087
FXUS63 KLOT 272331
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
631 PM CDT SUN JUL 27 2025
KEY MESSAGES
- ISOLATED TO SCATTERED SHOWERS AND STORMS MAY CONTINUE THROUGH
EARLY EVENING.
- POSSIBLE LINE OF DECAYING THUNDERSTORMS MONDAY MORNING WITH
STRONG/SEVERE STORMS POSSIBLE.
- DANGEROUS HEAT AND HUMIDITY MONDAY WITH A HEAT ADVISORY IN
EFFECT.
- PERIODS OF THUNDERSTORMS RETURN MONDAY THROUGH WEDNESDAY, SOME
POSSIBLY STRONG TO SEVERE WITH TORRENTIAL RAIN WHICH COULD
LEAD TO FLASH FLOODING.
- DANGEROUS SWIMMING CONDITIONS AT LAKE MICHIGAN BEACHES
WEDNESDAY INTO THURSDAY, POSSIBLY INTO FRIDAY NIGHT.
DISCUSSION
ISSUED AT 230 PM CDT SUN JUL 27 2025
THROUGH MONDAY:
TWO PRIMARY FORECAST CONCERNS WHICH ARE THUNDERSTORM CHANCES
MONDAY MORNING AND HEAT INDICES MONDAY AFTERNOON.
ISOLATED SHOWERS CONTINUE TO FESTER ACROSS PART OF THE CWA THIS
AFTERNOON WITH A WEAK FRONTAL BOUNDARY NEAR I-80 ALONG WITH WEAK
OUTFLOWS FROM THESE SHOWERS. ALSO A LAKE BREEZE BOUNDARY IS
SLOWLY MOVING IN FROM THE LAKE. ISOLATED SHOWERS WILL CONTINUE
THROUGH SUNSET, MAINLY FOCUSED AWAY FROM THE LAKE AND THROUGH
THE CENTER PART OF THE CWA. A FEW SHOWERS MAY PERSIST INTO THE
EVENING, MAINLY SOUTH OF I-80. SO FAR, THERE HASN'T BEEN ANY
DETECTED LIGHTNING, BUT AN ISOLATED THUNDERSTORM WILL REMAIN
POSSIBLE. WHILE VERY LOCALIZED AND BRIEF, HEAVY RAIN WILL ALSO
BE POSSIBLE.
ATTENTION THEN TURNS TO THUNDERSTORM DEVELOPMENT ALREADY
OCCURRING ACROSS WEST CENTRAL MN, WHICH IS ALSO A BIT EARLIER
THAN CURRENT GUIDANCE WAS SUGGESTING FOR DEVELOPMENT.
ADDITIONAL ACTIVITY IS EXPECTED LATER THIS EVENING WITH A
CONSENSUS FOR A TURN TO THE SOUTHEAST INTO SOUTHWEST WI/
NORTHEAST IA IN THE PREDAWN HOURS AND THEN INTO NORTHWEST IL
AROUND DAYBREAK. THE HRRR REMAINS THE MOST CONSISTENT AND ALSO
THE MOST BULLISH FOR THIS ACTIVITY REMAINING TOGETHER AS IT
MOVES ACROSS AT LEAST THE WESTERN THIRD/HALF OF THE CWA JUST
AFTER DAYBREAK. THERE IS PLENTY OF INSTABILITY EVEN AT THIS
EARLY HOUR WITH THE LIMITING FACTOR PERHAPS BEING SHEAR, THOUGH
SOMETIMES THESE COMPLEXES BECOME WELL ORGANIZED AND PERSIST
LONGER AND STRONGER THEN THE MODELS SHOW. GIVEN THE POTENTIAL,
HAVE INCREASED POPS INTO THE 40 PERCENT RANGE, GENERALLY WEST OF
THE FOX VALLEY AND THEN ALONG I-55. THESE WILL NEED TO BE
ADJUSTED AS TRENDS EMERGE LATER THIS EVENING AND TONIGHT. THE
MORE ORGANIZED THE STORMS ARE, THE BETTER THE CHANCE FOR STRONG/
SEVERE STORMS WITH DAMAGING WINDS THE MAIN THREAT. AHEAD OF
THIS POTENTIAL LINE, THE AIRMASS IS SO MOIST AND UNSTABLE, THERE
MAY BE ISOLATED SHOWERS OR THUNDERSTORMS AROUND DAYBREAK ACROSS
THE LOCAL AREA. HAVE ADDED SLIGHT CHANCE POPS STARTING AT 10Z,
MAINLY FOR THE IL COUNTIES FOR THIS POTENTIAL. REGARDLESS OF
COVERAGE, THE STORMS ARE EXPECTED TO QUICKLY DISSIPATE BY MID
MORNING, WITH THE REST OF MONDAY LOOKING DRY.
