807
FXUS63 KLOT 290841
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
341 AM CDT TUE JUL 29 2025
KEY MESSAGES
- PATCHY, SHALLOW DENSE FOG NEAR AND SOUTH OF THE KANKAKEE RIVER
EARLY THIS MORNING.
- PERIODS OF THUNDERSTORMS THROUGH THURSDAY MORNING, SOME
POSSIBLY STRONG TO SEVERE WITH TORRENTIAL RAIN WHICH COULD
LEAD TO LOCALIZED FLASH FLOODING.
- DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
WEDNESDAY INTO THURSDAY, POSSIBLY INTO FRIDAY NIGHT.
DISCUSSION
ISSUED AT 340 AM CDT TUE JUL 29 2025
THROUGH WEDNESDAY:
PATCHY FOG HAS DEVELOPED ACROSS SOUTHEAST PORTIONS OF THE
FORECAST AREA EARLY THIS MORNING, MAINLY ALONG/SOUTHEAST OF THE
KANKAKEE RIVER VALLEY. FORECAST SOUNDINGS INDICATE THE MOIST
BOUNDARY LAYER IS FAIRLY SHALLOW, THOUGH A COMBINATION OF CALM
WINDS AND MAINLY CLEAR SKIES HAS ALLOWED SOME ISOLATED DENSE FOG
IN SPOTS. EXPECT THIS WILL PERSIST THROUGH SUNRISE, AND WILL
CONTINUE TO MONITOR EXTENT OF SUB-1/2SM VISIBILITIES IN CASE A
SMALL ADVISORY MAY BE NEEDED.
TURNING ATTENTION QUICKLY TO OUR WEST AND NORTHWEST, A SEVERE
MCS ACROSS EASTERN IA HAS BEGUN TO PROPAGATE MORE TO THE
SOUTHEAST RECENTLY, ALONG AN INSTABILITY GRADIENT WHICH IS
ORIENTED FROM NORTHEAST IA INTO WESTERN/CENTRAL IL. THIS SYSTEM
CONTINUES TO PRODUCE SEVERE WEATHER, WITH A FEW 65-75 MPH GUSTS
MEASURED ACROSS PARTS OF EASTERN IA OVER THE PAST HOUR OR SO.
PRIMARY PUSH OF STRONGER STORMS AND WIND SHOULD CONTINUE TO THE
SOUTHEAST INTO WESTERN IL, FOLLOWING THE INSTABILITY GRADIENT.
INSTABILITY DECREASES QUICKLY TO THE EAST, WHICH ALONG WITH
STRONGER CAPPING AND DRIER AIR ALOFT NOTED IN 00Z RAOBS SHOULD
ALLOW FOR WEAKENING AND EVENTUAL DISSIPATION OF CELLS WHICH
PROPAGATE OR DEVELOP INTO THE WFO LOT FORECAST AREA. THAT SAID,
CAM GUIDANCE CONTINUES TO BE TOO SLOW AND LIKELY TOO QUICK IN
DISSIPATING CONVECTION AS IT MOVES EASTWARD THIS MORNING,
PARTICULARLY WITH AN EMBEDDED MCV WHICH WILL LIKELY TRACK
NEAR/ACROSS OUR SOUTHWESTERN CWA. WILL CONTINUE TO MONITOR THESE
STORMS FOR WIND POTENTIAL EARLY THIS MORNING, WITH AT LEAST
SOME 25-35 MPH CONVECTIVELY ENHANCED SYNOPTIC WINDS LIKELY FROM
THE NNW ACROSS THE WESTERN HALF OF THE FORECAST AREA, AND
POTENTIALLY SOME STRONGER CONVECTIVE GUSTS. HAVE TRENDED POPS UP
FROM LIKELY IN THE WEST TO CHANCE INTO THE CHICAGO METRO TOWARD
SUNRISE.
