397
FXUS63 KLOT 280824
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
324 AM CDT MON JUL 28 2025
KEY MESSAGES
- STORMS KEY MESSAGE?
- DANGEROUS HEAT AND HUMIDITY TODAY WITH A HEAT ADVISORY IN
EFFECT.
- PERIODS OF THUNDERSTORMS EXPECTED 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 324 AM CDT MON JUL 28 2025
THROUGH TUESDAY:
PRIMARY FORECAST CONCERNS CONTINUE TO FOCUS ON THUNDERSTORM
TRENDS THIS MORNING, AND IMPACTS OF OUTFLOW AND RESIDUAL CLOUD
COVER ON TEMPERATURES AND HEAT INDICES INTO THIS AFTERNOON.
SIMILAR CONCERNS EXTEND INTO TUESDAY, WITH ANOTHER POTENTIAL FOR
AN MCS TO REACH THE AREA IN THE MORNING.
REGIONAL RADAR DEPICTS A LARGE MCS DEVELOPING SOUTH-SOUTHEAST
ACROSS SOUTHERN MN/IA EARLY THIS MORNING, WITH AN APPARENT MCV
OVER NORTHEAST IA. THE STRONGEST CONVECTION AND SEVERE WIND
THREAT WAS GENERALLY PROPAGATING SOUTHEAST INTO CENTRAL/EASTERN
IA, WHERE SPC/RAP MESOANALYSIS DEPICTS A 4000 J/KG MLCAPE AXIS.
AN AREA OF ISOLATED TO SCATTERED THUNDERSTORMS WERE ALSO NOTED
EXTENDING FROM NEAR THE MCV INTO WESTERN IL, WITHIN MODEST WEST-
SOUTHWEST 850 MB FLOW/WAA WING ALONG THE EASTERN GRADIENT IN
MUCAPE AND AN 850 MB MOISTURE GRADIENT. WHILE THE PRIMARY PUSH
OF THE MCS IS EXPECTED TO PUSH ACROSS EASTERN IA INTO WESTERN IL
THIS MORNING, THE EXPECTATION PER CAM TRENDS IS THAT SCATTERED
THUNDERSTORMS WILL CONTINUE THIS TO SPREAD EAST INTO AREAS
GENERALLY ALONG/WEST OF THE FOX RIVER VALLEY AND I-55 CORRIDOR
REGIONS THROUGH SUNRISE THIS MORNING, BEFORE
WEAKENING/DISSIPATING DURING THE MORNING. PRECIPITABLE WATER
VALUES AROUND 1.50" WILL SUPPORT TORRENTIAL DOWNPOURS WITH THESE
STORMS, AS WELL AS LOCALIZED WIND GUSTS OF 40-45 MPH ACROSS OUR
FAR WESTERN CWA EARLY THIS MORNING. CAMS ARE LARGELY DRY ACROSS
OUR FORECAST AREA THIS AFTERNOON, THOUGH CAN'T RULE OUT
ISOLATED SHOWER/THUNDERSTORM DEVELOPMENT ALONG LINGERING
OUTFLOW BOUNDARIES OR THE LAKE BREEZE. THERE APPEARS TO BE
LITTLE IN THE WAY OF UPPER LEVEL SUPPORT HOWEVER, AND HAVE
MAINTAINED A DRY FORECAST AT THIS TIME WITH WEAK
SUBSIDENCE/RISING MID-LEVEL HEIGHTS DEPICTED BY GUIDANCE IN THE
WAKE OF THE MORNING MCV.
