004
FXUS63 KLOT 171659
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1159 AM CDT WED SEP 17 2025
KEY MESSAGES
- WARM AND DRY CONDITIONS PERSIST THROUGH FRIDAY.
- PERIODIC SHOWER AND THUNDERSTORM CHANCES RETURN THIS WEEKEND
INTO EARLY NEXT WEEK (20-40%).
DISCUSSION
ISSUED AT 339 AM CDT WED SEP 17 2025
THROUGH FRIDAY...
OUR BLOCKING PATTERN WILL REMAIN IN PLACE THROUGH THURSDAY WITH
HIGH PRESSURE EXPECTED TO BE THE DOMINANT FEATURE OVER THE
GREAT LAKES. THEREFORE, DRY AND WARM CONDITIONS WILL PREVAIL
THROUGH THURSDAY WITH AFTERNOON HIGHS IN THE MID TO UPPER 80S
INLAND, BUT ONLY IN THE UPPER 70S TO AROUND 80 NEAR THE LAKE DUE
TO ONSHORE WINDS. THAT SAID, SOME PATCHY FOG IS DEVELOPING
ACROSS PORTIONS OF NORTHEAST IL AND NORTHWEST IN. WITH
TEMPERATURE-DEW POINT SPREADS ACROSS THE AREA STILL 3-5 DEGREES
IN MANY PLACES FOG HAS BEEN STRUGGLING, BUT AS TEMPERATURES
CONTINUE TO COOL UNDER CLEAR SKIES SUSPECT FOG COVERAGE WILL
INCREASE TOWARDS DAYBREAK. GIVEN THAT SHALLOW NATURE OF THE
MOISTURE PROFILES SUSPECT MOST FOG WILL BE THIN, HOWEVER,
LOCALLY DENSE FOG (VISIBILITIES UNDER 1/4 MILE) IS POSSIBLE
PARTICULARLY EAST OF THE FOX VALLEY AND NEAR THE LAKE WHERE AN
AXIS OF HIGHER DEW POINTS RESIDES. ANY FOG THAT DEVELOPS EARLY
THIS MORNING WILL ERODE BY 8-9 AM CDT AND GIVE WAY TO
AFOREMENTIONED SUNNY AND WARM CONDITIONS.
HEADING INTO FRIDAY, THE BLOCKING PATTERN IS FORECAST TO BEGIN
TO BREAK DOWN AS THE BROAD TROUGH OVER THE NORTH-CENTRAL CONUS
PROGRESSES EASTWARD INTO THE MISSISSIPPI RIVER VALLEY. WHILE
THIS MEANS OUR DOME OF HIGH PRESSURE WILL BEGIN TO EXIT ON
FRIDAY, LINGERING SUBSIDENCE AND DRY AIR LOOKS TO KEEP THE AREA
RAIN-FREE FOR AT LEAST ONE MORE DAY. THOUGH, SOME SCATTERED
SHOWERS AND THUNDERSTORMS MAY ATTEMPT TO DEVELOP (20% CHANCE)
BETWEEN I-39 AND THE MISSISSIPPI RIVER CLOSER TO THE TROUGH AND
A DEVELOPING FRONTAL BOUNDARY. AS FOR TEMPERATURES, HIGHS ON
FRIDAY LOOK TO BE IN THE MID TO UPPER 80S ONCE AGAIN BUT A
BACKDOOR COLD FRONT IS FORECAST TO SURGE DOWN THE LAKE AND
RESULT IN COOLING TEMPERATURES NEAR THE LAKE FRIDAY AFTERNOON
AND EVENING, ESPECIALLY ALONG THE IL SHORELINE.
FRIDAY NIGHT THROUGH TUESDAY...