ONCE SHOWERS AND STORMS EVENTUALLY DISSIPATE ACROSS THE AREA BY
MID- LATE MORNING, LITTLE IN THE WAY OF FORCING IS APPARENT FOR
MUCH IN THE WAY OF ORGANIZED CONVECTION THROUGH MUCH OF THE
DAY. COOLER OUTFLOW FOOTPRINT, COMBINED WITH INCREASED CLOUD
COVER THROUGH THE MORNING IS EXPECTED TO INHIBIT TEMPERATURES,
THOUGH WE SHOULD SEE EVENTUAL RECOVERY THIS AFTERNOON WITH HIGHS
IN THE UPPER 80S TO AROUND 90 EXPECTED. CONVECTIVELY DISTURBED
WIND FIELD LOOKS TO EVENTUALLY SUPPORT A LIGHT SOUTHEAST WIND,
THOUGH GUIDANCE HAS BACKED OFF OF A STRONGER LAKE BREEZE
REACHING TOO FAR INLAND. IT'LL STILL BE WARM AND HUMID, THOUGH
FORECAST HEAT INDICES TOP OUT IN THE 95-100F RANGE AND THUS NO
HEAT HEADLINES ARE ANTICIPATED.
SOME CAM GUIDANCE DOES HINT AT SCATTERED CONVECTIVE DEVELOPMENT
THIS AFTERNOON ACROSS PARTS OF IA, ACROSS SLOWLY RETREATING
OUTFLOW/EFFECTIVE FRONT TO OUR WEST. 4KM NAM AND RAP SUGGEST
THERE COULD BE SOME SPOTTY WEAKER SHRA/TS INTO OUR NORTHWESTERN
CWA, AND HAVE CARRIED SOME LOW-CONFIDENCE SLIGHT CHANCES (<25%)
MAINLY NORTHWEST OF I-88 ACROSS NORTH/NORTHWEST IL LATE THIS
AFTERNOON AND THIS EVENING. GREATER SHOWER AND THUNDERSTORM
POTENTIAL APPEARS TO HOLD OFF UNTIL TOWARD SUNRISE WEDNESDAY
HOWEVER, AS AN UPSTREAM MID- LEVEL SHORT WAVE TROUGH PROPAGATES
SLOWLY EASTWARD AND FURTHER FLATTENS THE UPPER LEVEL RIDGE
ACROSS THE MIDWEST. SHOWER AND THUNDERSTORM CHANCES WILL
CONTINUE THROUGH THE DAY WEDNESDAY IN FACT, AS THE WAVE SLOWLY
TRANSITS THE REGION ATOP A SOUTHWARD SAGGING BAROCLINIC/FRONTAL
ZONE. BLENDED NBM GUIDANCE HAS MAINTAINED LIKELY (~60%) POPS
ACROSS THE AREA DURING THE DAY, WHICH APPEARS REASONABLE GIVEN
PERSISTENT FORCING ALONG THE BAROCLINIC ZONE, BENEATH THE
DIVERGENT RIGHT REAR ENTRANCE TO AN UPPER LEVEL JET STREAK OVER
THE NORTHERN LAKES/ONTARIO. P-WATS APPROACHING 2" SUGGEST SOME
POTENTIAL FOR SOME TORRENTIAL DOWNPOURS AND LOCALLY HEAVY
RAINFALL WHICH COULD LEAD TO SOME FLOODING. THE JUXTAPOSITION OF
THE SOUTHERN PERIPHERY OF STRONGER WESTERLY FLOW ALOFT AND LOW-
LEVEL INSTABILITY ALONG/SOUTH OF THE SURFACE FRONTAL ZONE MAY
ALSO SUPPORT SOME WIND POTENTIAL WITH STRONGER CONVECTIVE CELLS.
