888
FXUS63 KLOT 170158
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
858 PM CDT SAT AUG 16 2025
KEY MESSAGES
- STRONG TO DAMAGING-WIND PRODUCING STORMS (SOME ALSO WITH LARGE
HAIL) ARE LIKELY LATE AFTERNOON THROUGH EARLY EVENING ACROSS
THE AREA.
- ADDITIONAL STORMS MAY DEVELOP TONIGHT, THOUGH THE FAVORED ZONE
IS CURRENTLY JUST NORTHWEST OF THE AREA.
- A BRIEF BREAK IN THE PATTERN IS EXPECTED SUNDAY BEFORE HOT AND
HUMID CONDITIONS, AS WELL AS A THREAT FOR STORMS, RETURNS
MONDAY AND TUESDAY.
MESOSCALE
ISSUED AT 858 PM CDT SAT AUG 16 2025
AFTER SEVERAL ROUNDS OF STORMS THIS EVENING, A CONGLOMERATE
OUTFLOW BOUNDARY EXTENDS FROM NEAR RENSSELAER, IN WESTWARD TO
PONTIAC, IL NORTHWARD TOWARD DUBUQUE, IA. THE AIRMASS JUST TO
THE SOUTHWEST OF THE OUTFLOW BOUNDARY IS EXTREMELY UNSTABLE WITH
SURFACE DEW POINTS IN THE UPPER 70S, TEMPERATURES IN THE LOWER
80S, AND MID-LEVEL LAPSE RATES OF 8.2 K/KM (MOST RECENTLY
SAMPLED BY THE 00Z DVN RAOB) CONTRIBUTING TO MLCAPE OF 3000 TO
4500 J/KG. THIS AIRMASS CONTINUES TO SUPPORT CLUSTERS OF STRONG
TO SEVERE STORMS NEAR THE OUTFLOW BOUNDARY AT PRESS TIME.
IN THE WAKE OF A SUBTLE UPPER-LEVEL TROUGH PROGRESSING EASTWARD
TOWARD LOWER MICHIGAN THIS EVENING, RAP GUIDANCE DEPICTS A
MODEST INCREASE SOUTHWESTER 925 TO 850MB FLOW ACROSS THE OUTFLOW
BOUNDARY AND INTO NORTHEASTERN ILLINOIS AND SOUTHERN WISCONSIN
OVER THE NEXT FEW HOURS. SUCH FLOW WILL CONTINUE TO ADVECT THE
AFOREMENTIONED UNSTABLE AIRMASS ATOP THE OUTFLOW BOUNDARY,
LEADING TO A THREAT OF REGENERATING CONVECTION THROUGH THE
OVERNIGHT HOURS. CONCEPTUALLY, WOULD HAVE TO THINK THAT THE
ONGOING STORMS EXTENDING FROM FAR SOUTHWEST WISCONSIN TO FAR
NORTHWEST ILLINOIS WILL INTERCEPT THE INCREASING LOW-LEVEL JET,
THOUGH EXPERIMENTAL WOFS GUIDANCE SUGGESTS AN ADDITIONAL AXIS OF
STORMS MAY CONTINUE TO DEVELOP FROM SOUTHERN WISCONSIN INTO
NORTHERN IOWA IN THE COMING HOURS. WHEREVER THUNDERSTORMS
REGENERATE OVERNIGHT, THEY WILL PROVIDE A THREAT FOR FLASH
FLOODING GIVEN NORTHWESTERLY STEERING FLOW PARALLEL TO THE
OUTFLOW BOUNDARY AND A STEADY FEED OF EXCEPTIONALLY MOIST (PWATS
NEARING 1.8") AND UNSTABLE AIR. AT THIS POINT, WE FAVOR SUCH A
THREAT TO BE JUST OUTSIDE OUR FORECAST AREA EITHER IN SOUTHERN
WISCONSIN OR FAR NORTHWESTERN ILLINOIS. HOWEVER, WE WILL
CONTINUE TO MONITOR TRENDS TO EVALUATE IF A TARGETED FLASH FLOOD
WATCH WILL BE NEEDED ACROSS A PORTION OF OUR AREA.
