696
FXUS63 KLOT 161122
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
622 AM CDT SAT AUG 16 2025
KEY MESSAGES
- TODAY WILL BE HOT AND HUMID WITH PEAK HEAT INDEX VALUES OF 100
TO LOCALLY 110, WITH THE HOTTEST READINGS GENERALLY ALONG AND
SOUTH OF I-80.
- A HEAT ADVISORY IS IN EFFECT GENERALLY ALONG AND SOUTH OF
I-80, BUT IT MAY BE EXPANDED NORTHWARD BY MID-MORNING BASED ON
OBSERVATIONAL TRENDS.
- THUNDERSTORMS CANNOT BE RULED OUT AT ANY TIME TODAY, THOUGH
CONFIDENCE IS INCREASING IN AN EPISODE OF STRONG TO DAMAGING-
WIND PRODUCING STORMS FROM LATE AFTERNOON THROUGH EARLY
EVENING ACROSS THE ENTIRE 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.
DISCUSSION
ISSUED AT 325 AM CDT SAT AUG 16 2025
THROUGH SUNDAY:
SHOWERS CONTINUE TO PERCOLATE AT PRESS TIME ACROSS EASTERN IOWA
ALONG THE NOSE OF A MODEST 30KT 925MB LOW-LEVEL JET. MEANWHILE,
A SEPARATE AREA OF SHOWERS IS FESTERING ACROSS SOUTHWESTERN
WISCONSIN NEAR A REMNANT MCV. AS THE LOW-LEVEL JET VEERS
WESTERLY AND WEAKENS LATER THIS MORNING, THE ACTIVITY SHOULD
SHIFT EAST-SOUTHEASTWARD AND OVER NORTHERN ILLINOIS IN DECAYING
FASHION. WITH THAT SAID, A WEAKENING TREND IS NOT GUARANTEED
GIVEN THE ACTIVITY APPEARS ROOTED AT THE BASE OF AN EASTWARD-
ADVECTING EML (THE 00Z DVN RAOB SAMPLED 8 K/KM LAPSE RATES FROM
ABOUT 700 TO 400MB). FOR NOW, WILL CARRY LOW-END CHANCE
(20-30%) FOR SHOWERS THROUGH THE MORNING HOURS JUST TO BE SAFE,
THOUGH MANY (MOST) AREAS MAY VERY WELL END UP DRY.
ONCE THE MORNING ACTIVITY ERODES, ATTENTION WILL TURN SOLIDLY
TO THE COMPLEX OF STORMS RACING EAST-NORTHEASTWARD INTO SOUTHERN
MINNESOTA AT PRESS TIME. THE COMPLEX IS TIED TO A STRONG UPPER-
LEVEL SHORTWAVE EMBEDDED IN BROAD ANTI-CYCLONIC FLOW GOING
AROUND A PRONOUNCED UPPER- LEVEL RIDGE CENTERED OVER THE LOWER
MISSISSIPPI RIVER VALLEY. HRRR GUIDANCE APPEARS TO HAVE A GOOD
HANDLE ON THE EVOLUTION OF THE COMPLEX OF STORMS, LEVERAGING
HIGHER THAN AVERAGE CONFIDENCE THAT IT WILL ROLL RIGHT THROUGH
MINNESOTA THROUGH THE MORNING HOURS AND INTO CENTRAL WISCONSIN
BY EARLY AFTERNOON. WE'LL HAVE TO WATCH FOR CONVECTION TO FIRE
AHEAD OF THE COMPLEX INTO SOUTHERN WISCONSIN TIED TO ISENTROPIC
ASCENT ATOP THE EASTWARD-ADVECTION LAPSE RATE PLUME, THOUGH AT
THIS POINT, ALL INDICATIONS ARE THAT SUCH ACTIVITY SHOULD REMAIN
NORTH OF THE STATE LINE.
AS A RESULT, IT APPEARS MUCH OF OUR AREA IS POISED TO ENDURE A
HOT AND HUMID DAY. FORECAST 850MB TEMPERATURES OF +20 TO +21C
BENEATH FILTERED SUNSHINE SHOULD LEAD TO HIGHS IN THE LOWER 90S.