CLOUDS, PRECIPITATION AND INCREASING NORTH-NORTHEASTERLY
SURFACE WINDS BEHIND THE SOUTHWARD-SAGGING COLD FRONT WILL
RESULT IN RANGE IN TEMPERATURES WEDNESDAY, FROM THE UPPER 70S
NORTH AND ALONG LAKE MICHIGAN TO THE MID-UPPER 80S FARTHER
SOUTH. STRENGTHENING NORTHERLY WINDS AND BUILDING WAVES WILL
ALSO LEAD TO THE DEVELOPMENT OF DANGEROUS SWIMMING CONDITIONS AT
LAKE MICHIGAN BEACHES BY AFTERNOON.
RATZER
WEDNESDAY NIGHT THROUGH MONDAY:
A COLD FRONT WILL BE IN THE PROCESS OF MOVING THROUGH NORTHEAST
IL AND NORTHWEST IN WEDNESDAY NIGHT WITH A FAIR COVERAGE OF
SHOWERS AND THUNDERSTORMS ALONG AND AHEAD OF IT. IN TERMS OF
SEVERITY, THE MODEST MID-LEVEL FLOW (500MB WINDS AROUND 25-30
KTS) DOES LOOK TO GENERATE AROUND 30 KTS OF DEEP LAYER SHEAR
WHICH WOULD BE SUFFICIENT TO SUPPORT ROBUST UPDRAFTS, HOWEVER,
THE EFFECTIVE SHEAR (THE SHEAR THE STORMS ACTUALLY UTILIZE) IS
FORECAST TO ONLY BE AROUND 20 KTS OR SO. COUPLE THIS WITH THE
MODEST INSTABILITY AND THE THREAT FOR SEVERE WEATHER DOESN'T
LOOK OVERALL IMPRESSIVE AT THE CURRENT TIME. THAT SAID, IF A
STRONGER STORM IS TO DEVELOP THEN A LOCALIZED THREAT FOR GUSTY
WINDS COULD MATERIALIZE. THE BIGGER CONCERN WITH THE SHOWERS AND
STORMS WEDNESDAY NIGHT WILL BE THE TORRENTIAL RAIN POTENTIAL
GIVEN THE NEARLY 2.0 INCH PWATS. NOT TO MENTION THE STEERING
FLOW IS EXPECTED TO BE ORIENTED PARALLEL TO THE FRONTAL BOUNDARY
WHICH WILL SUPPORT TRAINING STORMS. WHILE THERE IS OBVIOUSLY A
FAIR BIT OF UNCERTAINTY AS TO WHERE THE HEAVIEST RAIN WILL SET
UP, THERE IS A NOTEWORTHY SIGNAL IN GUIDANCE FOR RAINFALL TOTALS
TO EXCEED 1 INCH SOMEWHERE IN OUR AREA (50-60% CHANCE) WITH A
20% CHANCE FOR AMOUNTS OVER 2 INCHES TO BE SEEN. GIVEN THE
RECENT WET WEATHER ACROSS MUCH OF THE AREA THIS WILL DEFINITELY
BE A PERIOD TO WATCH FOR THE RISK OF FLOODING.
HEADING INTO THURSDAY, THE FRONT AND ASSOCIATED SHOWERS/STORMS
WILL BE EXITING THE AREA DURING THE MORNING HOURS AS A CANADIAN
SURFACE HIGH MOVES INTO THE GREAT LAKES. THIS HIGH IN
COMBINATION WITH THE NORTHWEST FLOW ALOFT WILL USHER IN NOTABLY
COOLER AND DRIER AIR WHICH IS EXPECTED TO CAP HIGHS THURSDAY
AFTERNOON IN THE MID TO UPPER 70S DESPITE THE CLEARING SKIES.
