCORDINGLY, FOG DEVELOPMENT APPEARS LIKELY (70%+
CHANCE) OVERNIGHT INTO EARLY FRIDAY MORNING, AND THE FOG COULD
BECOME DENSE (VISIBILITY UNDER A 1/4 MILE) AND IMPACT EARLY
MORNING TRAVEL IN PARTS OF THE AREA. WITH THIS IN MIND, I
CONTINUED THE TREND OF HITTING THE FOG A BIT HARDER IN THE GRIDS
AND IN OUR WEATHER STORY.
FOLLOWING THE EARLY MORNING FOG ON FRIDAY, EXPECT ANOTHER
RATHER PLEASANT LATE AUGUST DAY ACROSS THE AREA. HIGH PRESSURE
WILL REMAIN OVERHEAD DURING THE DAY AND WILL CONTINUE TO PROMOTE
LIGHT WINDS. TEMPERATURES ON FRIDAY WILL BE SIMILAR TO TODAY,
WITH AFTERNOONS READINGS TOPPING OUT AROUND 80 (SLIGHTLY COOLER
LAKESIDE). DRY WEATHER IS EXPECTED, THOUGH PARTLY CLOUDY SKIES
ARE EXPECTED TO DEVELOP INTO THE AFTERNOON AS SOME FAIR WEATHER
STRATO CU DEVELOPS.
KJB
FRIDAY NIGHT THROUGH THURSDAY:
A STRONG UPPER LOW AND AN ASSOCIATED SURFACE LOW WILL SHIFT
EAST FROM MANITOBA INTO ONTARIO FRIDAY NIGHT. AS IT DOES, THE
ASSOCIATED COLD FRONT WILL BE APPROACHING OUR REGION FROM THE
UPPER MIDWEST BY EARLY SATURDAY MORNING. SHOWERS AND
THUNDERSTORMS ARE LIKELY TO ACCOMPANY THIS FRONTAL BOUNDARY AS
IT MOVES ACROSS THE UPPER MIDWEST LATE FRIDAY INTO FRIDAY NIGHT.
HOWEVER, WITH THE FRONT LOOKING TO MOVE INTO OUR AREA INTO
EARLY SATURDAY MORNING DURING A DIURNALLY UNFAVORABLE TIME, THE
ACTIVITY LOOKS TO WEAKEN CONSIDERABLY, IF NOT DIMINISH
ALTOGETHER, AS THE FRONT MOVES INTO NORTHERN IL SATURDAY
MORNING. ACCORDINGLY, THE THREAT FOR SHOWERS AND STORMS IN OUR
AREA IS LOW (20% OR LESS) AT THIS POINT, AND COULD BECOME MORE
FOCUSED SOUTHEAST OF I-55 SATURDAY AFTERNOON. OTHERWISE, EXPECT
ANOTHER MILD DAY SATURDAY WITH HIGHS AGAIN TOPPING OUT AROUND
80.
THE AFOREMENTIONED CANADIAN UPPER LOW WILL CARVE OUT DEEP MID-
UPPER LEVEL TROUGH ACROSS THE GREAT LAKES SUNDAY INTO THE FIRST
HALF OF NEXT WEEK. THIS WILL FAVOR A PERIOD OF BELOW AVERAGE
TEMPERATURES NEXT WEEK AS A MUCH COOLER AUTUMN-LIKE AIRMASS
SHIFTS INTO THE AREA ON BREEZY WEST-NORTHWESTERLY WINDS. HIGH
TEMPERATURES IN THIS AIRMASS WILL BE FAVORED TO ONLY BE IN THE
LOW TO MID 70S. OVERNIGHT LOWS WILL BE IN THE UPPER 40S (A FEW
LOCALIZED MID 40S?) TO LOWER 50S OUTSIDE OF CHICAGO! IN
ADDITION, AN EARLY SEASON LAKE EFFECT RAIN (AND EVEN
THUNDERSTORM) EPISODE MAY UNFOLD PRIMARILY TO OUR EAST AND
NORTHEAST OVER WESTERN LOWER MICHIGAN AND NORTH CENTRAL INDIANA
SUNDAY THROUGH MONDAY NIGHT. SOME LAKE EFFECT SHOWERS MAY SNEAK
INTO PORTER COUNTY INDIANA AT TIMES DEPENDENT UPON LOW-LEVEL
FLOW/BOUNDARY LAYER CONVERGENCE ORIENTATION SHIFTING MORE
NORTHERLY.
