696
FXUS63 KLOT 180830
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
330 AM CDT MON AUG 18 2025
KEY MESSAGES
- TODAY WILL BE HOT AND HUMID WITH PEAK HEAT INDEX VALUES NEAR
100 DEGREES.
- A ROUND OF SHOWERS AND STORMS IS EXPECTED 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 330 AM CDT MON AUG 18 2025
THROUGH TUESDAY:
AS IS COMMON IN THE SUMMER AND PARTICULARLY SO THIS SUMMER
(SEEMINGLY), TODAY'S CONVECTIVE FORECAST IS, NEEDLESS TO SAY,
UNCERTAIN. HOWEVER, WE'VE BEEN ABLE TO GLEAN SOME POTENTIALLY
KEY OBSERVATIONAL DETAILS THAT MAY HELP TIP MOTHER NATURE'S
HANDS AS TO HOW THINGS MAY PLAY OUT, OR AT LEAST A MORE FAVORED
SCENARIO AMONGST A LARGER SPECTRUM OF STILL PLAUSIBLE OUTCOMES.
THE STUBBORN CLUSTER OF STORMS THAT PUSHED ACROSS THE ROCKFORD
METRO OVERNIGHT IS FINALLY LOSING ITS STEAM AS OF THIS WRITING.
OBJECTIVE MESOANALYSIS INDICATES THAT INSTABILITY IS GREATLY
MUTED WITH NORTHEASTWARD EXTENT, WHICH SHOULD CONTINUE TO BE THE
CASE PAST SUNRISE, UNTIL LOW-LEVEL MOISTURE TRAJECTORIES RETURN
TO SOUTHWESTERLY (VS. CURRENT SOUTHEASTERLY). WHILE THE MORNING
HOURS *SHOULD* BE PRIMARILY DRY, WE CAN'T COMPLETELY RULE OUT
ISOLATED (15-20% POPS FOR A GOOD CHUNK OF THE AREA) FESTERING
SHOWERS AND NON- SEVERE STORMS FOR A LOCALIZED LIGHTNING STRIKE
AND DOWNPOUR THREAT.
OUR MAIN FEATURE OF INTEREST THAT WILL LIKELY PLAY A LARGE ROLE
IN THE CONVECTIVE EVOLUTION THIS AFTERNOON AND EVENING IS A
LOOSELY ORGANIZED MCS APPROACHING AND CROSSING THE MID MO RIVER
VALLEY EARLY THIS MORNING. OVERNIGHT CAM GUIDANCE THAT
ADEQUATELY INITIALIZED THIS MCS RESOLVED A 700 MB WIND RESPONSE
POINTING TOWARDS A REMNANT MCV LIKELY EMERGING AS THE *PROBABLY
DECAYING* COMPLEX TRACKS EASTWARD TODAY TOWARD THE MID AND UPPER
MS VALLEY.
ONE THING THAT IS OF HIGH CONFIDENCE IS AN OPPRESSIVELY HUMID
AIR MASS SLOSHING BACK INTO THE AREA TODAY, ESPECIALLY OVER AND
NEAR FARMING AREAS DUE TO EVAPOTRANSPIRATION. DEW POINTS WELL
INTO THE 70S, AND POSSIBLY NEARING 80F, ESPECIALLY WELL INLAND,
PAIRED WITH TEMPS IN THE UPPER 80S TO AROUND 90F WILL YIELD PEAK
HEAT INDICES NEAR TO AROUND 100F, WITH SOME LOCALIZED READINGS
APPROACHING 105F IN THE SOUTHWEST 1/3 OR SO OF THE CWA. LEANED
TOWARD THE LESS MIXED OUT GUIDANCE FOR DEW POINTS, WHICH HAS
BEEN THE WAY TO GO MUCH OF THIS SUMMER (VS. THE OVERLY MIXED OUT
HRRR DEPICTIONS).