IF THE AREA REMAINS DRY MONDAY, TEMPS SHOULD QUICKLY RECOVER BY
EARLY AFTERNOON WITH HIGHS INTO THE LOWER 90S FOR MOST AREAS,
POSSIBLY INTO THE MID 90S FOR SOME AREAS INCLUDING THE CHICAGO
METRO. WITH NO CHANGE TO THE AIRMASS, DEWPOINTS IN THE MID TO
UPPER 70S WILL PUSH PEAK HEAT INDICES INTO THE 105-109 RANGE FOR
MOST OF THE AREA AND HAVE ISSUED A HEAT ADVISORY FROM 17Z TO
01Z. THE WIND FIELD MAY BECOME QUITE LIGHT, DEPENDENT ON STORM
COVERAGE IN THE MORNING AND THIS MAY ALLOW FOR A LAKE BREEZE
NEAR THE LAKE MICHIGAN SHORE. QUITE A BIT OF UNCERTAINTY FOR HOW
FAR THIS MAY MOVE INLAND AND EVEN MORE UNCERTAINTY FOR HOW
TEMPS MAY BE IMPACTED. CURRENT TEMPS (SUNDAY AFTERNOON) ARE
STILL IN THE MID 80S NOT FAR FROM THE SHORE. IF ANYTHING IT MAY
LIMIT PEAK HEAT INDICES RIGHT NEAR THE SHORE.
FINALLY, FOG IS POSSIBLE OVERNIGHT INTO EARLY MONDAY MORNING,
THOUGH CONFIDENCE IS LOW WITH THE EXPECTATION FOR INCREASING
CLOUD COVER AND THEN POSSIBLE STORMS INTO THE NORTHWEST CWA BY
DAYBREAK. FOR NOW, HAVE NO FOG MENTION IN THE FORECAST. CMS
MONDAY NIGHT THROUGH SUNDAY:
REMNANT OUTFLOW FROM MONDAY'S POTENTIAL MCS WOULD LIKELY DRIFT
BACK NORTH ACROSS IOWA THROUGH EARLY MONDAY EVENING. WHILE
OVERALL LARGE SCALE FORCING LOOKS MEAGER, A PLUME OF EXTREMELY
STEEP MID-LEVEL LAPSE RATES WILL CONTINUE TO ADVECT EASTWARD
ATOP SEASONABLY MOIST LOW-LEVELS. EVEN A GLANCING BLOW FROM A
MODEST REMNANT MCV/VORT MAX OR OTHER SHORTWAVE(S) EMBEDDED
ROTATING AROUND THE NORTHERN PERIPHERY OF THE SUBTROPICAL HIGH
COULD SERVE AS A FOCUS TO REIGNITE STORMS OFF TO OUR NORTH AND
WEST. EXTENDED CAM GUIDANCE SEEMS TO GENERALLY SUPPORT THE IDEA
OF ANOTHER CLUSTER OF THUNDERSTORMS ORGANIZING SOMEWHERE
UPSTREAM OF US AND SUBSEQUENTLY EVOLVING INTO A FORWARD-
PROPAGATING MCS MONDAY NIGHT, ALTHOUGH SPECIFICS (AS USUAL AT
THIS RANGE) REMAIN UNCLEAR. SPECIFICALLY, HOW FAR EAST AND NORTH
ANY SUCH COMPLEX INITIALLY DEVELOPS WILL PLAY A LARGE ROLE IN
WHETHER STORMS MANAGE TO PUSH BACK INTO OUR REGION THROUGH THE
LATE OVERNIGHT AND INTO THE MORNING HOURS ON TUESDAY.
