303
FXUS63 KLOT 182057
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
357 PM CDT MON AUG 18 2025
KEY MESSAGES
- SEVERAL ROUNDS OF SHOWERS AND STORMS ARE LIKELY THIS
AFTERNOON AND EVENING. SOME STORMS MAY BE SEVERE WITH DAMAGING
WINDS AND TORRENTIAL DOWNPOURS.
- ADDITIONAL SHOWERS AND STORMS MAY DEVELOP OVERNIGHT, PRIMARILY
ALONG AND SOUTH OF I-80. THE MAIN THREAT WITH ANY ADDITIONAL
STORMS WOULD BE TORRENTIAL DOWNPOURS.
- A PATTERN CHANGE TOWARD COOLER AND LESS HUMID CONDITIONS IS
EXPECTED BY MIDWEEK. EVEN COOLER AND LESS HUMID CONDITIONS MAY
ARRIVE OVER THE WEEKEND.
DISCUSSION
ISSUED AT 239 PM CDT MON AUG 18 2025
THROUGH TUESDAY:
TWO ROUNDS OF THUNDERSTORMS LATE THIS AFTERNOON AND EVENING
STILL APPEAR TO BE IN STORE, THOUGH THE GAP THAT WAS EXPECTED
BETWEEN THESE MAY BE MORE SHORT LIVED THAN PREVIOUSLY
ANTICIPATED. THE INITIAL LINE OF STORMS STRETCHING SOUTH OUT OF
WI HAS BEEN VERY SLOW TO EXPAND ACROSS NORTHERN IL BUT THE LAST
FEW RADAR SCANS FINALLY SHOW MORE ORGANIZATION, AND EVEN THE
POTENTIAL DEVELOPMENT OF A REAR INFLOW JET IN THE STORMS ALONG
THE I-39 CORRIDOR. IN THE WAKE OF THE SOMEWHAT DISORGANIZED
EARLIER AFTERNOON ACTIVITY, MORE WIDESPREAD AND LONGER LIVED
SHOWERS AND STORMS ARE EXPECTED TO DEVELOP UPSTREAM AND MOVE
THROUGH THIS EVENING.
PLENTY OF BOUNDARY LAYER SUPPORT FOR CONVECTIVE DEVELOPMENT IS
ALREADY IN PLACE WITH WIDESPREAD DEWPOINTS IN THE MID TO UPPER
70S ACROSS THE CWA, AND ANY CONVERGENT OUTFLOW FROM NEW CELLS
SHOULD ALSO HELP TO TRIGGER NEW STORMS IN THE NEAR TERM. SUPPORT
FOR SEVERE STORMS REMAINS SOMEWHAT MARGINAL AND WOULD MOSTLY BE
DRIVEN BY THERMODYNAMICS GIVEN THAT THE MORE FAVORABLE SHEAR
AXIS IS FARTHER TO THE NORTH OVER WI. RAINFALL RATES UP TO 2
INCHES PER HOUR ARE CERTAINLY POSSIBLE.
THE SECOND ROUND OF ACTIVITY, INITIALLY EXPECTED TO TRAIL THE
FIRST BY AN HOUR OR TWO, IS ALSO SHOWING SIGNS OF DEVELOPING AND
EXPANDING IN COVERAGE ACROSS EASTERN IA INTO NORTHWEST IL ALONG
WITH BETTER BACKGROUND SYNOPTIC AND MESOSCALE SUPPORT FROM AN
UPPER TROUGH AND MORE PRONOUNCED SHEAR. THE ADDITIONAL CONCERN
WITH THIS SECOND ROUND WOULD INCLUDE HEAVY RAIN AND THE
POSSIBILITY FOR FLOODING. THIS WOULD ARISE FROM A COMBINATION OF
SEASONABLY HIGH PWATS, THE RELATIVELY SLOW EXPECTED CELL
MOTIONS, AND THE OVERALL EXTENT OF EXPECTED COVERAGE.
AFTER THESE TWO ROUNDS MOVE OUT, WE CONTINUE TO MONITOR FOR THE
CHANCE OVERNIGHT INTO EARLY TOMORROW OF A FINAL ROUND OF
SHOWERS AND THUNDERSTORMS AS THE MAIN UPPER TROUGH, DEVELOPING
SURFACE LOW, AND TRAILING FRONTAL ZONE PUSH THROUGH. THERE ALSO
APPEARS TO BE SOME SUPPORT FROM A LOW LEVEL JET TOWARD THE
PREDAWN HOURS AHEAD OF THE UPPER TROUGH. WITH THE HIGH
PRECIPITABLE WATER VALUES AND REPEATED ROUNDS OF RAIN, A FLOOD
WATCH FOR FLASH FLOODING IS IN EFFECT THROUGH LATE TONIGHT.
AFTER HIGHS TODAY IN THE MID 80S, TEMPERATURES TOMORROW MAY BE
SIMILAR TO TODAY'S, ESPECIALLY AHEAD OF THE FRONT, BUT GIVEN THE
MORNING CONVECTION AND POSSIBLE LINGERING CLOUD COVER ALONG
WITH DEWPOINTS A FEW DEGREES LOWER, HEAT INDICES SHOULD POSE
LESS OF A CONCERN.