THE RESIDUAL OUTFLOW AND CLOUD COVER FOOTPRINT FROM MORNING
STORMS DOES LOWER CONFIDENCE A BIT IN TEMPERATURE AND HEAT INDEX
TRENDS ACROSS THE AREA INTO THIS AFTERNOON. FAR NORTH/NORTHWEST
CWA (ESPECIALLY NORTHWEST OF I-88) APPEARS TO HAVE THE GREATEST
POTENTIAL FOR OUTFLOW TO LOWER SURFACE DEW POINTS, ALONG WITH
THE IMMEDIATE LAKE MICHIGAN SHORE AREAS WHERE A WEAK LAKE BREEZE
IS EXPECTED TO DEVELOP LATER THIS AFTERNOON. HIGH-RES GUIDANCE
SUGGESTS THAT THE AREA SHOULD BE GENERALLY CONVECTION-FREE FROM
MIDDAY THROUGH THIS AFTERNOON HOWEVER, WHICH SHOULD ALLOW
TEMPERATURES TO WARM INTO THE LOW-MID 90S BY PEAK HEATING.
CURRENT FORECAST AFTERNOON HEAT INDICES (AWAY FROM IMMEDIATE
LAKE SHORE) RANGE FROM 100-105 NORTH OF I-88, TO 105+ TO THE
SOUTH, AND THUS AM PLANNING NO CHANGES TO CURRENT HEAT ADVISORY
WHICH RUNS FROM NOON TO 8 PM CDT.
ATTENTION THEN TURNS TO ANOTHER SHORT WAVE TROUGH WHICH IS
FORECAST TO PROPAGATE INTO THE NORTHERN PLAINS, ALONG THE
NORTHERN PERIPHERY OF THE UPPER LEVEL RIDGE. INITIAL
THUNDERSTORM DEVELOPMENT OVER THAT AREA IS EXPECTED TO
EVENTUALLY EVOLVE INTO A FORWARD-PROPAGATING MCS THIS AFTERNOON,
TRACKING EAST-SOUTHEAST ACROSS THE DAKOTAS/MINNESOTA AND
NORTHWEST/NORTHERN IA INTO THIS EVENING. THERE REMAINS SOME
SPREAD IN CAM SOLUTIONS WITH THIS, HOWEVER THERE IS GENERAL
CONSENSUS IN RECENT 4KM NAM, HIGH-RES ARW, RAP AND HRRR RUNS
INDICATING THIS LINEAR MCS MAY APPROACH THE FORECAST AREA TOWARD
SUNRISE TUESDAY MORNING, THOUGH LIKELY IN A GRADUALLY WEAKENING
MODE. SPC HAS INCLUDED OUR NORTHWESTERN CWA IN THE NEW DAY 1
MARGINAL (1 OF 5) SEVERE RISK FOR WIND, WHICH APPEARS VERY
REASONABLE AT THIS TIME. THUNDERSTORM TRENDS BEYOND TUESDAY
MORNING THEN DECREASE IN CONFIDENCE, AGAIN DUE TO POTENTIAL FOR
RESIDUAL OUTFLOW AND CLOUD COVER WHICH WILL LIKELY IMPACT THE
POTENTIAL FOR REDEVELOPMENT LATER IN THE DAY. THESE SAME IMPACTS
WOULD LIKELY AFFECT TEMPERATURES/HEAT INDICES AS WELL, AND FOR
THAT REASON WILL CONTINUE TO ADDRESS ANY HEAT HEADLINES ON A DAY
BY DAY BASIS. IF OUTFLOW AND CLOUDS ARE LESS IMPACTFUL (OR
DON'T APPRECIABLY MAKE IT INTO OUR AREA), THE POTENTIAL REMAINS
FOR US TO HEAT INTO THE LOW- MID 90S WITH PEAK HEAT INDICES
AROUND/ABOVE 105.
RATZER
TUESDAY NIGHT THROUGH SUNDAY:
OUR BRIEF HEAT WAVE WILL BE IN THE PROCESS OF COMING TO A CLOSE
TUESDAY EVENING AS A COLD FRONT WILL BE BEGINNING TO ENTER
NORTHERN IL. WHILE THERE COULD BE SOME ONGOING SHOWERS AND
THUNDERSTORMS WITH THE FRONT AS IT ARRIVES, GUIDANCE CONTINUES
TO VARY AS TO THE COVERAGE LIKELY OWING TO DIFFERENCES IN THE
AMOUNT OF CAPPING IN PLACE AHEAD OF THE FRONT. REGARDLESS,
SHOWER AND THUNDERSTORM COVERAGE IS FORECAST TO INCREASE LATE
TUESDAY NIGHT INTO WEDNESDAY AS A SHORTWAVE TROUGH PIVOTS INTO
NORTHERN IL AND PROVIDES MORE ROBUST FORCING. AS PER TYPICAL
WITH CONVECTION, THERE WILL LIKELY BE A BREAK BETWEEN THE
SHOWERS/STORMS TUESDAY NIGHT/WEDNESDAY MORNING AND THE NEXT
ROUND THAT DEVELOPS ALONG THE FRONT WEDNESDAY AFTERNOON/EVENING.