THIS ELEMENT WILL BE VERY IMPORTANT AS TO HOW MUCH INSTABILITY
CAN BE REALIZED OUT AHEAD OF THE MCV THIS AFTERNOON AND HOW
EXTENSIVE THUNDERSTORM COVERAGE BECOMES. DESPITE MARGINAL MID-
LEVEL LAPSE RATES (6C/KM OR LESS), UNLIKE THIS PAST SATURDAY AND
SATURDAY NIGHT, 200 MB ELS IN THE HIGH DEW POINT AND HIGH PWAT
ENVIRONMENT CAN STILL EASILY YIELD MLCAPE OF 3500 J/KG OR MORE.
IN ADDITION, THE MARGINAL MID-LEVEL LAPSE RATES WILL
COMMENSURATELY YIELD LESS MLCIN TO ERODE DURING PEAK HEATING.
WITH THE HIGHER DEW POINT, LARGER INSTABILITY, AND LESS CAPPED
SCENARIO BEING OUR CURRENT FAVORED SCENARIO AS OF THIS WRITING,
THE FORECAST POPS THIS AFTERNOON AND EVENING WERE TAILORED TO A
AN EARLIER CONVECTIVE INITIATION AND QUICK INCREASE IN COVERAGE
AS OUTFLOW FROM THE EARLIER MENTIONED KEY MCS (AND MCV)
APPROACHES THE MS RIVER EARLY THIS AFTERNOON. 55-60% "LIKELY"
POPS (NUMEROUS STORMS) WERE FOCUSED NEAR AND NORTH OF THE
KANKAKEE RIVER VALLEY BETWEEN ~2PM-10PM CDT.
STRONG INSTABILITY, STEEP LOW-LEVEL LAPSE RATES, EXTREME DEW
POINTS, AND HIGH PWATS ARE A FAVORABLE MIX FOR DCAPE PUSHING
1500 J/KG WITH INTENSE PRECIP LOADED DOWNDRAFTS SUPPORTIVE OF AT
LEAST LOCALIZED DAMAGING DOWNBURST WIND DAMAGE. IF EFFECTIVE
SHEAR CAN PUSH INTO THE 25-35 KT RANGE AIDED BY THE MCV WIND
RESPONSE, THAT MAY UP THE ANTE FOR UPSCALE GROWTH AND A MORE
ORGANIZED LINE OF STORMS WITH SCATTERED DOWNBURST WIND DAMAGE.
IF THIS CURRENTLY MORE FAVORED SCENARIO COMES TO PASS,
INSTABILITY MAY BE SUFFICIENTLY EXHAUSTED IN THE WAKE OF THE
AFTERNOON-EVENING ACTIVITY TO BRING A LATE EVENING AND EARLY
OVERNIGHT MINIMA. SINCE WE CAN'T RULE OUT THE ALTERNATIVE OR
HYBRID SCENARIOS IN WHICH A LARGER CHUNK OF THE CWA HAS A LESS
WORKED OVER ENVIRONMENT BY THE LATE EVENING, WE HELD ONTO CHANCE
POPS TO ACCOUNT FOR THIS POSSIBILITY.
A MODEST SOUTHWESTERLY LOW-LEVEL JET OVERNIGHT CONCEPTUALLY
SHOULD REPLENISH MUCAPE MOST EFFECTIVELY WITH SOUTHWARD EXTENT,
WHERE EVENING T-STORM COVERAGE MAY END UP A BIT LESS. ADDITIONAL
IMPULSES SHOULD FUEL AN UPTICK IN SHOWERS AND STORMS,
PARTICULARLY NEAR AND SOUTH OF I-80 (WHERE 55-60% POPS WERE
FOCUSED). THE HIGH TO VERY HIGH PWATS IN PLACE TODAY CERTAINLY
DO LEND TO CONCERN FOR FLASH FLOODING, PARTICULARLY FOR THE
HARDEST HIT AREAS ON SATURDAY NIGHT. WITH THIS BEING SAID, THE
POTENTIAL FOR FAIRLY PROGRESSIVE CONVECTION THIS AFTERNOON AND
EVENING MAY KEEP THE THREAT MORE LOCALIZED. THE SETUP OVERNIGHT
MAY FAVOR A BETTER CHANCE FOR TRAINING STORMS AND FLOODING,
DEPENDING ON WHERE THE AXIS OF HIGHEST COVERAGE SETS UP,
CURRENTLY LOOKING TO BE NEAR AND SOUTH OF I-80.