WINDS IN THE WAKE OF THE FRONT WILL ALSO BECOME BREEZY OUT OF
THE NORTHEAST ESPECIALLY OVER LAKE MICHIGAN WHERE A PERIOD OF
20-25 KT WINDS IS EXPECTED LATE WEDNESDAY THROUGH THURSDAY
EVENING. BASED ON LOCAL WIND-WAVE CLIMATOLOGY, WINDS OF THIS
MAGNITUDE WILL LEAD TO WAVES IN THE 4-7 FT RANGE WHICH IS MORE
THAN SUFFICIENT TO CAUSE DANGEROUS AND LIFE THREATENING RIP
CURRENTS. WHILE IT IS STILL A BIT TOO SOON TO ISSUE A BEACH
HAZARD STATEMENT FOR THIS PERIOD, ONE WILL LIKELY BE COMING WITH
FUTURE FORECASTS. WINDS AND WAVES ARE EXPECTED TO DIMINISH
HEADING INTO FRIDAY, BUT WITH 10-15 KTS OF ONSHORE FLOW
PERSISTING SUSPECT THAT HIGH WAVES AND CURRENTS WILL PERSIST
THROUGH AT LEAST FRIDAY EVENING AS WELL.
WITH THE SURFACE HIGH EXPECTED TO REMAIN IN PLACE THROUGH THE
UPCOMING WEEKEND, THE PLEASANT WEATHER IS EXPECTED TO PREVAIL
INTO AT LEAST EARLY NEXT WEEK WITH HIGHS WARMING SLIGHTLY EACH
DAY INTO THE UPPER 70S AND LOWER 80S BY MONDAY. UNFORTUNATELY
THE NICE WEATHER CAN'T LAST FOREVER AS THE UPPER PATTERN IS
FORECAST TO SHIFT BACK INTO A SOUTHWEST FLOW REGIME TOWARDS THE
MIDDLE OF NEXT WEEK. WHILE THIS PATTERN WILL RETURN MORE TYPICAL
SUMMER WARMTH AND HUMIDITY, IT WILL ALSO BRING INCREASING
CHANCES FOR SHOWERS AND THUNDERSTORMS.
YACK
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 116 AM CDT TUE JUL 29 2025
FORECAST CONCERNS FOR THE TERMINALS...
- PATCHY FOG IN NORTHWEST IN AND EASTERN IL TONIGHT.
- SCATTERED SHOWERS AND THUNDERSTORMS POSSIBLE LATE TONIGHT
INTO EARLY TUESDAY MORNING, ESPECIALLY IN NORTHWEST IL.
- BRIEF PERIOD OF BREEZY EASTERLY WINDS BEHIND SHOWERS/STORMS
TUESDAY MORNING.
- ANOTHER PERIOD OF SHOWERS AND THUNDERSTORMS POSSIBLE TUESDAY
NIGHT INTO WEDNESDAY.
A COMPLEX PERIOD OF WEATHER IS EXPECTED AT THE TERMINALS OVER
THE NEXT 30 HOURS. TO START, PATCHY FOG (SOME DENSE WITH
VISIBILITY AROUND 1/2-1/4 MILE) HAS CONTINUED TO DEVELOP ACROSS
NORTHWEST IN AND ADJACENT PORTIONS OF EAST AND NORTHEAST IL
WHERE SURFACE DEW POINTS REMAIN IN THE LOWER 70S. THE FOG SHOULD
BE ABLE TO PERSIST THROUGH MUCH OF THE NIGHT BEFORE GRADUALLY
IMPROVING AFTER 10Z.
OUR ATTENTION THEN TURNS TO THE LINE OF SEVERE THUNDERSTORMS
ONGOING IN SOUTHERN MN AND EASTERN IA THAT IS EXPECTED TO MOVE
INTO NORTHWEST IL AROUND 08-09Z TONIGHT. THE STORMS ARE EXPECTED
TO ARRIVE IN A DECAYING STATE AS THEY OUTRUN THE BETTER
INSTABILITY AND ENCOUNTER A POCKET OF DRIER AIR ALOFT. HOWEVER,
THERE ARE A COUPLE OF CONVECTIVELY DRIVEN LOW PRESSURE CENTERS
WITHIN THE STORM COMPLEX THAT SHOULD ALLOW AT LEAST SOME
THUNDERSTORMS TO REACH RFD BEFORE THEY COMPLETELY DISSIPATE AND
BECOME SCATTERED SHOWERS. THEREFORE, HAVE OPTED TO CONVERT THE
PROB30 TO A TEMPO GROUP AT RFD TO ACCOUNT FOR THIS EXPECTATION.