BORCHARDT
DISCUSSION
ISSUED AT 307 PM CDT SAT AUG 16 2025
TONIGHT THROUGH SUNDAY:
CONVECTION IS EXPECTED TO SAG SOUTHWARD INTO CENTRAL AND
SOUTHERN ILLINOIS LATER THIS EVENING. IN ITS WAKE, MODEST
ISENTROPIC UPGLIDE WILL PERSIST INTO TONIGHT ATOP THE RESIDUAL
COLD POOL. ACCORDINGLY, THIS MAY SUPPORT ADDITIONAL SCATTERED
SHOWER AND STORMS DEVELOPMENT IN OUR REGION INTO TONIGHT.
HOWEVER, THE CORE OF THE LOW-LEVEL JET IS EXPECTED TO REMAIN
POSITIONED JUST NORTHWEST OF OUR AREA, AND THUS SUSPECT THAT
MUST OF THE SUBSTANTIAL RENEWED DEVELOPMENT WILL BE SOMEWHERE
IN THE SOUTHERN MINNESOTA/NORTHERN IOWA/SOUTHERN WISCONSIN
VICINITY. GIVEN A CONTINUATION OF PWATS NEAR 2" AND
NORTHWESTERLY STEERING FLOW TO ENCOURAGE TRAINING, A THREAT FOR
FLASH FLOODING MAY MATERIALIZE. HOWEVER, DEPENDING UPON HOW
THIS EVENINGS STORMS EVOLVE, THE HIGHER THREAT AREA FOR FLASH
FLOODING COULD REMAIN DISPLACED TO OUR NORTHWEST.
TOMORROW, THE COMPOSITE OUTFLOW IS EXPECTED TO TEMPORARILY
DISPLACE THE MOST UNSTABLE AIRMASS SOUTHWEST OF OUR AREA.
ONSHORE FLOW SHOULD PROVIDE RELIEF TO HOT CONDITIONS WITH HIGHS
EXPECTED TO TOP OFF IN THE UPPER 70S LAKESIDE TO UPPER 80S WELL
INLAND. HEAT INDICES MAY APPROACH 100 SOUTHWEST OF A LINE FROM
MENDOTA TO PAXTON, IN CLOSER PROXIMITY TO THE RESIDUAL FRONT. A
FEW STORMS MAY DEVELOP BY MID-AFTERNOON IN PULSE-LIKE FASHION
ALONG THE FRONT AS WELL, WARRANTING 20% POPS MAINLY ACROSS
CENTRAL IL.
KJB/BORCHARDT
SUNDAY NIGHT THROUGH SATURDAY:
FORECAST THINKING DURING THE LONG TERM PERIOD HAS NOT CHANGED.
PLEASE REFERENCE THE PREVIOUS DISCUSSION BELOW FOR MORE INFO.
THE STORY FOR THE COMING WEEK WILL BE CONTINUED HEAT AND
HUMIDITY, ESPECIALLY ON MONDAY, FOLLOWED BY A COOLDOWN STARTING
WEDNESDAY AND LIKELY PERSISTING INTO THE WEEKEND.
REGARDING THE HEAT, MONDAY LOOKS WORSE THAN TUESDAY, WITH THE
SOUTHWEST PORTION OF OUR FORECAST AREA PERHAPS AGAIN APPROACHING
HEAT ADVISORY LEVELS FOR A BRIEF PERIOD DURING THE AFTERNOON.
TEMPERATURES TUESDAY WILL STILL BE QUITE SUMMER-LIKE, BUT LOW
LEVEL FLOW WILL BE MORE WESTERLY THAN SOUTHWESTERLY AND
THUNDERSTORM CHANCES APPEAR TO BE MORE WIDESPREAD AS A SURFACE
LOW MOVING THROUGH THE GREAT LAKES DRAGS A TRAILING COLD FRONT
TOWARD THE AREA. BOTH DAYS ACTUALLY LOOK TO BE SUPPORTIVE OF
THUNDERSTORMS AS THE RIDGE ALOFT PUSHES MORE TO OUR SOUTH
RELATIVE TO ITS POSITION ON SUNDAY WHICH SHOULD ALLOW SHORTWAVES
TRAVERSING THE RIDGE AXIS TO MORE DIRECTLY INFLUENCE THE AREA.