ON TUESDAY, A COLD FRONT HERALDING A LONG AWAITED PATTERN CHANGE
TO COOLER AND LESS HUMID MIDWEEK AND ONWARD WILL PUSH SOUTHWARD.
FOLLOWING THE EFFECTS OF LINGERING STORMS PAST SUNRISE, FOCUS
NEAR/SOUTHEAST OF I-55 BY THEN?), THE EFFECTIVE AIR MASS SHOULD
BE SHUNTED SOUTHWARD, KEEPING THE HIGHEST CHANCE FOR RENEWED PM
STORMS PRIMARILY IN OUR SOUTHEAST 1/3 OR SO. A FEW SHOWERS AND
MAYBE AN ISOLATED T-STORM MAY NONETHELESS BE POSSIBLE NEAR THE
WI BORDER FROM CYCLONIC FLOW ALOFT. HIGHS WILL BE IN THE LOWER-
MID 80S I-80 AND NORTH AND UPPER 80S TO NEAR 90F SOUTH OF I-80
ON TUESDAY, WITH 70S DEW POINTS MAKING FOR ANOTHER UNCOMFORTABLE
DAY.
CASTRO
TUESDAY NIGHT THROUGH SUNDAY:
WEDNESDAY THROUGH THE END OF THE WORKWEEK, A SOUTHEASTWARD-
DRIFTING HIGH PRESSURE SYSTEM WILL SERVE AS THE PRIMARY
INFLUENCE OVER THE WEATHER IN THE GREAT LAKES. THE RESULT SHOULD
BE A MUCH-NEEDED BREAK IN ACTIVE WEATHER, WITH GENERALLY
SEASONABLY TEMPERATURES (HIGHS IN THE LOWER 80S, LOWS IN THE
LOWER 60S), LIGHT WINDS, AND LOWER HUMIDITY LEVELS. THE ONLY
ONLY IMPACTFUL WEATHER OF NOTE WILL BE A THREAT FOR HAZARDOUS
WAVES ON LAKE MICHIGAN BEACHES WEDNESDAY OWING TO A PUSH OF
NORTHEASTERLY WINDS AS THE SURFACE HIGH APPROACHES THE REGION.
ALSO, WOULD NOT BE SURPRISED TO SEE A FEW SPRINKLES IN THE
REGION ON WEDNESDAY ESPECIALLY IF LOW-LEVEL CAPPING ENDS UP
WEAKER THAN CURRENTLY FORECASTED. OTHERWISE, THE MIDDLE TO END
OF THE WEEK IS SHAPING UP TO BE PRETTY QUIET.
ENSEMBLE MODEL GUIDANCE IS ADVERTISING A STRONG UPPER-LEVEL LOW
BARRELING TOWARD THE GREAT LAKE 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 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.
BORCHARDT
AVIATION /06Z TAFS THROUGH 12Z TUESDAY/
ISSUED AT 1243 AM CDT MON AUG 18 2025
A CLUSTER OF SHOWERS AND THUNDERSTORMS JUST EAST OF THE
MISSISSIPPI RIVER SHOULD CONTINUE WEAKENING OVER THE NEXT FEW
HOURS AS IT ENCOUNTERS DECREASING INSTABILITY. GIVEN PROXIMITY
TO RFD, WILL CONVERT THE INHERITED PROB30 GROUP TO A TEMPO GROUP
FOR -SHRA (NOTE A FLASH OF LIGHTNING CANNOT BE RULED OUT). A
BRIEF WESTERLY WIND SHIFT MAY OCCUR AS WELL. THE CLUSTER IS
EXPECTED TO FULLY DECAY BEFORE REACHING DPA/ORD/MDW/GYY.
