609
FXUS63 KLOT 162007
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
307 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 215 PM CDT SAT AUG 16 2025
A CLUSTER OF THUNDERSTORMS CONTINUES TO FESTER ACROSS SOUTHERN
WI THIS AFTERNOON AS A MID-LEVEL SHORTWAVE INTERACTS WITH THE
HUMID AND UNSTABLE AIR MASS. RADAR TRENDS OVER THE PAST FEW
HOURS HAVE SHOWN THE CLUSTER OF STORMS, ESPECIALLY THE STORMS
NEAR AND WEST OF MADISON, TO BE STARTING TO TURN MORE
SOUTHEASTERLY WHICH PUTS THEM MORE ON TRACK TO MOVE INTO
PORTIONS OF NORTHERN IL. ADDITIONALLY, THERE IS AN OUTFLOW
BOUNDARY RACING OUT AHEAD OF THE STORMS WHICH SHOULD SERVE AS
THE FOCUS FOR NEW STORM DEVELOPMENT AS IT MOVES SOUTHWARD INTO
NORTHERN IL THIS AFTERNOON.
GIVEN THAT THE ENVIRONMENT ACROSS NORTHERN IL IS SOMEWHAT
UNSTABLE (MLCAPE AROUND 3500 J/KG) AND VERY HUMID (DEW POINTS IN
THE LOW TO MID-70S), THE OUTFLOW SHOULD BE ABLE TO KICK OFF
ADDITIONAL THUNDERSTORMS OVER THE NEXT 1-3 HOURS. THE MAIN AREA
FOR THUNDERSTORMS LOOKS TO BE FAVORED ALONG AND NORTHEAST OF A
LINE FROM ROCKFORD, IL TO RENSSELAER, IN BUT SCATTERED STORMS
COULD DEVELOP OUTSIDE OF THIS AREA IF SUFFICIENT FORCING IS
REALIZED. FROM A SEVERE PERSPECTIVE, WIND SHEAR OVERHEAD IS
RATHER WEAK AROUND 20-25 KTS AS OF 2 PM BUT SHOULD INCREASE A
BIT AS THE AFOREMENTIONED SHORTWAVE DIVES TOWARDS LAKE MICHIGAN
THIS EVENING. THEREFORE, THERE IS A CHANCE FOR SOME STRONGER
STORMS TO MATERIALIZE AND RESULT IN WINDS GUSTS UP TO 60-70 MPH
AS THE MAIN THREAT. IT WAS FOR THIS REASON THAT A SEVERE
THUNDERSTORM WATCH WAS ISSUED FOR PORTIONS OF NORTHEAST IL AND
NORTHWEST IN THROUGH 7 PM THIS EVENING.
ADDITIONAL THUNDERSTORMS WILL BE POSSIBLE BEHIND THIS INITIAL
CLUSTER AS A ANOTHER DISTURBANCE MOVES OUT OF SOUTHEAST MN
OVERNIGHT, BUT CONFIDENCE ON THE COVERAGE AND SEVERITY OF THIS
SECOND WAVE IS LOW AND WILL DEPEND ON WHAT THE INITIAL WAVE
DOES. THUS WE WILL HAVE PROVIDE MORE UPDATES ON THIS PART OF THE
FORECAST AT A LATER TIME.
YACK
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 /18Z TAFS THROUGH 00Z MONDAY/
ISSUED AT 121 PM CDT SAT AUG 16 2025
KEY MESSAGES:
- STRONG GUSTY THUNDERSTORMS MOVING INTO THE TERMINALS 19Z TO
22Z THIS AFTERNOON. GUSTS IN EXCESS OF 40 POSSIBLE WITH
NORTHERLY OUTFLOW.
- SCATTERED HEAVY RAIN PRODUCING STORMS MAY PERSIST/REDEVELOP
ACROSS PARTS OF THE AREA THIS EVENING. CONFIDENCE ON THE
COVERAGE OF THESE REMAINS LOW AT THIS TIME.
- POTENTIAL FOR A PERIOD OF LOW MVFR TO IFR CIGS EARLY SUNDAY
MORNING.
A CLUSTER OF THUNDERSTORMS CONTINUES TO DROP SOUTHWARD WITH AN
OUTFLOW BOUNDARY ACROSS SOUTHERN WI. THIS OUTFLOW BOUNDARY IS
EXPECTED TO MOVE INTO RFD, DPA AND ORD AROUND 1930Z (AFTER 20Z
AT MDW AND GYY), WITH A GUSTY NORTHERLY WIND SHIFT UP TO 40 KT.
RECENT TRENDS WITH THE CONVECTION INDICATE THAT THE OUTFLOW
BOUNDARY IS RUNNING OUT WELL AHEAD OF THE AREAS OF STORMS,
LIKELY A RESULT OF WEAKER DEEP LAYER WIND SHEAR WITH SOUTHWARD
EXTENT INTO NORTHERN IL. FOR THIS REASON, THE OUTFLOW MAY BEGIN
TO SLOW AND POTENTIALLY RESULT IN A LATER TIMING AT THE
TERMINALS. EITHER WAY, THE THREAT FOR THUNDERSTORMS WILL
INCREASE ACROSS THE AREA AFTER 20Z THIS AFTERNOON.
THUNDERSTORMS MAY PERSIST AND OUR REDEVELOP ACROSS PARTS OF
NORTHERN IL THIS EVENING. THE EXACT PLACEMENT OF THIS
REDEVELOPMENT REMAINS OF LOW CONFIDENCE, AND WILL ULTIMATELY BE
DEPENDENT UPON THE OVERALL EVOLUTION OF THE ONGOING ACTIVITY
MOVE TOWARDS THE AREA THIS AFTERNOON. FOR THIS REASON, I HAVE
CONTINUED TO HOLD ON TO THE PROB30 GROUP FOR TS THIS EVENING.
LATER TONIGHT, WINDS WILL TURN NORTH-NORTHEASTERLY OFF THE LAKE.
WHILE THIS SHOULD PUSH THE PRIMARY FOCUS FOR THUNDERSTORMS SOUTH
OF THE TERMINALS OVERNIGHT, WE WILL HAVE TO MONITOR THE THREAT
FOR SOME LOW MVFR TO IFR STRATUS FOR A COUPLE HOURS SUNDAY
MORNING.
KJB
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY UNTIL 8 PM CDT THIS EVENING FOR ILZ010-ILZ019-
ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107-
ILZ108.
IN...NONE.
LM...NONE.
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