WHILE OVERALL STORM SEVERITY SHOULD DIMINISH BEFORE THE STORMS
REACH RFD, STILL CANNOT RULE OUT SOME LOCALLY GUSTY WINDS IN
EXCESS OF 30 KTS. AS FOR THE CHICAGO AREA TERMINALS, THE STORMS
SHOULD WEAKEN ENOUGH THAT THEY WILL BE IN THE FORM OF SCATTERED
SHOWERS IF THEY MAKE IT AT ALL. SINCE CONFIDENCE ON THE COVERAGE
OF SHOWERS AT THE CHICAGO TERMINALS IS STILL UNCERTAIN, HAVE
OPTED TO MAINTAIN THE PROB30 GROUPS AT DPA, MDW, AND ORD FOR NOW
BUT WILL CONTINUE TO ASSESS TRENDS AND UPDATE AS NEEDED. THAT
SAID, STILL CANNOT COMPLETELY RULE OUT THAT AN ISOLATED
LIGHTNING STRIKE COULD BE SEEN IN THE VICINITY OF THE CHICAGO
TERMINALS BETWEEN 11Z AND 13Z TUESDAY MORNING.
REGARDLESS OF WHAT HAPPENS WITH THE SHOWERS/STORMS, A MESO-HIGH
ON THE BACK SIDE OF THE STORM COMPLEX IS EXPECTED TO PIVOT
THROUGH NORTHEAST IL AND NORTHWEST IN EARLY TUESDAY MORNING.
THIS SHOULD RESULT IN A 2-3 HOUR PERIOD OF BREEZY (10-12 KT)
EASTERLY WINDS BEFORE WINDS EASE BACK INTO THE 5-10 KT RANGE AND
DIRECTIONS TURN SOUTHWEST TOWARDS MIDDAY. OTHERWISE, IT LOOKS
AS A MOSTLY DRY DAY WILL FOLLOW THE MORNING RAIN CHANCES WITH
VFR CONDITIONS EXPECTED THROUGH THE PERIOD. THAT SAID, GUIDANCE
HAS TRENDED A FEW DEGREES COOLER WITH TEMPERATURES TODAY DUE TO
THE BROADER CLOUD COVER FROM THE MORNING SHOWERS/STORMS WHICH IN
TURN LOOKS TO LOWER THE LIKELIHOOD THAT A LAKE BREEZE WILL
DEVELOP AND MAKE IT TO ORD AND MDW. WHILE IT IS STILL NOT
COMPLETELY OUT OF THE QUESTION THAT A LAKE BREEZE DEVELOPS, THE
NEW CONSENSUS IS THAT WINDS SHOULD BE REMAIN WEST-SOUTHWEST
THROUGH 06Z TUESDAY NIGHT BEFORE TURNING NORTHEAST BEHIND A COLD
FRONT AND THE TAFS HAVE BEEN ADJUSTED TO REFLECT THIS THINKING.
FINALLY, THERE IS YET ANOTHER POTENTIAL FOR SHOWERS AND
THUNDERSTORMS TUESDAY NIGHT INTO WEDNESDAY AS A COLD FRONT MOVES
THROUGH THE AREA. GIVEN THE DRIER AIR FORECAST TO DEVELOP
OVERHEAD TODAY, SUSPECT ANY SHOWERS STORMS WILL HOLD OFF UNTIL
AFTER THIS TAF PERIOD SO NO FORMAL MENTIONS HAVE BEEN ADDED BUT
WILL REASSESS FOR FUTURE ISSUANCES.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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