ATTENTION THEN TURNS TO THE THREAT FOR ANOTHER ROUND OF SHOWERS
AND STORMS AS AN UPPER-LEVEL SHORTWAVE CURRENTLY MOVING ACROSS
THE CENTRAL PLAINS ARRIVES THIS AFTERNOON. AT THIS POINT, THE
EXPECTATION IS FOR EITHER THE REINVIGORATION OF THUNDERSTORMS
CURRENTLY ONGOING ALONG THE NE/KS BORDER OR FOR NEW STORMS TO
DEVELOP ALONG RESIDUAL OUTFLOW NEAR THE MISSISSIPPI RIVER BY
EARLY AFTERNOON. SUCH AN EVOLUTION WOULD SET THE STAGE FOR THE
RESULTING LINE OF STORMS TO SWEEP ACROSS THE TERMINALS THROUGH
EARLY EVENING VANQUISHING THE INSTABILITY POOL. NOTE THAT SOME
GUIDANCE OFFERS A SEPARATE SCENARIO WHERE COVERAGE OF AFTERNOON
CONVECTION IS MINIMIZED, IN WHICH CASE A PROLONGED PERIOD OF
THUNDER WOULD MATERIALIZE AFTER DARK INTO EARLY TUESDAY MORNING
AS SHORTWAVES PARADE OVER THE UNDISTURBED UNSTABLE AIRMASS. AT
THIS POINT, WE BELIEVE THE FORMER (LINE OF STORMS) SCENARIO IS
MORE LIKELY, AS THE CAMS THAT SUPPORT THE LATTER SCENARIO
(PROLONGED OVERNIGHT THUNDER) APPEAR TO BE TOO LOW ON DEW POINTS
DURING THE AFTERNOON PREVENTING THUNDERSTORMS FROM
REDEVELOPING ON A WIDESPREAD BASIS. SO, WILL GO AHEAD AND OFFER
TARGETED TEMPO GROUPS FOR THUNDER AT ALL TERMINALS TIED TO THE
LATE AFTERNOON TO EARLY EVENING HOURS. A STRONG WESTERLY WIND
SHIFT WITH GUSTS TO 40KT (LOCALLY HIGHER) IS PROBABLE AS THE
LINE OF STORMS SWEEPS ACROSS THE AIRSPACE.
EASTERLY WINDS AT THE START OF THE TAF PERIOD SHOULD GRADUALLY
VEER SOUTHEASTERLY THROUGHOUT THE NIGHT AND THROUGH MONDAY
MORNING. AT ORD/MDW/DPA/GYY, LAKE INFLUENCE MAY KEEP THE WIND
DIRECTION EAST OF SOUTH EVEN THROUGH EARLY AFTERNOON, WHILE THE
WIND DIRECTION SHOULD VEER SOUTH TO SOUTHWESTERLY AT RFD. WIND
DIRECTIONS SHOULD "GO AROUND THE CLOCK" BEHIND THE LINE OF
STORMS FROM NORTHEAST TO SOUTHEAST BEFORE SETTLING ON
SOUTHWESTERLY BY THE VERY END OF THE TAF PERIOD.
FINALLY, THERE MAY BE PERIODIC PERIODS OF MVFR CIGS (AOA 2KFT)
THROUGHOUT THE TAF PERIOD.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT UNTIL 10 AM CDT THIS MORNING FOR
ILZ006-ILZ103-ILZ104.
IN...BEACH HAZARDS STATEMENT UNTIL 10 AM CDT THIS MORNING FOR
INZ001-INZ002.
LM...SMALL CRAFT ADVISORY UNTIL 10 AM CDT THIS MORNING FOR THE IL
NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page