IN TERMS OF SEVERITY, MODEST 500 MB FLOW ASSOCIATED WITH THE
SHORTWAVE IS EXPECTED TO GENERATE AROUND 30-35 KTS OF DEEP LAYER
SHEAR ACROSS NORTHERN IL AND NORTHWEST IN WHICH WILL BE
SUFFICIENT TO SUPPORT MORE ROBUST UPDRAFTS AND A SEVERE WEATHER
THREAT MAINLY IN THE FORM OF DAMAGING WINDS. ADDITIONALLY, THE
HUMID AIR MASS AHEAD OF THE FRONT (PWATS AROUND 1.75-2.0 INCHES)
WILL ALLOW ANY SHOWERS/STORMS TO BE EFFICIENT RAINFALL
PRODUCERS WHICH MAY RESULT IN INSTANCES OF FLOODING ESPECIALLY
IF STORMS TRAIN OVER THE SAME AREA.
THE SHOWERS AND THUNDERSTORMS WILL GRADUALLY TAPER FROM NORTH
TO SOUTH THURSDAY MORNING AS THE FRONT EXITS INTO SOUTHERN IL,
BUT SOME LINGERING RAIN CHANCE WILL BE POSSIBLE SOUTH OF I-80
THURSDAY AFTERNOON. OTHERWISE, EXPECT NOTABLY COOLER, LESS
HUMID, AND DRIER CONDITIONS TO MOVE INTO THE GREAT LAKES BEHIND
THE FRONT AS WE BECOME SEATED UNDER THE WESTERN PERIPHERY OF AN
UPPER TROUGH FORECAST TO BE CENTERED OVER EASTERN ONTARIO AND
THE NORTHEAST CONUS AND A CANADIAN SURFACE HIGH. THEREFORE,
EXPECT HIGHS THURSDAY THROUGH THE WEEKEND TO BE IN THE UPPER 70S
TO AROUND 80 WITH DEW POINTS IN THE MID TO UPPER 50S.
HOWEVER, THE COLD ADVECTION WILL ALSO RESULT DEEPER MIXING AND
AS A RESULT BREEZIER WINDS ESPECIALLY OVER LAKE MICHIGAN. COUPLE
THIS WITH THE ONSHORE WIND DIRECTIONS AND CONDITIONS ARE IN
PLACE FOR HIGH WAVES AND DANGEROUS RIP CURRENTS AT SOUTHERN LAKE
MICHIGAN BEACHES. WHILE THE PEAK OF THE STRONGEST WINDS AND
ASSOCIATED HIGHEST WAVES ARE EXPECTED WEDNESDAY THROUGH THURSDAY
NIGHT, CONTINUED 10-15 KT ONSHORE WINDS ON FRIDAY MAY SLOW THE
SUBSIDING WAVE ACTION AND THUS PROLONG THE RIP CURRENT THREAT
THROUGH FRIDAY AS WELL. SO IF YOU ARE PLANNING A TRIP TO THE
BEACHES THIS WEEKEND BE SURE TO CHECK THE FORECAST BEFORE
HEADING OUT.
YACK
AVIATION /06Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 104 AM CDT MON JUL 28 2025
FORECAST CONCERNS FOR THE TERMINALS...
- PERIOD OF THUNDERSTORMS ACROSS NORTHWESTERN IL EARLY MONDAY
MORNING. A FEW COULD BE STRONG TO SEVERE WITH WINDS IN EXCESS
OF 30 KTS.