SIMILAR TO TONIGHT/MONDAY'S SETUP, FORWARD-PROPAGATING CORFIDI
VECTORS SEEM TO BE ORIENTED A BIT MORE EASTERLY AS OPPOSED TO
SOUTH OR SOUTHEAST, AND 850-300 MB THICKNESS LINES ALSO APPEAR
TO BE LAID OUT IN A WNW-ESE ORIENTATION, BOTH OF WHICH SUGGEST
MCS MOTION THAT COULD END UP STAYING NORTH OF THE LOCAL AREA,
PARTICULARLY IF INITIAL DEVELOP TAKES PLACE ACROSS THE
MINNEAPOLIS AREA. HOWEVER, THE INSTABILITY/MUCAPE AXIS IS
FORECAST TO TAKE ON MORE OF A NW-SE ORIENTATION AND BUILD MORE
SOLIDLY INTO OUR REGION, AND WITH THE PROPENSITY FOR MATURE MCSS
TO FAVOR THESE GRADIENTS, THAT DOES PORTEND SOME POTENTIAL FOR
ACTIVITY TO ATTEMPT TO BUILD INTO OUR REGION. WHETHER ANY
COMPLEX MANAGES TO SUSTAIN AN APPRECIABLE SEVERE WEATHER THREAT
INTO OUR AREA ALSO REMAINS UNCERTAIN, GIVEN THE UNFAVORABLE TIME
OF NIGHT WITH INCREASED NOCTURNAL CONVECTIVE INHIBITION AND
GENERALLY EASTERLY-DIRECTED DEEP LAYER SHEAR (0-7, 0- 8 KM
LAYERS). THE LATTER WOULD SUPPORT OUTFLOW ATTEMPTING TO DEVELOP
AHEAD OF ANY NW-SE MOVING COMPLEX. STILL, THE PRESENCE OF
POTENTIALLY 2500-3500 J/KG OF MUCAPE, AT LEAST MODEST HIGH LEVEL
FLOW, AND SIGNIFICANT UPSTREAM DCAPE (1000-1500 J/KG), THE
THREAT FOR AN MCS WITH SOME STRONG-SEVERE STORM POTENTIAL EXISTS
LATE MONDAY NIGHT INTO EARLY TUESDAY MORNING.
GIVEN THE POTENTIAL FOR MORNING CONVECTION AND DEBRIS CLOUD COVER,
THIS REALLY MUDDIES THE HEAT INDEX FORECAST ON TUESDAY. WITHOUT
ANY STORMS, THINGS ARE EASILY PRIMED TO PUSH WELL INTO THE
105-110 RANGE (PERHAPS EVEN A THREAT FOR SOME ISOLATED 115
READINGS), MAINLY AWAY FROM THE LAKE WITH SIGNS OF A LATE-
MORNING/MIDDAY LAKE BREEZE PUSH. WITH A MORNING MCS, HEAT
INDICES COULD STILL RECOVER TO NEAR ADVISORY LEVELS, BUT THE
AREAL EXTENT WOULD LIKELY BE DIMINISHED, AND PERHAPS RELEGATED
TO OUR SOUTHWEST HALF/THIRD. GIVEN ALL OF THIS, WE'RE TAKING THE
STANCE OF HANDLING HEAT HEADLINES ON A DAY-BY- DAY BASIS, BUT
WILL CONTINUE TO MESSAGE THE POTENTIAL FOR DANGEROUS
HEAT/HUMIDITY. AS IT STANDS RIGHT NOW, THE GREATEST CHANCES FOR
HEAT INDICES OVER 105 F ON TUESDAY EXIST SOUTH AND WEST OF A
ROCKFORD TO RENSSELAER LINE.
FOR COOK COUNTY WHERE THERE IS A SEPARATE 2-DAY EXTREME HEAT
WARNING THRESHOLD OF HEAT INDICES OVER 105 F, THERE IS SOME
POTENTIAL WE THREATEN THIS IF STORMS DON'T DISRUPT THE
ENVIRONMENT AS IT LOOKS LIKE WE'LL BE CLOSE TO 105 F ON MONDAY.
AT THIS POINT, WITH AN AFTERNOON LAKE BREEZE LOOKING LIKE A
GREATER POSSIBILITY ON TUESDAY, THE EXTENT/DURATION OF THESE
HIGHER READINGS SEEMS MORE LIMITED ON TUESDAY, LESSENING THE
POTENTIAL FOR BUSTING THE 2-DAY CRITERIA.
FINALLY, IF NO MCS MATERIALIZES TUESDAY MORNING, ISOLATED TO
WIDELY SCATTERED STORMS COULD DEVELOP IN THE AFTERNOON IN A
WEAKLY CAPPED, HIGHLY UNSTABLE ENVIRONMENT. COVERAGE WOULD
LIKELY REMAIN QUITE LIMITED THOUGH GIVEN WEAK OVERALL FORCING.
BETTER SIGNAL FOR STORMS IN THE GUIDANCE SUITE EXISTS DURING THE
TUESDAY EVENING INTO WEDNESDAY TIMEFRAME AS A COLD FRONT PUSHES
SOUTH OUT OF WISCONSIN. ANY STORMS OR SURGING COMPLEX WOULD
CARRY WITH IT ANOTHER THREAT FOR SEVERE WEATHER GIVEN THE HIGHLY
UNSTABLE ENVIRONMENT, AND INCREASING SHEAR THROUGH THE 0-6 AND
0-8 KM LAYERS.