WHEN PAIRED WITH EVAPOTRANSPIRATION-BOLSTERED DEW POINTS IN THE
MID TO LOCALLY UPPER 70S, THE STAGE IS SET FOR PEAK AFTERNOON
HEAT INDICES TO RANGE FROM 100 TO LOCALLY 110 DEGREES BY EARLY
AFTERNOON. AN OTHERWISE STRAIGHT-FORWARD DECISION TO ISSUE A
HEAT ADVISORY FOR THE ENTIRE AREA IS COMPLICATED BY THE THREAT
FOR CONVECTION THIS AFTERNOON, AS NORTHWESTERLY STEERING FLOW
ALONG THE NORTHEASTERN PERIPHERY OF THE UPPER-LEVEL RIDGE WILL
ENCOURAGE THE AFOREMENTIONED COMPLEX OF STORMS IN CENTRAL
WISCONSIN TO DIVE SOUTHEASTWARD INTO NORTHERN ILLINOIS BY MID-
AFTERNOON. SUCH AN EVOLUTION MAY BE ACCELERATED IF WAA-DRIVEN
CONVECTION AHEAD OF THE MAIN COMPLEX IS ESPECIALLY EFFICIENT.
THIS PRESENTS A THREAT TO BUST OUR HIGH TEMPERATURE AND HEAT
INDEX FORECAST WITH NORTHWARD EXTENT. WITH THIS IN MIND, WILL GO
AHEAD AND ISSUE A HEAT ADVISORY ROUGHLY ALONG I-80 AND POINTS
SOUTHWARD IN ILLINOIS WHERE CONFIDENCE IS RELATIVELY HIGHEST IN
IMPACTS FROM CONVECTION BEING MINIMIZED. THE DAY SHIFT MAY HAVE
TO EXPAND THE HEAT ADVISORY NORTHWARD LATER THIS MORNING IF IT
BECOMES CLEAR CONVECTION WILL REMAIN CONFINED IN WISCONSIN
THROUGH AT LEAST MID-AFTERNOON.
IN TERMS OF THE THUNDERSTORM THREAT THIS AFTERNOON, THE
EXPECTATION IS FOR THE CONGLOMERATE THUNDERSTORM COMPLEX IN
WISCONSIN TO ROLL SOUTHWARD INTO NORTHERN ILLINOIS AND
NORTHWESTERN INDIANA. WHILE THE COMPLEX MAY TEND TO OUTRUN THE
BEST DEEP-LAYER SHEAR LAGGING IN WISCONSIN, MOISTURE-LADEN
MLCAPE OF 3000 TO 4000 J/KG AND PWATS >2" WILL SUPPORT STRONG TO
DAMAGING WINDS IN THE TALLEST EPISODIC CORES WITHIN THE
COMPLEX. IF ANY LOCALIZED BOWING SEGMENT CAN DEVELOP (ESPECIALLY
IF ANY STORMS CAN DEVELOP AHEAD OF AND MERGE WITH THE COMPLEX),
A LOCALIZED THREAT FOR A CORRIDOR OF SIGNIFICANTLY DAMAGING
WINDS OF 70 TO 75 MPH MAY MATERIALIZE, AS WELL. IN ALL, THIS
AFTERNOON AND EVENING ARE SHAPING UP TO BE QUITE ACTIVE ACROSS
OUR AREA. TORRENTIAL RAIN RATES WOULD ALSO BE EXPECTED GIVEN THE
AFOREMENTIONED HIGH PWATS, THOUGH THE PROGRESSIVE PROPAGATION
OF THE COMPLEX SHOULD MITIGATE A WIDESPREAD FLASH FLOOD THREAT.
NOTE THAT WHILE HRRR GUIDANCE IS PERFORMING WELL WITH THE
COMPLEX AT PRESS TIME, IT DEPICTS THE COMPLEX WEAKENING BY THE
TIME IT REACHES NORTHERN ILLINOIS THIS AFTERNOON. THIS APPEARS
TO BE A CONSEQUENCE OF ERRONEOUSLY LOW DEW POINTS, AND HENCE,
INSTABILITY. THE 06Z NAM3 BETTER MATCHES OUR EXPECTATIONS FOR
THIS AFTERNOON.