BELOW NORMAL, BUT COMFORTABLE/PLEASANT, TEMPERATURES SHOULD
PERSIST THROUGH AT LEAST MID NEXT WEEK BEFORE READINGS RETURN
BACK TO NEAR TO SLIGHTLY ABOVE SEASONAL NORMS HEADING INTO LABOR
DAY WEEKEND. ALSO IMPORTANTLY, PRIMARILY DRY CONDITIONS SHOULD
PREVAIL THROUGH NEXT WORK WEEK, GIVING MORE TIME TO DRY OUT FROM
THE RECENT HEAVY RAINFALL.
KJB/CASTRO
AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1221 PM CDT THU AUG 21 2025
FORECAST CONCERNS FOR THE TERMINALS...
- LINGERING MVFR CEILINGS THROUGH 19Z.
- FOG TO DEVELOP ACROSS INTERIOR IL AND NORTHWEST IN TONIGHT
RESULTING IFR VISIBILITIES, LIFR VISIBILITY POSSIBLE.
- LAKE BREEZE TO MOVE THROUGH CHICAGO TERMINALS AND GYY FRIDAY
AFTERNOON.
SCATTERED TO BROKEN 2500-3500 FT CEILINGS CONTINUE TO BE
OBSERVED THIS AFTERNOON, BUT CLOUD BASES WILL RISE CLOSER TO
3500 FT BY 19Z. OTHERWISE, 10-12 KT EASTERLY WINDS CAN BE
EXPECTED THIS AFTERNOON BEFORE WINDS BECOME LIGHT AND VARIABLE
THIS EVENING AND OVERNIGHT AS A SURFACE HIGH MOVES OVERHEAD.
AS WINDS EASE, TEMPERATURES SHOULD QUICKLY COOL NEAR THE DEW
POINTS AND RESULT IN THE FORMATION OF FOG ACROSS MUCH OF
INTERIOR IL AND NORTHWEST IN SOME OF WHICH COULD BE DENSE. IN
FACT, THERE IS A STRONG SIGNAL IN GUIDANCE (50-60% CHANCE) FOR
IFR VISIBILITIES TO DEVELOP TONIGHT WITH A 20% CHANCE FOR LIFR
VISIBILITIES ESPECIALLY NEAR RFD. OVERALL PREVAILING TAFS SEEM
TO CAPTURE FOG TRENDS WELL SO HAVE MAINTAINED THE GOING FORECAST
WITH IFR AND LIFR CONDITIONS FORECAST FOR DPA AND RFD
RESPECTIVELY. WHILE SOME PATCHY FOG CANNOT BE RULE OUT AT THE
CHICAGO TERMINALS, THE LARGE TEMPERATURE-DEW POINT SPREADS
FORECAST MAKE FOG FORMATION HERE LOWER CONFIDENCE AND THUS HAVE
FOREGONE A FORMAL MENTION. REGARDLESS, FOG IS EXPECTED TO ERODE
BETWEEN 13-15Z FRIDAY MORNING RESULTING IN VFR CONDITIONS FOR
THE REST OF THE PERIOD.
FINALLY, A LAKE BREEZE IS EXPECTED TO MOVE INLAND FRIDAY
AFTERNOON WHICH WILL RESULT IN AN EASTERLY WIND SHIFT AT THE
CHICAGO TERMINALS AND GYY. THOUGH SPEEDS SHOULD GENERALLY REMAIN
LIGHT IN THE 5-7 KT RANGE.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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