- 15-20% CHANCE FOR SCATTERED SHOWERS AND THUNDERSTORMS TO
DEVELOP ALONG THE LAKE BREEZE THIS AFTERNOON/EVENING.
A THUNDERSTORM COMPLEX WILL CONTINUE TO RACE SOUTH-SOUTHEAST
ACROSS EASTERN IA TONIGHT INTO MONDAY MORNING. AHEAD OF THE
COMPLEX IS A SURFACE CONVERGENCE ZONE THAT IS EXPECTED TO DRIFT
EASTWARD INTO WESTERN AND NORTHWEST IL LATER TONIGHT. AS THE
OUTFLOW FROM THE COMPLEX INTERACTS WITH THE CONVERGENCE ZONE A
BAND OF SHOWERS AND THUNDERSTORMS IS EXPECTED TO DEVELOP AND
GRADUALLY MOVE SOUTHWARD THROUGH MONDAY MORNING. WHILE
CONFIDENCE ON HOW FAR EAST THIS CONVERGENCE ZONE WILL GET
REMAINS SOMEWHAT LOW, CONFIDENCE CONTINUES TO INCREASE THAT RFD
WILL SEE A PERIOD OF THUNDERSTORMS BETWEEN 10Z AND 13Z MONDAY
MORNING AND THUS A TEMPO GROUP HAS BEEN ADDED. FURTHERMORE, THE
STRONGEST WINDS WITH THE THUNDERSTORM COMPLEX ARE FORECAST TO
REMAIN WEST OF RFD, BUT A LOCALIZED GUST OVER 30 KTS CANNOT BE
RULED OUT JUST YET.
AS FOR THE CHICAGO AREA TERMINALS, IT APPEARS THAT ANY
THUNDERSTORM ACTIVITY TONIGHT SHOULD REMAIN WELL TO THE WEST OF
THE TAF SITES AND THUS DRY FORECASTS HAVE BEEN MAINTAINED.
HOWEVER, THE IS ABOUT A 15% CHANCE THAT A SHOWER OR STORM
DEVELOPS IN THE VICINITY OF DPA; A TREND THAT WE WILL CONTINUE
TO MONITOR. ANY SHOWERS STORMS THAT DO MATERIALIZE IN NORTHERN
IL WILL BEGIN TO TAPER BY 15Z RESULTING IN MOSTLY DRY CONDITIONS
THROUGH THIS AFTERNOON. THAT SAID, THERE IS A 15-20% CHANCE FOR
ADDITIONAL ISOLATED TO WIDELY SCATTERED SHOWERS AND
THUNDERSTORMS TO DEVELOP ALONG THE LAKE BREEZE THIS AFTERNOON
GIVEN THE WARM MOIST AIR MASS AND WEAK MID-LEVEL CAPPING. SINCE
THE COVERAGE OF ANY LAKE BREEZE STORMS IS LOWER CONFIDENCE HAVE
FOREGONE A FORMAL MENTION IN THE CHICAGO TAFS FOR NOW.
OTHERWISE, EXPECT VFR CONDITIONS TO PREVAIL THROUGH THE PERIOD.
WINDS WILL START OFF LIGHT OUT OF THE SOUTH-SOUTHWEST BEFORE
SHIFTING TO NORTHWEST AROUND 10 KTS BEHIND THE AFOREMENTIONED
OUTFLOW AFTER 17Z (14Z AT RFD). AS FOR THE LAKE BREEZE, THE 10
KT WINDS SHOULD HELP KEEP THE LAKE BREEZE EAST OF ORD AND MDW
UNTIL WINDS EASE AFTER 00Z BUT IT SHOULD MAKE IT THROUGH GYY
AROUND 20-22Z THIS AFTERNOON.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY FROM NOON TODAY TO 8 PM CDT THIS EVENING 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/ TODAY TO 8 PM CDT /9 PM
EDT/ THIS EVENING FOR INZ001-INZ002-INZ010-INZ011-INZ019.
LM...NONE.
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