TONIGHT, CONVECTION IS EXPECTED TO RACE SOUTHWARD INTO SOUTHERN
ILLINOIS. INCREASING ISENTROPIC ASCENT ATOP THE RESIDUAL COLD
POOL BY VIRTUE OF A BROAD 925- TO 850MB LOW-LEVEL JET ORIENTED
INTO THE UPPER MISSISSIPPI RIVER VALLEY MAY ALLOW FOR ADDITIONAL
STORMS TO DEVELOP IN THE REGION OVERNIGHT. WITH THE CORE OF THE
LOW-LEVEL JET EXPECTED TO BE POSITIONED JUST NORTHWEST OF OUR
AREA SUSPECT THAT RENEWED DEVELOPMENT WILL BE SOMEWHERE IN THE
SOUTHERN MINNESOTA/NORTHERN IOWA/SOUTHERN WISCONSIN VICINITY.
WITH THAT SAID, THE SPECIFIC MESOSCALE GEOMETRY OF THE COLD POOL
CONFIGURATION AND LOW-LEVEL JET WILL ULTIMATELY DEPEND ON THE
EVOLUTION OF CONVECTION THIS AFTERNOON AND EVENING, MEANING
NORTHERN ILLINOIS IS VERY MUCH IN THE THREAT ZONE FOR ADDITIONAL
STORMS OVERNIGHT. GIVEN A CONTINUATION OF PWATS NEAR 2" AND
NORTHWESTERLY STEERING FLOW TO ENCOURAGE TRAINING, A THREAT FOR
FLASH FLOODING MAY MATERIALIZE WHEREVER (AND IF) CONVECTION
REDEVELOPS OVERNIGHT.
TOMORROW, THE COMPOSITE OUTFLOW IS EXPECTED TO TEMPORARILY
DISPLACE THE MOST UNSTABLE AIRMASS SOUTHWEST OF OUR AREA.
ONSHORE FLOW SHOULD PROVIDE RELIEF TO HOT AND HUMID 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.
BORCHARDT
SUNDAY NIGHT THROUGH FRIDAY:
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 /12Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 622 AM CDT SAT AUG 16 2025
THE MESSAGE ATTEMPTING TO BE CONVEYED IN THE MORNING SET OF TAFS
IS AN ONGOING CONCERN FOR THUNDERSTORM DEVELOPMENT THIS
AFTERNOON INTO TONIGHT, WITH THE MOST CONFIDENCE BEING IN THE
AFTERNOON TO EVENING PERIOD. THUS THERE IS A LACK OF PROB30
GROUPS AFTER 06Z, THOUGH THIS DOES NOT MEAN THAT ADDITIONAL
THUNDERSTORM DEVELOPMENT IS NOT POSSIBLE AFTER MIDNIGHT.
WE WILL BE WATCHING THE MOVEMENT OF THE THUNDERSTORM COMPLEX
PRESENTLY OVER MINNESOTA EARLY THIS MORNING FOR A CLUE ABOUT THE
TRENDS LOCALLY THIS AFTERNOON. MULTIPLE SCENARIOS ARE IN PLAY,
SO THE FORECAST ATTEMPTS TO ACCOUNT FOR SOME OF THIS UNCERTAINTY.
ONE OPTION IS FOR THE BULK OF THE STORMS TO MISS US JUST TO THE
NORTH AND EAST, WITH PRIMARILY THE OUTFLOW PUSHING ONSHORE AS
AN ENHANCED LAKE BREEZE. WE COULD ALSO SEE A PERIOD OF ONSHORE
WINDS EVOLVE EVEN IF THE STORMS MOVE MORE DIRECTLY OVERHEAD,
WHICH WE ARE CONCERNED ABOUT AS A HIGHER IMPACT SCENARIO AND
CHOOSING TO LEAN TOWARD IN THE CURRENT FORECAST PACKAGE.
AT LEAST IT APPEARS THAT THE INITIAL ROUND OF STORMS WE THOUGHT
MIGHT BE POSSIBLE THIS MORNING DOES NOT SEEM TO BE DEVELOPING.
AS MENTIONED IN THE PREVIOUS DISCUSSION AND ALLUDED TO ABOVE,
WIND TRENDS ARE THE OTHER BIG PUZZLE TODAY. SOUTHERLY FLOW WILL
BE THE DEFAULT, BUT ANY STORMS THAT DEVELOP WILL OF COURSE THROW
THOSE INTO DISARRAY, EVEN TO THE POINT OF ALLOWING THAT ONSHORE
LAKE BREEZE FOR PART OF THE AFTERNOON.
LENNING
LOT WATCHES/WARNINGS/ADVISORIES
IL...HEAT ADVISORY FROM NOON TODAY TO 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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