MOISTURE DOESN'T LOOK LIKE IT'LL GET FULLY SCOURED OUT ON
WEDNESDAY BEHIND THE FRONT, WITH LOW TO MID 70S DEWPOINTS
LINGERING AND PWATS STILL PUSHING 1.5 TO 2 INCHES. A SHORTWAVE
RIDING ATOP THE FRONTAL ZONE COULD YIELD A CONTINUED THREAT FOR
SHOWERS AND STORMS THROUGH THE DAY AS A RESULT.
A PERIOD OF MUCH MORE TRANQUIL AND PLEASANT CONDITIONS IS FORECAST
TO DEVELOP FROM ABOUT THURSDAY AND THROUGH THE UPCOMING WEEKEND
AS A SPRAWLING 1020-1030 MB HIGH BUILDS OVERHEAD. STRONG/GUSTY
NORTHERLY WINDS DOWN THE LAKE WILL PROMOTE BUILDING WAVES AND A
PROLONGED THREAT FOR RIP CURRENTS/DANGEROUS SWIMMING CONDITIONS
WEDNESDAY AND THURSDAY, AND POTENTIALLY EVEN LINGERING INTO
FRIDAY AND FRIDAY NIGHT.
CARLAW
AVIATION /00Z TAFS THROUGH 06Z MONDAY/
ISSUED AT 630 PM CDT SUN JUL 27 2025
KEY AVIATION MESSAGES FOR THE CURRENT TAF PERIOD:
- CHANCE FOR SHOWERS AND STORMS AROUND KRFD MONDAY MORNING
- POTENTIAL LAKE BREEZE MONDAY LATE AFTERNOON/EARLY EVENING
SHOWERS AND STORMS THAT FORMED THIS AFTERNOON ARE SLOWLY
DIMINISHING. LINGERING VCTS AROUND KDPA IS POSSIBLE THROUGH
01Z. THINGS TURN DRIER THIS EVENING AS WINDS BECOME LIGHT AND
VARIABLE. WITH THE AMOUNT OF MOISTURE STICKING AROUND OVERNIGHT,
CANNOT RULE OUT SOME 2 TO 4 SM BR DEVELOPING LOCALLY IN RURAL
AREAS. DENSE FOG IS NOT CURRENTLY EXPECTED AND THUS ANY LOWER
VIS WAS KEPT OUT OF THE TAF.
MODELS ARE CONTINUING TO DEPICTING A DECAYING LINE OF STORMS
ENTERING NORTHERN ILLINOIS MONDAY MORNING. MADE NO CHANGES TO
THE PROB30 FOR KRFD TOMORROW MORNING AS THAT REMAINS THE HIGHEST
CHANCE FOR SHOWERS/STORMS. HOWEVER, CHICAGO TERMINALS WERE KEPT
DRY FOR NOW. IT WOULD NOT BE SURPRISING IF A VCSH/VCTS NEEDS TO
BE ADDED, BUT THERE IS STILL UNCERTAINTY ON WHETHER OR NOT THE
SYSTEM WILL MAINTAIN ITS STRENGTH ENOUGH TO MERIT THE ADDITION,
OR IF IT FALLS APART ENTIRELY AS IT CROSSES THE STATELINE. IF
THE SHOWERS WERE TO MATERIALIZE, MVFR CONDITIONS COULD DEVELOP
AS THEY PASS OVER.
WINDS WILL DEVELOP OUT OF THE SOUTHWEST MONDAY MORNING. THEY
WILL THEN SWITCH TO THE NORTHWEST BEHIND THE DECAYING SYSTEM
(REGARDLESS OF RAIN OR DRY). MODELS ARE SUGGESTING A LAKE BREEZE
COULD DEVELOP LATE BEHIND THE EXITING SYSTEM, BUT THERE IS LOW
CONFIDENCE ON THE EXACT TIME IT WOULD ARRIVE AT CHICAGO
TERMINALS.
DK
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY FROM NOON TO 8 PM CDT MONDAY FOR ILZ003-ILZ004-
ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108.
IN...HEAT ADVISORY FROM NOON CDT /1 PM EDT/ TO 8 PM CDT /9 PM EDT/
MONDAY FOR INZ001-INZ002-INZ010-INZ011-INZ019.
LM...NONE.
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