FOR MONDAY THIS ACTIVITY PRESENTLY APPEARS TO BE MORE OF A
GLANCING BLOW FAVORING ESPECIALLY NORTHERN PORTIONS OF THE AREA
WHERE LAPSE RATES WILL ALSO BE MOST PRONOUNCED.
AFTER THE COLD FRONT PASSES THROUGH DURING THE DAY ON TUESDAY,
THE TRAILING SURFACE HIGH SETTLES OVER THE WESTERN GREAT LAKES
FOR THE REMAINDER OF THE WEEK. BY NEXT WEEKEND, MODELS HAVE THIS
FEATURE SLOWLY SLIDING EAST AND ALLOWING WARMER AND MORE HUMID
AIR TO START WORKING BACK INTO THE AREA AROUND ITS WESTERN
PERIPHERY. WEDNESDAY AND THURSDAY THUS APPEAR TO BE THE COOLEST
DAYS DURING THE COMING WEEK, WITH HIGHS TOPPING OUT IN THE MID
70S IN OUR NORTH AND NEAR THE LAKE, TO MID 80S SOUTH. ASIDE FROM
THE POSSIBILITY OF LINGERING ACTIVITY EARLY WEDNESDAY, SHOWER
AND THUNDERSTORM CHANGES SHOULD ALSO BE MOST LIMITED FROM
MIDWEEK ONWARD, AT LEAST UNTIL THE RIDGE MOVES EAST.
LENNING
AVIATION /00Z TAFS THROUGH 06Z MONDAY/
ISSUED AT 650 PM CDT SAT AUG 16 2025
FORECAST CONCERNS INCLUDE...
SCATTERED THUNDERSTORMS THIS EVENING.
LOW MVFR/IFR CIGS SUNDAY MORNING.
WIND SHIFT TO NORTHEAST SUNDAY MORNING.
A LINE OF THUNDERSTORMS ACROSS FAR NORTHERN IL WILL MOVE
SOUTHEAST ACROSS THE TERMINALS OVER THE NEXT FEW HOURS. AN
OUTFLOW BOUNDARY AHEAD OF THIS LINE OF STORMS MAY ALLOW FOR
NORTHERLY WINDS GUSTING TO 30KT. CONFIDENCE IS LOW FOR
ADDITIONAL THUNDERSTORMS LATER THIS EVENING AND OVERNIGHT, WHICH
MAY END UP FOCUSING SOUTHWEST OF THE TERMINALS AND TRENDS WILL
NEED TO BE MONITORED THIS EVENING. ANY SHOWERS OR THUNDERSTORMS
ON SUNDAY ARE ALSO EXPECTED TO REMAIN SOUTHWEST OF THE TERMINALS
AND HAVE MAINTAINED A DRY FORECAST.
GUIDANCE IS IN GOOD AGREEMENT FOR LOW MVFR OR IFR CIGS TO
DEVELOP IN THE PREDAWN HOURS/DAYBREAK SUNDAY MORNING ACROSS
NORTHERN IL, THE TERMINALS AND SOUTHERN LAKE MICHIGAN. CURRENT
TRENDS SUGGEST CIGS MAY REMAIN LOW MVFR BUT OPTED TO MAINTAIN
THE GOING IFR FOR NOW. THESE CIGS MAY BE SLOW TO LIFT UNTIL
MID/LATE MORNING BUT LIFTING AND SCATTERING IS EXPECTED BY LATE
MORNING/EARLY AFTERNOON.
AFTER THE THUNDERSTORMS END THIS EVENING AND THE OUTFLOW
BOUNDARIES DISSIPATE, WINDS SHOULD FAVOR A LIGHT SOUTHERLY
DIRECTION. WINDS WILL EVENTUALLY SHIFT TO THE NORTHEAST IN THE
PREDAWN HOURS OR AROUND SUNRISE AND THEN INCREASE INTO THE
10-12KT RANGE DURING THE MORNING AND AFTERNOON. SOME GUSTS INTO
THE 15-20KT RANGE ARE POSSIBLE IN THE AFTERNOON/EARLY EVENING.
CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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