THE AFOREMENTIONED TROUGH WILL CONTINUE TO MOVE EAST FRIDAY
NIGHT INTO SATURDAY WHICH WILL FORCE THE ASSOCIATED FRONTAL
BOUNDARY INTO NORTHEAST IL AND NORTHWEST IN. THIS SHOULD RESULT
IN INCREASING CHANCES FOR SHOWERS AND THUNDERSTORMS THROUGH THE
WEEKEND AS THE FRONT STALLS OVER THE AREA AND THE TROUGH PHASES
WITH A SECONDARY SHORTWAVE THAT IS FORECAST TO DIVE ACROSS THE
UPPER MIDWEST SATURDAY NIGHT INTO SUNDAY. WHILE A SURGE OF
MOISTURE IS EXPECTED TO ADVECT INTO IL AND IN AHEAD OF THE
FRONT, LOW-LEVEL WIND FIELDS THROUGH THE WEEKEND GENERALLY LOOK
TO REMAIN OUT OF THE EAST-SOUTHEAST WHICH MAY CONTINUE TO ADVECT
IN SOME DRIER AIR AS WELL. THEREFORE, THE COVERAGE OF SHOWERS
AND THUNDERSTORMS EACH DAY MAY BE MORE SCATTERED (HIT AND MISS)
IN NATURE AND OFFER PERIODS OF DRY WEATHER TOO. IT WAS FOR THIS
REASON THAT 20-30% POPS HAVE BEEN MAINTAINED FOR SATURDAY AND
SUNDAY.
THAT SAID, A MORE AMPLIFIED TROUGH LOOKS TO EJECT OUT OF THE
NORTHERN PLAINS ON MONDAY AND MOVE ACROSS THE GREAT LAKES MONDAY
NIGHT THROUGH TUESDAY. ASSOCIATED WITH THIS FEATURE SHOULD BE A
MORE FORMIDABLE COLD FRONT WHICH SHOULD BRING A BROADER
COVERAGE OF SHOWERS AND THUNDERSTORMS TO THE AREA DURING THIS
TIMEFRAME. ON THE BRIGHT SIDE, WIND SHEAR VALUES AND PWATS THIS
WEEKEND THROUGH TUESDAY LOOK TO BE RATHER MODEST SO THE THREAT
FOR ANY SEVERE WEATHER AND/OR HEAVY RAINFALL LOOKS LOW AT THIS
TIME. ADDITIONALLY, THE AFOREMENTIONED SOUTHEAST WINDS LOOK TO
KEEP HIGH TEMPERATURES IN THE 80S THROUGH THE EARLY PART OF NEXT
WEEK.
LOOKING TOWARDS THE MIDDLE OF NEXT WEEK, THE FORECAST BECOMES
MUCH MORE UNCERTAIN AS GUIDANCE IS STRUGGLING WITH THE EVOLUTION
OF THE MONDAY/TUESDAY TROUGH. SOME GUIDANCE SUGGESTS THE TROUGH
WILL MORPH INTO AN UPPER LOW AND REMAIN PARKED OVER THE GREAT
LAKES THROUGH A GOOD PORTION OF NEXT WEEK WHILE OTHERS SUGGEST A
RETURN OF UPPER RIDGING. IF THE UPPER LOW SOLUTIONS MATERIALIZE
THEN COOLER AND WETTER CONDITIONS WOULD BE FAVORED, BUT IF THE
RIDGING COMES TO FRUITION THEN A RETURN TO WARM AND DRY
CONDITIONS WOULD BE FAVORED. WITH THE HIGH DEGREE OF UNCERTAINTY
AT THIS RANGE RECOMMEND CHECKING BACK FOR MORE DETAILS AS WE
GET CLOSER IN TIME.
YACK
AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 1159 AM CDT WED SEP 17 2025
THERE ARE NO SIGNIFICANT AVIATION WEATHER CONCERNS ANTICIPATED
THROUGH THE PERIOD.
SURFACE HIGH PRESSURE WILL REMAIN DOMINANT ACROSS THE GREAT
LAKES REGION THROUGH THE PERIOD. THIS WILL FOSTER LIGHT WINDS,
AND FAVOR INLAND SHIFTING LAKE BREEZES WITH EASTERLY WINDS BOTH
TODAY AND AGAIN ON THURSDAY. OTHERWISE, ASIDE FROM SOME PATCHY
GROUND FOG POTENTIAL OUTSIDE OF THE MAIN CHICAGO TERMINALS
TOWARDS DAYBREAK THURSDAY MORNING, VFR CONDITIONS ARE EXPECTED
THROUGH THE PERIOD.
KJB
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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