LENNING
TUESDAY NIGHT THROUGH MONDAY:
HIGH PRESSURE MOVING OVERHEAD IN THE WAKE OF THE FRONTAL PASSAGE
SHOULD BRING AN END TO THE HOT, HUMID, AND ACTIVE PATTERN WE
HAVE BEEN EXPERIENCING THESE PAST FEW DAYS. TEMPERATURES RETURN
TO A MORE SEASONABLE LEVEL AND HUMIDITY LEVELS ALSO FALL. THE
MAIN CONCERN FROM MID- TO LATE-WEEK WOULD BE POTENTIAL WAVE
GROWTH ON LAKE MICHIGAN AND AN ELEVATED SWIM RISK WITH ONSHORE
FLOW.
ENSEMBLE MODEL GUIDANCE IS ADVERTISING A STRONG UPPER-LEVEL LOW
BARRELING TOWARD THE GREAT LAKES THIS WEEKEND, WHICH SHOULD
USHER IN A SEASONABLY COOL AND RELATIVELY DRY AIRMASS. WITH
MEAGER MOISTURE RETURN AHEAD OF THE PRINCIPAL COLD FRONT ON
SATURDAY, WE CURRENTLY FAVOR A DRY FRONTAL PASSAGE. FALLING
850MB TEMPERATURES TOWARD THE UPPER SINGLE DIGITS (+7 TO +9C)
WILL SUPPORT HIGHS IN THE MID TO UPPER 70S AND OVERNIGHT LOWS IN
THE 50S. PERHAPS EVEN MORE NOTICEABLE WILL BE THE LOWER HUMIDITY
LEVELS, AS DEW POINTS WILL BE POISED TO FALL INTO THE 50S. THE
COOL AND DRY PATTERN SHOULD CONTINUE INTO EARLY NEXT WEEK BEFORE
A PERIOD OF WARMER, AND POTENTIALLY STORMY, CONDITIONS RETURN
TO CLOSE THE MONTH OF AUGUST.
LENNING/BORCHARDT
AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/
ISSUED AT 1245 PM CDT MON AUG 18 2025
FOR THE AFTERNOON AND EVENING WE ARE LOOKING FOR TWO ROUNDS OF
THUNDERSTORM ACTIVITY, ONE WITH THE INITIAL LINE OF STORMS
APPROACHING THE RFD AREA AT FORECAST ISSUANCE TIME, AND THEN
MORE WIDESPREAD AND LONGER LIVED ACTIVITY EXPECTED TO DEVELOP
UPSTREAM AND MOVE THROUGH IN THE WAKE OF THE FIRST LINE.
IT IS ACTUALLY NOT COMPLETELY CERTAIN THAT THE INITIAL LINE
HEADING TOWARD RFD WILL HANG TOGETHER OR BUILD SOUTH ENOUGH TO
REACH THE CHICAGO AREA TERMINALS, THOUGH THE CONVECTIVE
INHIBITION THAT HAS BEEN IN PLACE FOR MOST OF THE DAY APPEARS TO
BE ERODING. THUS IT WOULD SEEM THAT CONVERGENCE ALONG THE
PRONOUNCED LINE OF CUMULUS STRETCHING SOUTH OUT OF WISCONSIN
WOULD BE ENOUGH TO TRIGGER EXPANDED DEVELOPMENT. THERE IS ALSO A
DECENT POSSIBILITY OF RANDOM POP UP CONVECTION OUT AHEAD OF
THIS LINE. THUS THE STRATEGY WAS TO EXTRAPOLATE THIS FIRST LINE
THROUGH RFD, DPA, AND ORD/MDW AND ADD A SMALL BUFFER AROUND THE
MOST LIKELY ARRIVAL TIMES.
THE SECOND ROUND OF ACTIVITY WOULD BE EXPECTED TO TRAIL THE
FIRST BY AN HOUR OR TWO, ASSUMING IT ACTUALLY MANAGES TO
DEVELOP AT ALL. UPSTREAM ACROSS IOWA THERE IS A BROAD REGION OF
AGITATED CUMULUS ALONG WITH HINTS OF A MCV CIRCULATION SHOWING
SIGNS OF TRYING TO INTENSIFY. BOUNDARY LAYER DEWPOINTS IN THE
MID TO UPPER 70S WILL CERTAINLY PROVIDE SUFFICIENT MOISTURE,
AND THIS IS ALSO THE AREA WITH BETTER SYNOPTIC AND MESOSCALE
SUPPORT IN TERMS OF AN UPPER TROUGH AND MORE PRONOUNCED SHEAR.
AFTER THESE TWO ROUNDS MOVE OUT, THERE IS ONE MORE CHANCE
OVERNIGHT INTO EARLY TOMORROW FOR A FINAL ROUND AS THE MAIN
UPPER TROUGH AND LOW LEVEL FRONTAL ZONE PUSH THROUGH.
LENNING
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT UNTIL 3 PM CDT THIS AFTERNOON FOR
ILZ006-ILZ103-ILZ104.
FLOOD WATCH UNTIL 4 AM CDT FOR
ILZ003-ILX004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
ILZ019-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
ILZ108
IN...FLOOD WATCH UNTIL 4 AM CDT FOR INZ001-INZ002
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page